Hydraulic Impacts of Quarries and Gravel Pits

Hydraulic Impacts of Quarries and Gravel Pits

I. Research Sections
1. Scope of the Issue
2. Study Design
3. Kraemer Quarry-Prairie du Chien Limestone
4. Golberg Quarry-Prairie du Chien Limestone
5. Spinier Quarry-Galena Limestone
6. Fountain Quarry-Galena Limestone
7. Harmony Big Spring Quarry-Galena Limestone
8. Donovan Pit-Alluvial Sand and Gravel
9. Leitzen-Grabau Pit-Alluvial Sand and Gravel
10. Felton Pit-Beach Ridge Sand and Gravel
SECTION 1. SCOPE OF THE ISSUE
Crushed stone and sand and gravel, generally defined as natural aggregate resources, are the building blocks for much of modern society. Rock quarries and gravel pits are common features on Minnesota's landscape. Aggregate mining is an extractive use of resources that may result in the landscape and its hydrology being altered. Operation of quarries and pits has the potential to cause impacts on ground-water and surface-water systems in various ways. This potential causes concerns about mining operations by citizens and state and local officials, as well as mine and pit operators. Quarries and gravel pits often are located in the aquifer itself; thus, water quality impacts can be direct and unmitigated. Ground-water levels surrounding many of these quarries and pits are drawn down to allow dry quarry operations, converting billions of gallons of ground water into surface water annually and reducing ground-water availability for nearby wells, wetlands, springs, streams, and lakes (Figure I .1 ). DNR Waters staff have ongoing involvement in the investigation of the hydrologic impacts of quarries and pits. DNR water resource professionals in all parts of the state are called on to make decisions related to aggregate extraction as part of water appropriation permitting and environmental review and during complaint resolution. The burden on state and local staffs is only exacerbated by the lack of definitive information about the impacts of mining and quarrying. Most information on these topics is anecdotal and little can be found in the literature about the impacts of quarries and pits on water resources. 14 Ir:l::l 12 0 C'lS 10 C) - 8 0 Ill 6 r::: 0 4 m 2 - 0 Quarry and Pit Dewatering Southeastern Minnesota and Twin Cities Metro Area Year Figure 1.1. The extent of dewatering. In 1990, the Minerals Division of DNR estimated the number of active and inactive operations at 1500 with the view that this number was likely too low (Dennis Martin, pers. comm.). Quarries for mining limestone, dolomite, sandstone, and hard rock (granite and quartzite) are found in 34 counties. A 1990 Minerals Division inventory found 165 active operations with 88% of those 3 being limestone quarries (Nelson and others, 1990). That same inventory counted 1 ,367 inactive operations with 70% of those being limestone quarries. Aggregate is an important element of our infrastructure; roads, bridges, streets, bricks, concrete, tile, paint, wallboard, roofing products, and glass are some of the commodities requiring aggregate. Concrete pavement and asphalt consist of90% and 80% aggregate, respectively. Crushed limestone is used for agriculture, medicine, and household products. Nationwide, aggregate mining in 1996 yielded about 3.25 billion tons, approximately two-thirds of nonfuel minerals produced in the United States (USGS Fact Sheet FS 14-97, 1999). Iron ore, dimension stone, and aggregate compose the predominant Minnesota nonfuel mining. Minnesota's nonfuel mineral production ranked fifth in the nation in 2003 (Ewell, 2003) and Minnesota's 2003 production of construction sand and gravel ranked sixth (Bolen, 2004). Minnesota's aggregate industry is a vital component of Minnesota's economy and serves to maintain Minnesota'.s standard of living. Aggregate demand is expected to continue to expand as Minnesota's economy remains strong and its population grows. Quarries Limestone deposits are found in southeastern Minnesota from the Twin Cities south to Iowa and west to Mankato (Figure 1.2). These deposits are fractured; as water moves through the soil it Faribault Freeborn • llliantiad Karst. Areas underlain by carbonate bedrock but wilh more than 100 ft. of sediment cover. Transition Karat. Areas underlain by carbonate bedrock with 50 · 100ft. of sediment cover. Shallow Karst. Areas underlain by carbonate badrock with less than 50 ft. of sediment cover. Copyfight c 2001 by E. Calvin Alexander Jr. and Yongll Gao. May be reproduced with atlribulion. t
Figure 1.2. Carbonate rock (limestone and dolostone) areas of Minnesota; quarries are present in the red and yellow areas. mixes with carbon dioxide to form a mildly acidic solution. As this water moves through the fractures, it dissolves the rock, enlarging the fractures and forming a system of conduits to carry ground water. This rock dissolution and conduit creation is the driving process for karst. Drew (1999) described karst, as "an area of limestone or other highly soluble rock, in which the 4 landforms are of dominantly solutional origin and in which the drainage is underground in solutionally enlarged fissures and conduits." Whenever limestone is near the surface, it has been exposed to karst processes. Though most of Minnesota does not contain "underground streams", those portions of the state with karst conditions are the exception. Because ground water in these underground conduits can convey water as rapidly as surface streams, the potential for deterioration of water levels and quality is high. Prediction of flow direction and flow volumes is difficult where there are karst conditions. Ground-water watersheds in karst environments can be and usually are quite different than the land surface watersheds. Water supplies in karst areas are quite vulnerable to unwise land use and the impacts of that use can affect water supplies quite distant from the source. Limestone quarries are mining in karst aquifers; some of these operations mine below the water table. In order to do this, the quarries must be dewatered. Dewatering can locally depress the water table, altering ground-water flow paths and affecting nearby wells, springs, and surfacewater bodies. Interception of a ground-water conduit by a quarry can interfere with ground-water flow paths, pirating the flow and redirecting the discharge to a completely different location (Green and others, 2003). Two examples of the impacts of limestone quarries that require dewatering can be found in southeastern Minnesota. At Owatonna, Minnesota, the Fretham and Lundin quarries mine below the water table in the Galena limestone and are dewatered for mining. Between 1985 and 1992, DNR Waters staff received several complaints about wells near the quarries going dry or losing pressure. (Pressure loss can be a symptom of a water level that has dropped too close to the level at which the pump is set. Drawdown during active pumping then brings the water level to the pump intake causing the pump to suck in air.) The investigation determined that these wells were also in the Galena limestone and were in fact being impacted by the dewatering. In order to resolve the issue, the quarry operators paid to have the homes connected to the City of Owatonna's water system. A second example is the Osmundson quarry in the Lithograph City Formation at LeRoy, Minnesota. This below water table quarry requires seasonal dewatering at 250 gallons per minute to 800 gallons per minute. When the quarry is being dewatered, Sweets Spring, approximately 325 yards to the southeast, stops flowing. Dye traces in 1993 and 1994 verified that the quarry pirates the ground-water flow to the spring. Pits Generally, sand and gravel operations are found in deposits formed during the advance and retreat of glaciers and in alluvial floodplain deposits formed by streams. Both types of deposits often are critical ground-water aquifers and recharge areas in upland settings; they often are focused discharge zones in stream and river valleys where wetlands and springs depend on continued ground-water flows through the sand and gravel. Because sand and gravel deposits allow comparatively high infiltration rates and relatively rapid rates of water transfer within an aquifer, activities and land uses within and above granular aggregate can have negative effects on groundwater quantity and quality within aquifers. Where decisions are made to leave the sand and gravel deposits in place to provide natural resource values and ground water for human use, the availability of the aggregate resource will be limited. This fact concerns those who plan for the state's future aggregate production. One example of the impacts of sand and gravel mining can be found in Clay County wbere t/Je Buffalo aquifer is the primary potable water supply for the City of Moorhead during drought conditions. Lying within the flat lake bed of Glacial Lake Agassiz (now called the Red River Valley), aquifer recharge rates are very low; replenishment of ground water takes a long time 5 because the very dense clay sediments that encase the Buffalo aquifer prevent water from reaching the aquifer horizontally. Composed of coarse granular materials, the aquifer is also the closest source of aggregate materials for the Moorhead/Fargo area. Excavation of sand and gravel in this area below depths of about 30 feet actually removes aquifer material from beneath the water table. In the northern one-half of this area, the sands and gravels are protected from direct introduction of fluids and contaminants by a blanket of less permeable silt and clay. The gravel mines create openings (windows) through the overlying silts and clays into the aquifer. Along with direct contamination due to mining operations or neglect after mine closure, the potential of aquifer contamination due to introduction of contaminated floodwaters is significant because the Red River Valley regularly experiences broad overland flooding. Floodwaters incorporate everything from farm chemicals to tanker spills along two major roads (Interstate 94 and U.S. Highway 1 0). If contaminated floodwaters enter the pit, they can be readily introduced into Moorhead's primary drought period water supply. Along with providing drought supply to Moorhead, the Buffalo aquifer supplies water to surrounding rural farmsteads and several smaller communities in the area. In recognition of the importance of the aquifer, Moorhead has taken proactive steps to protect their source of water despite the fact that the aquifer is located outside of its jurisdictional boundaries. Acknowledging the low recharge rates into the aquifer due to the protective silts and clays, the city greatly reduced its use of the aquifer, saving it for drought conditions. It has conducted technical studies and developed a wellhead protection plan; it also recognizes that gravel pit windows provide direct conduits for contamination to reach the aquifer and its water supply. Recently, a company opened a new pit above the aquifer to provide fill for Interstate 394 connecting Interstate 94 and U.S. Highway 10 without a permit from Clay County. The city took legal action and worked with the county tlu-ough their conditional use permit process to substantially reduce the potential of contamination to the aquifer and limit future expansions of the pit. Study Purpose The Minnesota Legislature established the Aggregate Resources Task Force in 1998 because of the importance and dwindling supply of aggregate resources. The task force published findings and recommendations "for the management of aggregate resources throughout the state, helping to ensure the continued availability of these resources for future use at reasonable costs while maintaining existing environmental safeguards related to mining" (Southwick and others, 2000). The task force identified that local government units (LGUs) often lack the expertise to assess potential environmental impacts of mining proposals, and rarely do they have budgetary resources to hire consultants of their own to adequately evaluate mining proposals and ensure that environmental safeguards remain in place. This study and this report are intended to provide the following assistance: • help local officials, the public, and the mining industry understand the main issues surrounding mine establishment and • provide suggestions for monitoring and mitigating strategies to prevent significantly harmful impacts on water resources. The focus of this study was on the following impacts: • effects on ground-water levels from mining operations and mine dewatering, • turbidity in wells due to blasting and quarry operations, • interruption of conduit flow paths by rock removal, and 6 I • temperature change (thermal impacts) in springs and surface-water streams. This study was proposed for LCMR funding in order to begin systematic evaluation of aggregate mining impacts at test sites in several areas of the state. 7 8 100 SECTION 2. STUDY DESIGN Project Site Map -- 0 ~ Limestone Quarry X Sand & Gravel Pit D Counties in Minnesota N -~· s 100 Miles --- Figure 2.1. Site location map. 9 Site Selection Five quarries and three sand and gravel pits were studied (Figure 2.1 above). The limestone quarries are the following: • Kraemer quarry, Burnsville, Dakota County; • Golberg quarry, north of Rochester, Olmsted County; • Spinier quarry, southwest of Owatonna, Steele County; • Fountain quarry, Fountain, Fillmore County; and • Big Spring quarry, west of Harmony near the unincorporated village of Big Spring, Fillmore County. The sand and gravel pits are the following: • Donovan pit, Salem Township, Olmsted County; • Leitzen-Grabau pit, Salem Township, Olmsted County; and • Felton pit, near Felton, Clay County. Impact Monitoring Table 2.1 lists the sites and the impacts that were monitored during the project. The text following the table describes the monitoring at the sites. Ground-water impacts studied Site Mineral resource Temperature Spring Water level Turbidity change diversion Prairie du Kraemer Chien X X limestone Prairie du Golberg Chien X X limestone Spinier Galena limestone X Fountain Galena limestone X Big Spring Galena limestone X X Donovan Alluvial sand and gravel X X Leitzen- Alluvial sand Grabau and gravel X Glacial beach Felton ridge sand and X ravel Table 2.1. Summary table of sites and impacts studied. 10 Water Level Wells were monitored at three limestone quarries, Kraemer, Golberg, and Spinier, to measure the extent of the impact of dewatering on water levels in the areas around the quarries. All three of these sites are below-water table operations that require dewatering for mining operations to occur. Project funds were used to install wells at all three sites. The Spinier site had additional wells in place that had been installed by the quarry owner as part of it~ water appropriation permit requirements. At the Felton, Donovan, and Leitzen-Grabau sand and gravel pits, mining activities occur below the water table. This is "wet mining" involving no dewatering. Monitoring wells had been installed at the Felton mine as part of an ongoing research project to evaluate the mine's impact on a nearby calcareous seepage fen. At the Leitzen-Grabau and Donovan sites, Salem Township had required the mining companies to install monitoring wells as a stipulation in their conditional use permits. Project ftmds were used to install a second set of shallow wells at the Donovan pit. The wells at these sites were monitored for mining impacts on ground-water levels. At the Kraemer and Golberg sites, blasting impacts on ground-water levels were also monitored. Turbidity At the Kraemer, Golberg, and Fountain sites, blasting impacts on turbidity levels were monitored. The monitoring at Kraemer and Golberg sites was done with turbidity sensors in the monitoring wells. Most limestone and dolostone quarries in Minnesota are dry quarries; the quarrying activities occur above the water table. In these cases, the direct impacts of dewatering on the ground-water system are not manifested. There are, however, issues associated with these dry quarries. Concerns include the impacts these quarries can have on ground-water quality in the area and the particular impacts of mining and blasting on neighboring wells. In order to partially address these issues, we investigated the impacts of the quan-y operated by Milestone Materials Division of Mathy Construction near Harmony, Minnesota, on the Fountain Big Spring, which is near the quarry. Hydraulic Diversion of Spring Flows The Big Spring quan-y is west of Harmony near the unincorporated village of Big Spring. In the early 1960s, the quarry breached conduits carrying ground water to the Big Spring on Camp Creek. Water that fonnerly discharged directly from the spring now discharges into the quarry. Most of the flow sinks back into a conduit and then discharges from the Big Spring, and the remainder flows overland to Camp Creek. Previous dye tracing work had demonstrated that a significant portion of the springshed (the area contributing flow to a spring) of the Big Spring had been diverted to the quarry. During this project, we were able to use dye tracing to more accurately quantify the extent of this conduit piracy. Water Temperature At the Big Spring quarry we began the process of assessing the impact that spring diversions might have on the temperatures of the springs and stream. At the Donovan site, the shallow wells and the pond created by mining were monitored to determine if there were thermal impacts on these systems from mining activities. 11 Quarry Site Descriptions Kraemer Quarry Figure 2.2. Kraemer quarry site photograph. The Kraemer quarry (Figure 2.2) is located in township 27, range 24W, section 33, a half-mile north of State Highway 13, west of Interstate 35W, and a quarter-mile south of the Minnesota River in Burnsville, Dakota County. The primary resource being removed from the site by Kraemer and Sons is ctushed limestone of various grades. In a cooperative effort between Kraemer and Sons and DNR Waters, three wells were drilled on the southwest part of the property to a depth of about 120 feet. The area that is currently excavated is about 235 acres; however, the total disturbed area is closer to 500 acres. The precipitation normal for the site is 29.41 inches based on area data from the National Oceanic Oceanic and Atmospheric Administration (NOAA) from 1971 to 2000. Daily precipitation was collected at station 217538, located 1 mile from the site, by the state climatology program's highdensity network. The 2001 through 2005 precipitation is presented in Figure 2.3. 12 Precipitation Station 217538 with Cumulative Departure from Normal 30 ,-Figure 2.3. Precipitation data 2001-2005 near Kraemer Quarry. Cumulative departure from normal is a measure of long-term precipitation trends. The departure from normal is calculated by subtracting the 1971-2000 monthly precipitation normals from the monthly precipitation. This is summed over the period of interest providing a measure of precipitation trends. The precipitation in the area has been above normal for the study duration with big increases in precipitation in 2002. The precipitation in 2003 through the first few months of 2005 has been near average. 13 Golberg Quarry Figure 2.4. Golberg quarry site photograph. The Golberg quarry (Figure 2.4) is located in township 108, range 14W, section 36, north of County Road 14 and along the banks of the Zumbro River in Olmsted County. The site is 5 miles northeast of the city of Rochester. The primary resource being removed from the site by Milestone Materials Division of Mathy Construction is crushed limestone of various grades. In a cooperative effort between Mathy Construction and DNR Waters, two wells were drilled on the north property line to a depth of about 120 feet. The area that is currently being mined is about 51 acres; however, the total disturbed area is closer to 150 acres. The precipitation normal for the site is 31.40 inches based on area data from NOAA from 1971 to 2000. Daily precipitation was collected at station 217009, located 4 miles from the site, by the state climatology program's high-density network. The 2001 through 2005 precipitation is presented in Figure 2.5. 14 18 16 14 4 2 Precipitation Station 217009 with Cumulative Departure from Normal Date 7/1/03 1/1/04 7/1/04 Figure 2.5. Precipitation data 2001-2005 near Golberg quarry. 1/1105 Cumulative departure from normal is a measure of long-term precipitation trends. The departure from normal is calculated by subtracting the 1971-2000 monthly precipitation normals from the monthly precipitation. This is summed over the period of interest providing a measure of precipitation trends. The precipitation in the area has been above normal for the study duration with big increases in precipitation in the first halves of2001, 2002, and 2004. It was slightly drier in the second half of 2003, but the overaii departure from normal remained positive. 15 Spinier Quarry Figure 2.6. Spinier quarry site photograph. The Spinier quarry (Figure 2.6) is located in township I 06, range 21 W, section 1, a half-mile west of County Road 30, two miles west of Interstate Hwy 35, south of 51st Road, and a half-mile west ofthe Straight River in Steele County. The site is 6 miles southwest ofthe city of Owatonna. Crushed limestone is the primary resource being removed from the site by Milestone Materials Division of Mathy Construction. The site was initially operated by Crane Creek Construction as a sand and gravel pit but switched to limestone mining when bedrock was reached. In an effort to determine impacts on the local ground water and the Straight River, Crane Creek Construction was required to drill three bedrock wells around the quarry and six shallow sand and gravel wells between the quarry and the river. Two additional shallow wells were drilled by DNR Waters to supplement the infonnation previously gathered. The area that is currently excavated is about 34 acres; however, the total disturbed area is closer to 58 acres. The precipitation normal for the site is 31.64 inches based on area NOAA data from 1971 to 2000. Daily precipitation was collected at station 216287, located within a mile ofthe site, by the state climatology program's high-density network. The 2001 through 2005 precipitation is presented in Figure 2.7. 16 Precipitation Station 216287 with Cumulative Departure from Normal 18 O ~LL~~LLLI~~LL~~LLLI~~LL~~LLLI~~LL~~LLLI~~~ 1/1/01 7/1/01 1/1/02 7/1102 1/1/03 Date 7/1/03 1/1/04 711104 1/1/05 Figure 2.7. Precipitation data 2001-2005 near Spinier quarry. Cumulative departure from normal is a measure of long-term precipitation trends. The departure from normal is calculated by subtracting the 1971-2000 monthly precipitation normals from the monthly precipitation. This is summed over the period of interest providing a measure of precipitation trends. The precipitation in the area was above normal in the first halves of2001 and 2004. The precipitation during the remainder of 2001, 2002, and the first half of 2003 was about normal. The end of2003 was drier than normal. 17 Fountain Quarry Figure 2.8. Fountain quarry site photograph. The Fountain quarry (Figure 2.8) is located in township I 03, range 11 W, section 3, north of County Road 8, and a half-mile west of U.S. Highway 52 in Fillmore County. The site is 25 miles southeast of the city of Rochester. The primary resource being removed from the site by Milestone Materials Division of Mathy Construction is crushed limestone of various grades. No wells were installed in the vicinity of the quarry; however, turbidity was being monitored in a spring that was shown to be taking runoff from the quarry floor through a dye trace performed by University of Minnesota Geology Department in cooperation with DNR Waters. The area that is currently being mined is about 24 acres; however, the total disturbed area is closer to 47 acres. The Fountain quarry is in the flat-lying Stewartville and Prosser Members of the Ordovician Galena Group. The Prosser is fine-grained, thin-bedded limestone with minor shale partings while the Stewartville is fine-grained dolomitic limestone and dolostone (Mossier, 1995). Both of these formations exhibit classic karst features when they are in a shallow setting as they are here. Numerous sinkholes, stream sinks, springs, and caves are found in this area. In the quarry there are four open joints on the floor. These joints generally have water flowing through them except during the driest periods of the year. The precipitation nonnal for the site is 34.29 inches based on area NOAA data from 1971 to 2000. Daily precipitation was collected at station 216654, located 10 miles from the site, by the state climatology program's high-density network. The 200 1 through 2005 precipitation is presented in Figure 2.9. 18 15 10 5 ~ 0 c 0 E -5 Q.. u f -10 n.. -15 -20 -25 Precipitation Station 216654 with Cumulative Departure from Normal ,------------- -------------------, • Precip -L---------------------------1-.-cum. Departure Date Figure 2.9. Precipitation data 2001-2005 near Fountain quarry. Cumulative departure from normal is a measure oflong-term precipitation trends. The departure from normal is calculated by subtracting the 1971-2000 monthly precipitation normals from the monthly precipitation. This is summed over the period of interest providing a measure of precipitation trends. The precipitation in the area has been significantly below normal through spring 2004. Early summer 2004 was wet and precipitation has been about normal since then; however, a precipitation deficit still exists. 19 Big Springs Quarry Figure 2.1 0. Big Springs quarry site photograph (2003). The Big Springs quarry (Figure 2.10) is located in township 101, range lOW, section 9, south of County Road 22 and about 2 miles west of U.S. Highway 52 in Fillmore County. The site is near Harmony, Minnesota, 38 miles southeast of the city of Rochester. The primary resource being removed from the site by Pederson Brothers Construction is crushed limestone of various grades. The Big Spring quarry is in the flat-lying Stewartville and Prosser Members of the Ordovician Galena Group. The Prosser is fine-grained, thin-bedded limestone with minor shale partings while the Stewartville is fine-grained dolomitic limestone and dolostone (Mossier, 1995). Both of these formations exhibit classic karst features when they are in a shallow setting as they are here. In this area they have numerous sinkholes, stream sinks, springs, and caves. In the quarry face, solution conduits up to 3 yards in diameter have been exposed as the quarry has expanded. The area that is currently being mined is about 35 acres; however, the total disturbed area is closer to 47 acres. In the early 1960s, quarrying operations disrupted the conduits carrying flow to Big Spring (A24), the headwaters of a trout stream, Camp Creek, which lies 550 yards north of the quarry. The owners of the spring have stated that when this disruption occurred, the flow from the spring decreased. At that time, water started rising in the quarry at several different points; some flows overland to Camp Creek while the rest sinks back into the quarry and resurges in Big Spring (A24). This quarry is not actively dewatered. The ratio of overland flow versus resurging flow varies depending on spring stage and runoff events. As the quarry has expanded to the south, the points at which the water discharges in the quarry have migrated south also. Figure 2.11 below is a photograph of Big Spring East, A238, the main point where water discharges in the eastern part of the quarry. 20 \ ; \' \'\ ~··· \ .~. . I ,,I \ "l \ Figure 2.11. Big Spring East, the main discharge point in the eastern part of the quarry. The precipitation normal for the site is 34.29 inches based on area NOAA data from 1971 to 2000. Daily precipitation was collected at station 213520, located 1 mile from the site, by the state climatology program's high-density network. The 200 I through 2005 precipitation is presented in Figure 2.12. Cumulative departure from normal is a measure of long-term precipitation trends. The departure from normal is calculated by subtracting the 1971-2000 monthly precipitation normals from the monthly precipitation. This is summed over the period of interest providing a measure of precipitation trends. The precipitation in the area has been about average through summer 2002. In late summer 2002 through the following year, the precipitation was significantly below normal. The end of 2003 was about normal and was followed by a wet spring in 2004. Precipitation has been about normal since then. 21 Precipitation Station 213520 with Cumulative Departure from Normal 15 ------··--·----·· 10 -10 -15 -20 ------------------ ----- Date Figure 2.12. Precipitation data 2001-2005 near Big Spring quarry. 22 Sand and Gravel Pit Site Descriptions Donovan Pit Figure 2.13. Donovan pit site photograph. The Donovan pit (Figure 2.13) is located in township 106, range 15W, section 24, just west of County Road 104 and 250 feet south of the Zumbro River in Olmsted County. The site is in Salem Township, 5 miles southwest of the city ofRochester. Milestone Materials Division of Mathy Construction is removing sand and gravel from the pit for use in the reconstruction of U.S. Highway 52 through Rochester. As required by the conditional use permit through the township, Milestone Materials Division of Mathy Construction installed three wells to monitor ground-water levels around the pit. One of the company's wells was located upgradient of the pit and two were located downgradient; all were drilled to.about 50 feet. Four additional shallow wells (15 to 20 feet deep) were drilled by DNR Waters with project funds to supplement the information provided by the company wells. The area that is to be mined is about 33 acres; however, the total disturbed area currently is 15 acres. The most recent past use of the site was as an agricultural field on a soybean and com rotation. The site is located almost entirely within the Zumbro River floodplain and is subject to inundation when the river floods. The precipitation normal for the site is 31.4 inches based on area NOAA data from 1971 to 2000. Daily precipitation was collected at station 217 422, located 4 miles from the site, by the state climatology program's high-density network. The 2001 through 2005 precipitation is presented in Figure 2.14. 23 'E" :;. 1: 0 !·a () ! L 12 10 8 6 4 2 0 Precipitation Station 217422 with Cumulative Departure from Normal Date Figure 2.14. Precipitation data 2001-2005 near Donovan pit. Cumulative departure from normal is a measure of long-term precipitation trends. The departure from normal is calculated by subtracting the 1971-2000 monthly precipitation normals from the monthly precipitation. This is summed over the period of interest providing a measure of precipitation trends. The first half of 2001 was wetter than normal and the second half of 2001 through the first half of 2003 was about normal. The second half of 2003 was drier than normal; however, it recovered in the first part of 2004 and has been about normal through the first couple months of2005. 24 Leitzen-Grabau Pit Figure 2.15. Leitzen-Grabau pit site photograph. The Leitzen-Grabau pit (Figure 2.15) is located in township 106, range 15W, section 25,just west of 60th Ave SW and south of County Road I 17 along the Zumbro River in Olmsted County. The site is in Salem Township, 5.5 miles southwest of the city of Rochester. The Leitzen-Grabau pit is in an alluvial sand and gravel deposit in the floodplain of the South Fork Zumbro River. Pebbles and cobbles of banded iron ore, granite, and other rocks from northern Minnesota are clear evidence that at least some of the material was transported to the area by glaciers. Since this area was not covered by ice during the last glacial advance, these materials likely date back to an earlier glacial advance. The materials were reworked by the Zumbro River and deposited in the alluvial plain. The primary resource being removed from the site by Leitzen Concrete is sand and gravel being used in the reconstruction of U.S. Highway 52 through Rochester. As a condition of its conditional use permit through the township, Leitzen Concrete was required to install three wells to monitor ground-water levels around the pit. One well was located upgradient of the pit and two were located downgradient. The area that is to be mined is about 32 acres; however, the total disturbed area currently is 19 acres. The most recent past use of the site was as an agricultural field on a soybean and corn rotation. The precipitation normal for the site is 31.4 inches based on area NOAA data from 197 I to 2000. Daily precipitation was collected at station 217422, located 5 miles from the site, by the state climatology program's high-density network. The 2001 through 2005 precipitation is presented in Figure 2.16. 25 Precipitation Station 217422 with Cumulative Departure from Normal 12 ,---------------------------------------------------------------, 10 8 Date Figure 2.16. Precipitation data 2001-2005 near Leitzen-Grabau pit. Cumulative departure from normal is a measure of long-term precipitation trends. The departure from normal is calculated by subtracting the 1971-2000 monthly precipitation normals from the monthly precipitation. This is summed over the period of interest providing a measure of precipitation trends. The first half of 2001 was wetter than normal and the second half of 2001 through the first half of 2003 was about normal. The second half of 2003 was drier than normal; however, it recovered in the first part of2004 and has been about normal through the first couple months of2005. 26 Felton Pit Figure 2.17. Felton pit site photograph. The Felton study area has several active gravel pits, the largest of which are the Trust Fund Pit and the Clay County Pit. The first is a large open-water gravel pit (Figure 2.17), which is managed by the DNR Division of Forestry for the School Trust Fund. This pit is located in township 142N, range 45W, SW1/4 NW 1/4 section 32. Construction-grade gravel has been removed from the deposit on the Trust Fund parcel since 1959. The lease is currently held by Aggregate Industries. Mined to approximately the ground-water table, the Clay County Pit is situated in township 142N, range 45W, S1/2 section 6. Substantial gravel resources have been identified below the water table in and surrounding the pit. Both pits are at the western edge of the top of the Lake Agassiz beach ridge. The two calcareous fens, simplistically named North Fen and South Fen, are downslope of and 30 feet lower in elevation than the beach ridge top. The South Fen is located in township 142N, range 46W, SE 1/4 section 36; the North Fen is approximately 1,000 ft northeast of the South Fen in township 142N, range 45W, SW 1/4 NW 1/4 section 31. Contrasting with the relatively flat North Fen (elevation 967), the South Fen slopes approximately 18 feet from its eastern, upgradient edge (elevation 980) to its western edge (elevation 962). Ground-water gradients were monitored at four well nest locations: at each of the fens and upgradient of each of the fens between the gravel pits and the fens. Water levels were monitored in other single-well installations surrounding both the Trust Fund Pit and the County Pit. Water level monitoring provided the framework for a conceptual model used to assist in assessing the reasons for degradation of the North Fen. A weather station was installed on the southwestern edge of the South Fen, and a staff gage was installed in the open-water pit to record water levels. The two closest NOAA locations to the study area are Ada (15.5 miles north-northeast) and Halstad (26.7 miles northwest). The precipitation normal for the Ada site is 23.79 inches and for the Halstad site is 19.89 inches based on updated area NOAA data. 27 28 SECTION 3. KRAEMER QUARRY-PRAIRIE DU CHIEN LIMESTONE 2000~JI"""""''~iiiiiiil!-!'!'!"!!liiiiiiiiiiii'!JI"""""''~oiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii~2;ii!OOO~!'!'!"!!!'!'!"!!!'!'!"!!!'!'!"!!!!'!I4000 Feet Figure 3.1. Site by aerial photograph. Impacts on Water Levels Kraemer Quarry Burnsville -$- Monitoring wwll s Water level monitoring at the Kraemer quarry was accomplished by measuring water levels in three wells drilled for the project (Figure 3.1 ). The wells were measured manually several times per month during quarrying activities. Pressure transducers measured water levels at 15-minute intervals. Because the quarry personnel do not tum off the quarry pumps, water levels do not fluctuate very much from year to year. Seasonal variation is usually slight with an increase ofless than 3 feet occurring in early summer from snowmelt (Figure 3.2). A reconstruction of historic water levels (Figure 3.3), using available information from the landscape and all available data, reveals that water levels in the immediate vicinity of the quarry have declined by at least 70 feet since quarrying activities began in 1959 (Figure 3.4 ). This is largely due to pumping at the quarry, but there are other water users in the area that may also influence the current drawdown. The Kraemer quarry is not the only quarry that has existed in the area, and there may have been depressed water tables due to other quarry operations in the past. 29 Kraemer Quarry Wells 635.00 +-----~----~----~---~-----------1 110101 05a002 -e,oe,G2 0012403 Date 01004 0712804 Figure 3.2. Seasonal variation of water levels in Kraemer quarry wells. Kraemer Historic Water Levels " W~.E 40f!i0011!"'!!!!!11!"'!!!!!!iiiiiiiiiiiiiiiiiiil!!ll!"'!!!!!ll!"'!!!!!ll!"'!!!!!ll!"'!!!!!!!!!!4005i0i;;;;;;;;;iiiiiiiiiiiiiiiiiiiiiiiiil8000 Feet "' / Water Levels Figure 3.3. Historic water levels in Kraemer quarry. 30 02!13{)5 " W~E s 5000 0 5000 Feet N Water Level ® Monitoring wells Figure 3.4. Water levels at drawdown in Kraemer quarry wells. Prior to quarrying and pumping in this area, ground water flowed from the upland toward the Minnesota River, sustaining a series of wetlands parallel to the river along its flowpath. Currently, water flows radially into the quarry from at least the south and west. Flow information is lacking for areas around some of the quarry perimeter; however, given the seepage faces seen around the quarry, it can be assumed that water is flowing into the quarry from all sides. Most emerges from the south and west sides of the pit. Because water levels in the quarry are lower than the water level in the Minnesota River, ground water flows into the quarry from the river on the north side. Blasting Impacts Turbidity in the drilled wells was measured with an instrument that can be left in the well to record changes in turbidity. Throughout the monitoring period, no significant changes in turbidity were observed. It is possible that no significant changes occurred because of the lack of variability in pumping and water levels at the site. Monitoring wells such as those at the Kraemer quarry are rarely if ever pumped. The stability of ground-water levels in this quarry and the stability of the pumping level within the quany is not likely to suddenly dislodge particles that will result in higher turbidity because the water is moving at a relatively stable velocity. Little change in flow likely means little disruption to the matrix. 31 We were also interested in turbidity changes in response to blasting events. In theory, particles in or near the well could be dislodged by the energy of the blast and cause cloudiness in the water, that is, increased turbidity. Several blasting events were monitored during summer 2002 and 2003 along the southern portion of the west quarry face near the monitoring wells. Kraemer l_W ater Level and Turbidity 8/28/02 39.47 0 47:31.2 48:57.6 50:24.0 51:50.4 53:16.8 54:43.2 56:09.6 57:36.0 59:02.4 00:28.8 01:55.2 Time (Min:Sec) Figure 3.5. Comparison of water levels and turbidity in Kraemer quarry wells. On August 8, 2002, blasting occurred below the water table. This event is characteristic of most of the data collected during such events. No response is seen in turbidity levels. Because the wells that are monitored in this case are relatively new, there may be little material present to be dislodged. The blasting results shown in Figure 3.5 reveal a slight change in water level immediately following the blast. A drop in ground-water level is observed that we attribute to the removal of a portion of the rock matrix. Kraemer Quarry Downhole Camera Given the inconclusive results of our turbidity monitoring and our determination that the condition of the well may be key to understanding the results, we deployed a downhole camera at Kraemer Quarry on April 7, 2005. Well I is the well closest to the quarry. When it was drilled, the casing terminated below the water table. At the time the well was video logged, the water table was approximately 3 feet below the bottom ofthe casing. This was a very visible impact of rock removal in the quarry that lowered the water table. The well casing was intact with no apparent damage. One bedrock void 32 was noted 94 ft below land surface but there was no visible flow. The well had much particulate matter in the form of mats of orange-white matetial, particles, and areas where the particulate literally formed clouds in the well. The logging terminated at the bottom of the well at 99ft. Well2 is also on the quarry property. The water table was above the casing on this date. The well casing was intact with no apparent damage. Like well 1, this well had much particulate matter in the form of mats of orange-white material, particles, and areas where the particulate literally formed clouds in the well. Bedrock voids were noted at 73.4 ft, 78.7 ft, 81 ft, and 82.4 ft. The void at 73.4 ft was the only one where flow could be observed. As the camera went down the well, particles would slowly settle alongside the camera. At this void, the particles were being pushed into the well by inward flow of ground water. The logging terminated at the bottom of the well. Well 3 is off-site to the south of the quarry. The water table was above the casing on this date. The well casing was intact with no apparent damage. Like the other two wells, this well had much particulate matter in the form of mats of orange-white material, particles, and areas where the particulate literally formed clouds in the well. Voids were encountered at 63 ft to 65 ft with no visible flow. Below this depth, the water became significantly clearer as the visible amount of particulates dropped by several orders of magnitude. Voids were also encountered at 74.2 ft, 75 ft, 81 ft, and 82.5 ft. The void from 81 ft to 82.5 ft was so large that it encompassed about half of the drill hole. No flow was visible from these voids. At 87 ft, we encountered a larger bedrock ledge at the base of a void. This void appeared to have alluvial gravel material on its floor. This ledge was so large that it would have made camera retrieval problematic if the camera was lowered past it. Therefore, logging was terminated at 87 ft. Conclusions Dewatering at this quarry has profoundly affected ground-water levels in the Prairie du Chien aquifer in the area around the quarry. The primary impact of dewatering is the alteration of the water table and resulting draining of adjacent wetlands. The lack of turbidity change indicates that the turbidity levels were fairly constant and probably more related to the density of bacterial mats floating in the well than anything that happened in the aquifer. Even with frequent blasting, turbidity wasn't observed to be a problem at the site. Quarrying operations had no visible impact on the integrity of the observation well's casing. The casings all appeared to be intact and in good condition. No holes, ruptures, or seam failures were visible. Based on this work, future monitoring projects should use videologging prior to the start of monitoring. In this case and if monitoring is resumed in these wells, turbidity loggers could in particular be placed at the active conduit in Kraemer well 2 and in the large void zone in Kraemer well 3. This fine-tuning of instrument placement may allow more accurate data to be obtained. These video logs will also serve as a point of reference when DNR Waters staff run the camera down these wells again in 3 to 5 years. 33 34 SECTION 4. GOLBERG QUARRY-PRAIRIE DU CHIEN LIMESTONE Golberg Quarry -$- Monitoring well . w~.• 2000 Feet Figure 4.1. Site by aerial photograph. Impacts on Water Levels The two drilled wells on the quarry site (Figure 4.1) were measured several times per month during quarrying activities, and pressure transducers measured water levels at 15-minute intervals. The quarry company does not turn off the pumps during winter, and the quarry expansion toward the wells has removed rock, which has caused ground-water levels to decline. Ground-water levels usually rise seasonally during late spring or early summer from snowmelt (Figure 4.2). 35 Golberg Quarry Wells 907.00 ,.------------------------------,- 894 .00 882.00 898.00 .J------.------,------r-----~-----,.-----+ 880.00 12/25/01 07/13/02 01/29/03 08/17/03 03/04/04 09/20/04 04/08/05 Date Figure 4.2. Seasonal variation of water levels in Golberg quarry wells. Blasting Impacts Blasting events were monitored during 2002 while the pit was expanded north. Similar to results at the Kraemer site, ground-water levels changed immediately following the blast (Figure 4.3). We were also interested in turbidity changes in response to blasting events. In theory, particles in or near the well could be dislodged by the energy of the blast and cause cloudiness in the water, that is, increased turbidity. Several blasting events were monitored during summer 2002 while active blasting occurred along the quarry face near the monitoring wells. There was no effect from blasting on turbidity levels. Because the wells that are monitored in this case are relatively new, there may be little material present to be dislodged. 36 4.5 4.4 ; 4.3 -LL Qi > Q.l ...J. -Q.l ~ 4.2 4.1 4 3.9 3.8 52:48.0 __ , Jl. 00:00.0 Golberg West Water Level 5-21-02 - ----- D 1 CI:SL . .w. IIIIIJM 1m*-O.au~U I IIU II 07:12.0 14:24.0 Time 21:36.0 - I ,,.. I I I 1- Water Level l 28:48.0 36:00.0 Figure 4.3. Impacts of blasting on water levels in Golberg quarry wells. Historically, ground water flowed from the northeast to the southwest, toward the Zumbro River. The dewatering and excavation of the quarry has lowered ground-water levels about 40 feet at the north side of the quarry (Figure 4.4). This drawdown is a result of quarry operations since no other large water users are in the immediate area. 37 . -~.· 1000 0 1000 2000 3000 Feet N Water Level Figure 4.4. Historic water levels in Golberg Quarry. Golberg Quarry Downhole Camera Well 1 is on the quany property on the north side of the mining area. The water table was above the casing on this date. The well casing was intact with no apparent damage. The well had much particulate matter in the form of mats of orange-white material, particles, and areas where the particulate literally formed clouds in the well. Voids were visible at 109ft, 121ft, and 129ft. There was no evidence of flow at any ofthese points. Well2 is on the quany property on the nmth side of the mining area to the west of well 1. The water table was above the casing on this date. The well casing was intact with no apparent damage. The well had much particulate matter in the form of mats of orange-white material, particles, and areas where the particulate literally formed clouds in the well. Prominent voids were encountered at 127.5 ft and 151.5 ft; neither point had evidence of flow. 38 Conclusions The primary impact of the quarry is a continual decline in water levels in the Prairie du Chien aquifer from historical levels. Additionally, most wells were drilled after state rules were developed to prevent the drilling of wells into the first aquifer in karst terrains. These rules were designed to protect the wells from contamination from surface sources, but they also protect the wells from the quarry dewatering. Quarrying operations had no visible impact on the integrity of the observation well's casings. The casings all appeared to be intact and in good condition. No holes, ruptures, or seam failmes were visible. Blasting had no impact on turbidity levels in the monitoring wells onsite. 39 40 SECTION 5. SPINLER QUARRY-GALENA LIMESTONE Spinier Quarry -$- Monitoring well Figure 5.1. Site by aerial photograph. Impacts on Water Levels Over the course of the study, the water levels in II wells (six wells in three well nests and five other wells) at the site (Figure 5.1) were monitored for changes due to pumping and seasonal variation. Six wells in three well nests around the quarry provide information about vertical directions of ground-water movement (Figures 5.2, 5.3, and 5.4). 41 Spinier Pit Nest 1 --~ 1150 +-------------------------------------------~~~~--~ 1: 0 +:0 C'G ~ 1145 t-------.. r-----~~------~·-~~------,r---------r=L---, m ~ 1140 +-------------------------------------------------__':=r-----' ...J.. -Q) ~ 1135 1130+--------.--------.-------.--------.--------,-------~ 11/5/01 5/24/02 12/10/02 6/28/03 Date 1/14/04 8/1/04 Figure 5.2. Seasonal variation of water levels in Spinier well nest 1. 2/17/05 Pl is a shallow well in the sand and gravel aquifer, and Ml is a deep well in the Galena limestone. They are separated by a thick clay layer, which results in two distinct aquifers. The quarry company is pumping from the lower aquifer, represented by M 1, into a series of settling ponds that feed the upper aquifer, represented by Pl. Water levels do not fluctuate much from year to year because the company has not been operating any deeper and keeps the pumps running almost constantly. The pumping of water into the higher aquifer also keeps the cone of depression from extending too far west. 42 Spinier Pit Nest 2 1150 Z' 1149 !:!:. t: 0 :;::; cu > Q) w Qj > Q) .....J. .Q...) cu ~ Z' !:!:. t: 0 :;::; cu > Q) w Qj > Q) .....J. -Q) cu ~ 1148 1147 1146 1145 1144 1143 11/5/01 5/24/02 12/10/02 6/28/03 Date 1/14/04 8/1/04 2/17/05 Figure 5.3. Seasonal variation of water levels in Spinier well nest 2. Spinier Pit Nest 3 1146 1144 - 1142 1140 1138 1136 1134 +---------------------------~~--------------~ -ii- P3 t-----------~~==~~~~--~~----------j -+- M3 1132 1130 1128 1126 1124 11/5/01 5/24/02 12/10/02 6/28/03 1/14/04 8/1/04 2/17/05 Date Figure 5.4. Seasonal variation of water levels in Spinier well nest 3. 43 The ground-water levels at the site on September 15, 2004, were fairly representative of levels in late summer (Figure 5.5). Ground water flows from the surrounding areas into the pit; the groundwater model indicates that a portion of the flow from the Straight River is pirated by the quarry. Stream gaging was completed on the Straight River both upstream and downstream of the pit, and the differences were within the measurement error of the equipment. Results of stream gaging indicate that the loss of water from the river at this time is minimal. Water Table Drawdown 9/15/04 2000 0 2000 Drawdown 9-15-04 N o /\/10 e Well Locations 1'--., 4000 Feet ;~;~:~-r "" Figure 5.5. Water levels at drawdown in Spinier quarry wells. Ground-water levels measured June 1, 2005, show a much different scenario (Figure 5.6). The company apparently had allowed water levels within the quarry to rise to nearly nonnal levels. The water level was still depressed in the vicinity of the quarry, but there was much less water coming from the direction of the river. Since water levels have risen in the quarry and the lower aquifer, the lower aquifer has changed from being recharged by the upper aquifer to discharging to the upper aquifer in the vicinity of the quarry. 44 Water Table Drawdown 6-1-05 N -<>-· s 2000 0 2000 4000 Feet Figure 5.6. Water levels in Spinier quarry, 2005. Historically, water levels in the upper aquifer in the vicinity of the pit have been about 1155 ft above mean sea level, about 1 0 ft above the cunent level being maintained by quarry pumping (Figure 5.7). Prior to farming and quanying activities, water levels were probably just below the ground surface; however, ditches were dug throughout the area to lower the water table and improve crop productivity. Due to the complex site geology and hydrology, additional analysis of the impacts of the quany and its dewatering was done using three-dimensional software. Those results are presented in Appendix 2. 45 Spinier Pit Historic Water Level Upper Aquifer Figure 5.7. Historic water levels in upper aquifer, Spinier Quarry. Spinier Quarry Downhole Camera . w~.• Equipment breakdown prevented the videologging of these wells prior to the conclusion of the project. Conclusions Quarrying operations have penetrated two aquifers. Ground-water pumping has changed the hydraulic gradient in the vicinity of the quarry. The lower water levels could affect domestic wells in the immediate area. With ground-water levels dropping, water is flowing from the Straight River into the upper aquifer; historically, the river gained water from the local groundwater system. 46 SECTION 6. FOUNTAIN QUARRY-GALENA LIMESTONE Fountain Quarry ' w--<. r• 1D~DD~iiiil!!!!!!!!!'!liiiiii~!!!!!!!!'!!!!!!!!!'!!!!!!!!!'!"!!1~DDii;Diiiiiiiiiiiiiiiiiiiiiiiiiii2DDD Feet Figure 6.1. Site by aerial photograph. Fountain quarry operations nonnally take place above the water table although the quarry (Figure 6.1) occasionally floods from ground water discharging into it from the conduits below. In the quarry, there are four open joints in the floor. These joints have water flowing through them except during the driest parts of the year. Prior to this project, the University of Minnesota Department of Geology and Geophysics, in cooperation with DNR Waters, demonstrated by dye tracing that these joints connect to the nearby Fountain Big Spring on Riceford Creek. During this project, turbidity levels in the spring were monitored to assess the impact of dry quarry operations and blasting events. Turbidity Impacts Previous Investigations Several hundred yards north and west of the Fountain quarry lies the Fountain Big Spring complex (Figure 6.2). These springs are the discharge point for the Fountain springshed. Water sinks in the uplands in sinkholes and through the soil mantle and flows through fractures and solution-enlarged conduits to the springs. The Fountain Big Spring is part of a complex offour springs that has a mapped springshed of approximately 2900 acres. This basin was delineated by 47 dye tracing work for the Fillmore County Geologic Atlas; the longest straight-line dye trace was from a sinkhole 2.9 miles from the spring complex. Prior to this study, the University of Minnesota Department of Geology and Geophysics, in cooperation with DNR Waters, ran a dye trace from a joint in the quarry floor to the spring. The dye was poured into a flowing stream at the base of the joint in the east end of the quarry. Prior to the trace, background direct water samples were taken at the spring to check for natural fluorescence. After the dye was poured into the conduit, University of Minnesota students took direct water samples at the spring every I 5 minutes for 24 hours. The concentrations of those samples were analyzed with a Turner filter fluorometer. The dye took approximately I 2 hours to travel I 900 ft in straight-line distance to the spring (E.C. Alexander, Jr., University of Minnesota, oral commun., 2005). Figure 6.2. Fountain Big Spring. Project Investigations During our project studies, the discharge of the Big Spring was monitored for turbidity changes from blasting events at the quarry. Two blasting events were captured: one with a Global Water WQ series handheld turbidity meter and the second with a Global Water WQ series turbidity sensor, which was left in the spring. Additional blasts were measured with a Greenspan TS series turbidity sensor, but equipment failure prevented data retrieval. A weather station was installed at the quarry to measure precipitation falling on the quarry floor. On November 4, 2002, Milestone Materials Division of Mathy Construction blasted rock on the west end of the Fountain Quarry. DNR staff at the Big Spring using a handheld turbidity meter made measurements prior to the blast, for approximately 2.5 hours after the blast, and the next day. Results in Figure 6.3 indicate that the turbidity level in the spring increased rapidly after the 48 30 2S 20 Blasted ...--"1' 0 11/4/02 12:00 ll/4/02 13:00 ~ ~ Fountain Big Spring 11/4/02 ' \ \,~ ll/4/02 14:00 ll/4/02 15:00 ll/4/02 16:00 Date I I I I I I I ' I ~-•- Turbidity (NTU) I e Blast ' ll/4/02 17:00 11/4/02 18:00 Figure 6.3. Fountain Big Spring turbidity after November 4, 2002 blast. blast. In the first 20 minutes after the blast, the turbidity level more then tripled. Based on the ground-water travel time from the dye tracing work, this increase was not due to material moving from the quarry. Our explanation is that the shock wave from the blast put fine sediment in the conduit system into suspension. This suspended material then moved through the conduits and was discharged at the spring. On October 5, 2004, another blast on the west end of the quarry was captured with a Global Water WQ series turbidity sensor. The instrument was programmed to read turbidity every 15 minutes. Approximately 1.5 hours after the blast, the turbidity level increased by 4 nephelometric turbidity units (NTUs) from 23 to 27 (Figure 6.4). The turbidity peaked at 32 NTUs 7.5 hours after the blast. The level dropped back down to 27 NTU and then went back up to approximately 32 NTU. Our explanation for this event is similar to that of the November 4, 2002 blast. Material in the conduit is shaken loose and transported by turbulent flow to the spring. 49 Fountain Big Spring Turbidity 45 Blast 40 35 s 30 ~ 25 .~, :e 20 1\ \ ~ ~ ~ ~h~ ~L r lll'i = E-< 15 10 5 0 10/3/04 0:00 10/3/04 12:00 10/4/04 0:00 10/4/04 12:00 10/5/04 0:00 10/5/04 12:00 10/6/04 0:00 Time Figure 6.4. Fountain Big Spring turbidity after October 5, 2004 blast. Before it was stolen, the Greenspan TS series turbidity sensor recorded turbidity changes during spring from rain events (Figure 6.5). Several large rainfall events provided the opportunity to study the effects of precipitation on turbidity. The majority of the precipitation events caused the turbidity to rise between 12 hours and 16 hours following the onset of precipitation; however, when the system was primed by antecedent rainfall, the effect could be seen in less than 6 hours. Most increases in measurable turbidity were on the order of 10 NTUs, but the largest events increased the turbidity above 600 NTUs. The true magnitude of these events is unknown because the maximum turbidity exceeded the upper measurement limit of the turbidity sensor. In general, the response from precipitation is much larger than the response following blasting. 50 700 600 ;;;J 500 E- ~ 400 c ::ac 300 ;... = E- 200 100 0 .... l Storm Induced Turbidity Response Fountain Big Spring ·---~- ~ -Turbidity - Cumulative Precip 0 0 0 0 0 j. J- ~- 0 · ... ~ 0 .8 .7 .6 .s '-" .5 .e- Col ;<..I.I Q.. .4 ....~. 1:11 .3 = 13 .2 8 .I 6/22/03 6/24/03 6/26/03 6/28/03 6/30/03 7/2/03 7/4/03 7/6/03 7/8/03 7 /I 0/03 Date Figure 6.5. Fountain Big Spring turbidity response to rain. Conclusions The turbidity data indicate an impact from blasting on the water flowing through the conduit system to the spring. Based on our knowledge of ground-water travel time in this system, this material comes into suspension in the limestone's joints and fractures because of the blasting. Essentially, the shaking of the rock causes some fine-grained material to go into suspension where it is already present, then the material is carried to the spring. This same mechanism could also provide for turbidity spikes in domestic wells. Those wells would need to be finished in the surficial limestone layer and be open to solution-enlarged joints and fractures with significant flow. This describes many wells completed in southeastern Minnesota before enactment of the state well code; typically, owners of those wells have complained about the effects of blasting. These types of wells need to be considered when evaluating other quarrying operations in limestone terrains. Another mechanism of blasting-induced turbidity in domestic wells is the shaking of precipitates from the inside of the casing. Normal quarrying operations above the water table did not appear to increase turbidity in the Big Spring. Precipitation events can also increase turbidity in the conduit system; at this site, those impacts were much larger than those of blasting. In the case of a domestic well owner complaining about turbidity from blasting, this effect would also need to be monitored. 51 52 SECTION 7. HARMONY BIG SPRING QUARRY-GALENA LIMESTONE 0 -- 1000 2000 Feet Figure 7.1. Site by aerial photograph. Previous Investigations Big Spring Quarry . w~.• Dye traces were conducted in the area as part of the Fillmore County Geologic Atlas mapping effort (Alexander and others, 1995) by the Minnesota Department ofNatural Resources. This work was an attempt to map the basins feeding some of the large springs in the county. Boundaries between the Big Spring basin and other basins were partially established during this project. The Big Spring basin lies between the Odessa basin to the south, the Engle Spring basin to the west, and the Hart Spring and Buggywhip Spring basins to the east. During this research, we knew that we had not located all of the basin boundaries nor had we dye traced from the sinkhole plain area that extends north and east from the quarry site (Figure 7.1). Project Investigations Impacts on Ground-Water Flow Paths As part of this research project, we have been using dye tracing to refine the existing basin boundary map and determine how much of the basin area is being routed through the quarry (Green and others, 2003). Tracing was also done in the quarry to determine internal flow paths. The internal traces were run from sinking points on the west and east sides of the quarry. The trace from the west side, done under high-flow conditions, demonstrated that water sinking in the southwest part of the quarry emerges at the Big Spring. Because this trace was done under high- 53 flow conditions, some of the water and dye returned to the surface in the quarry itself. Under normal- and low-flow conditions, water does not return in the quarry. Water on the east side of the quarry discharges primarily from spring A238 (Big Spring East). The water flows into a pond where some of it sinks into a stream sink, B 11, in the bed of the pond. A dye trace from the pond confirmed that water does sink in the bed of the pond; the water that does not sink flows out of the quarry in a surface stream that joins Camp Creek. Dye traces were also run from four upland sinkholes. These traces were performed to further refine the boundaries of the Big Spring basin and document the extent of the basin area that has been pirated by the quarry. The rate of groundwater movement during these traces was 1,635 feet per day to 3,270 feet per day. The results of the tracing are shown in Figure 7 .2. The quarry has had a profound impact on the local ground-water flow system. Based on the 1995 springshed map in the geologic atlas and the additional tracing work done as part of this project, approximately 90% of the flow in the Big Spring basin is now being routed through the quarry. This exposes the conduit water to thermal impacts and makes it more vulnerable to pollution from quarrying activities. ' ' Big Spring Basin Mapping ' ' /~ -- ' / ' I ' I ' I ' I ' I ' I ' I ' :XI ,. - --...-"" '---,------"-----, ...... -----~ ... \ \ Dye trace point ..._ Sinkhole • Spring * stream sink N LCMR dye trace t\1 Geoatlas dye trace - Big Spring Quarry 0 Big Spring basln-Geoatlas mapping D Big Spring basin·LCMR mapping Figure 7.2. Springshed dye trace map. 54 Impacts on Water Temperature The impact of the conduit piracy on water temperature was also a concern. Trout are a cold-water species and, as noted previously, Camp Creek is a designated trout stream. In order for a stream to be considered cold water, the long-term temperature maximum cannot exceed 70 degrees Fahrenheit (F) (21 Celsius [C]). Peak daily temperatures cannot exceed 75 F (23 .8 C). If these thresholds are exceeded, the stream will not be able to sustain a viable trout population. Through this project, we have begun to document the quarry impacts on the spring and stream. A round oftemperature measurements was taken July 15,2003, to begin quantifying the thermal impacts on the ground-water system. Figure 7.3 shows the results of that monitoring. Water emanating from A238 (the main discharge point on the east side of the quarry) was about 49 F (9.3 C). As it flowed through the quarry and into the quarry pond, it warmed to a measured maximum of about 70 F (21.3 C). The water flowing out of the quarry via the surface stream was about 64.5 F (18.6 C) (several other springs in the quany also discharge to this stream). The temperature ofBig Spring, the headwater spring of Camp Creek, was about 56 F (13.6 C). In contrast, Little Big Spring, which has not had its basin pirated by the quarry, was 48.2 F (9 C). The warming of the water at the stream's headwater spring could have significant impacts on the stream's ecosystem. Project funds have been used to purchase thermochrons (button-size temperature recorders); they will be used to continue the thermal impact monitoring at this site after the project ends. July 15, 2003 Thermal monitoring e Temperature w~.• Figure 7.3. Temperature (degrees Celsius) monitoring results. 55 Conclusions By using dye tracing, we were able to document and quantifY the scope of spring piracy at the Big Spring quarry. Based on available information, roughly 90% of the basin is now being routed through the quarry. Without any dewatering occurring, this quarry has significantly altered ground-water flow paths. The water surfacing in the quarry is significantly affected by the surface air temperature, changing the thermal regime of the Big Spring and the upper reaches of Camp Creek. 56 SECTION 8. DONOVAN PIT-ALLUVIAL SAND AND GRAVEL * Monilorinll well ' w~• • Figure 8.1 . Site by aerial photograph. Monitoring began at the Donovan gravel pit (Figure 8.1) because local residents had expressed concerns regarding the impacts of gravel pits on local ground water. Operations in the pit started working below the ground water in March 2003. Sand and gravel pits in alluvial settings are vulnerable to flooding. This highlights the importance of good surveys of site elevations and site planning before a pit opens. These preparations will help prevent flooding from inundating equipment and buildings on the site. Floods in May 2003 and September 2004 overtopped the banks of the Zumbro River and flooded the Donovan pit (Figure 8.2). These events resulted in ground-water levels rising more than 6 feet; however, at the peak, the floodwaters were much deeper and had completely flooded over the wells. 57 Figure 8.2. Floodwaters in the Donovan Pit in 2003. Impacts on Water Levels Mining below the water table in the pit has done little to affect the quantity of water flowing to the Zumbro River (Figure 8.3) and has not affected upgradient domestic wells (Figures 8.4a, b, c, d based on an assumed datum of 100 ft). In areas of complex stratigraphy with abundant clay and a high topographic gradient, the loss of sand and gravel possibly could affect ground-water levels substantially; however, alluvial deposits are not very likely to have the significant deposits of clay and the topographic gradient is slight. The open water does increase the possibility of ground-water contamination if a spill were to occur. The ground water is only about 6 feet below the ground surface, so if a spill were to occur on land, it could contaminate the ground water because 6 feet does not allow enough time to remedy the contamination. 58 Donovan Pit Ground Water Elevations Figure 8.3. Ground-water levels in the Donovan pit. 59 . w~.• N Eievation ® Wells 97 96 95 --u: 94 "i 93 > 41 ..J "- 92 ..4..1. nl ~ 91 90 89 88 7/3/02 97 96 -95 ..... =u. 94 41 > 41 ..J "- 41 1ii ~ 93 92 91 90 89 7/3/02 Donovan Pit Nest 1 \ [- Well1 1 ~A ~ \ [- 18 A h A \ / ~ ""J Af\f--1 ....... 0 y v 12/30/02 6/28/03 12/25/03 6/22/04 12/19/04 Date Figure 8.4a. Water levels, nest 1. Donovan Pit Nest 2 ~ I- Well21 \ \ A l- 28 " \ ~~ 1\r\ -J \J \ _/ ~ ~ I "\! 12/30/02 6/28/03 12/25/03 6/22/04 12/19/04 Date Figure 8.4b. Water levels, nest 2. 60 98 97 Z' 96 -u. Qj 95 > Cl.l I '- 94 -Cl.l Ill 3: 93 92 91 7/3/02 96 95 94 --u:: 93 Qj > Cl.l -I '- Cl.l ~ 92 91 90 89 88 87 7/3/02 12/30/02 12/30/02 Donovan Pit Nest 3 6/28/03 12/25/03 6/22/04 12/19/04 Date Figure 8.4c. Water levels, nest 3. Donovan Pit 45 ! I : I I I : /\ \ !\_/ \rj I \ I \j I v 6/28/03 12/25/03 6/22/04 12/19/04 Date Figure 8.4d. Water levels, nest 4. 61 Impacts on Water Temperature Early in this project, the monitoring at this site focused on the influence of the pit pond on ground-water levels and ground-water quality. As the project progressed, the focus shifted to the thermal impacts of the pit pond on ground-water temperatures in the sand and gravel aquifer and the influence that had on the thermal characteristics of the Zumbro River. This change resulted after DNR Ecological Services voiced concerns about the impacts of mining on threatened mussel species that had been found during biological surveys of the Zumbro River in this area. In order to assess the thermal impacts, project staff began measuring water temperatures in the pit pond and the four shallow monitoring wells at the site. Ground-water temperatures in southern Minnesota typically are fairly stable at approximately 48 F. Data displayed in Figure 8.5 indicate there are significant fluctuations in ground-water temperature in the wells onsite. However, there was no apparent pattern to the variations. Wells 2S and 3S are upgradient of the pit while wells IS and 4S are downgradient. Since we could not draw any conclusions from this monitoring, staff purchased thermochrons (button-size temperature monitors) to deploy into the wells, pit, and river in order to continue this monitoring after the project concluded. These devices can measure the temperature fluctuations with minimal maintenance. They are designed to be left in the field yearround and will provide a more comprehensive picture of the thermal impacts of this pit. To enhance future thermal modeling efforts, project staff also directly measured ground-water time of travel in the sand and gravel deposit using a ground-water tracer. Using the observation well water level data and the water surface elevation of the pond, staff were able to determine the relative ground-water flow direction in the sand and gravel. Initially, project staff intended to pour a fluorescent dye into the pit pond and use well 2, 2S, and 4S as sampling points. After further review, that idea was abandoned, as there was no guarantee that the dye plume from the pond would intersect with the wells. Subsequently, staff decided to use open trenches in the water table perpendicular to the groundwater flow direction for dye injection and sampling. The trenches were excavated with a backhoe by Milestone Materials Division of Mathy Construction. Uranine C was poured into trench I (Figure 8.6); water samples were taken from trenches 2 and 3 daily. Six days after dye input, trench 2 had visible dye (Figure 8.7). The concentration visibly increased over the next several months. Sampling continued for several months after dye injection; no dye was detected in trench 3. Sample analysis was performed with an Aquafluor handheld fluorometer. In late December 2004, project staff had Milestone Materials staff fill the trenches so they would not remain open all winter and pose a safety hazard. 62 Donovan Gravel Pit Shallow Monitoring Well Temperatures 1 DO -----------------------------------------;:==========::J__~ -+-18 90+-----------~---------~ ~ 28 ~ 38 - 80+-------~~-~~-=~-------~ -- 48 lL - - Avg GW Temp cv ~ro -- ~ ~ ~ 60~~~~--~-~-~~--T~~~~~~~~~~~-~ E ~ 50 ~~~====~~~~~==~==~==================~~~i 40+-------------------------~ 30+---~~--~---~---~--~---~--~ 1/1/04 3/1/04 4/30/04 6/29/04 8/28/04 1 0/27/04 12/26/04 2/24/05 Date Figure 8.5. Fluctuations in ground-water temperatures. Figure 8.6. Trench 1, dye injection point. 63 Figure 8.7. Trench 1 (left) and trench 2 (right) dye sampling point. Conclusions Fluctuations in ground-water levels were observed in the monitoring wells onsite. There is no evidence that these fluctuations were due to mining activities. Based on the available data, the fluctuations resulted from precipitation events and are part of the normal response of a surficial aquifer to climatic variations. Temperature monitoring in the wells was inconclusive because of the sporadic nature of the monitoring. The breakthrough curve for the dye trace is displayed in Figure 8.8. The dye plume did not intersect with trench 3, but the dye broke through to trench 2 in 80 to 100 hours based on the increase in dye concentrations. That indicates a ground-water travel time of approximately I ft per 10 hours or 2.5 ft per day. This is a reasonable value for a sand and gravel aquifer. Prior to the dye trace, project staff had measured water levels in the shallow wells and pond and had determined the ground-water elevations at each site with an assumed datum. This allowed staff to determine that the hydraulic gradient at the time of the survey was 0.003 feet per ft; the porosity of a sample of the material at the trench site was 0.27. Putting these numbers into Darcy's equation along with the ground-water velocity gives a hydraulic conductivity of 1683 gallons per square ft per day. This number is well within the range of typical hydraulic conductivity values for an aquifer of this type (Heath, 1984 ). This value can be used in the future in coordination with the temperature monitoring to model the thermal flux from the pit pond to the Zumbro River. The dye did not break through to trench 3. Further investigation at the site by DNR Waters staff could not find any type of fine-grained sediments between trench 2 and trench 3 that would have slowed the ground-water velocity. Staff concluded that the dye plume traveled past the trench on its way to the Zumbro River. 64 :0 o._ o._ u (]) c c (IJ "-- ::) 60 40 20 Donovan Gravel Pit Dye TraceDye Travel Distance Is Ten Feet / / , / • J J J J l l I J J J J l J l J J J J l J l ,.J o -.~~~~~~~~-~-~-~~~~~~~~~ 0 40 80 120 160 Hours after dye input Figure 8.8. Breakthrough curve for the dye trace. This was the first dye trace in Minnesota where the analysis was performed with a handheld fluorometer (Figure 8.9). This device performed well and allowed rapid analysis of the samples. Without this device, completing the analysis would have required from weeks to months. Having the device and the ability to analyze the samples immediately after they were taken allowed staff to quickly determine the dye breakthrough curve. Using only visual analysis would have resulted in a slower ground-water velocity being calculated as the dye broke through at 2 orders of magnitude above background levels fully 24 hours prior to its being visible. The device was also very beneficial for the trench 3 monitoring; staff were able to know that the dye had not broken through and were able to end the trace prior to the end of mining activities in the pit for the winter. 65 Figure 8.9. Handheld fluorometer. 66 SECTION 9. LEITZEN-GRABAU PIT-ALLUVIAL SAND AND GRAVEL Leitzen-Grabau Pit -$- Monitoring well N w~. • 1000 0 1000 2000 Feet Figure 9.1. Site by aerial photograph. Monitoring started at the Leitzen-Grabau pit (Figure 9.1) about the same time as at the Donovan pit. Concerns were expressed regarding the impacts of gravel pits on local ground water. At the Leitzen pit, most of the sand and gravel is available above the ground water so excavation significantly below the water table is not necessary. The peaks in monitoring well3 (MW3) in 2003 and 2004 (Figure 9.2) are due to heavy rain events and show the natural variability of the ground water in the area. The ground water in the area functions similar to that found in the Donovan pit; however, it is higher on the landscape and above the floodplain. While ponding occurs frequently within the pit, it never floods more than 1 or 2 ft deep. There is no indication that the gravel mining has had any impacts on ground-water levels in the area. 67 Leitzen Pit Monitoring Wells 0 2 4 -~ 6 .... Q) - 8 ~ 10 0 ..... ..c: 12 a. Q) 14 0 16 18 \ /~ /"~~ • \ ~ / ~ I \ -..__,; ~ I \ ...- MW1 ~ 1\ I \ --MW2 \_ I\ I \ -.- MW3 .1. ~ I v 20 3/20/2003 6/28/2003 1 0/6/2003 1/14/2004 4/23/2004 8/1/2004 11 /9/2004 Date Figure 9.2. Depth to water in Leitzen-Grabau wells. 68 SECTION 10. FELTON PIT-GLACIAL BEACH RIDGE SAND AND GRAVEL Felton Pit Area 2000 0 2000 ~ Monitoring Wells • . Calcareous Fens IV Gravel Pits 4000 Feet Figure 1 0.1. Site by aerial photograph. The Felton study area has several active gravel pits, the largest of which is the Trust Fund Pit (Figure I 0.1 ). The mandate is to earn money for the School Trust Fund. Substantial high-value gravel resources have been identified below the water table in and surrounding the Felton area gravel mines. Monitoring work in this area was carried out in an effort to understand the impacts of sand and gravel mining on calcareous fen wetlands. Two such wetlands lie to the west of the mines and about 30 feet lower in elevation. Calcareous fen wetlands provide habitat for rare plants and are protected under Minnesota Statute 103G.223. The plants can only exist where there is a constant supply of nutrient-poor, carbonate-rich ground water and there must not be surface water inflows. It follows that mining-induced changes in ground-water and surface-water hydrology may be incompatible with the sustainability of calcareous fen wetlands. The Felton study area provided an opportunity to study a calcareous fen in proximity to an active gravel mine (North Fen) while a second calcareous fen could serve as a control (South Fen). Mining adjacent to the North Fen had begun in 1959, while basic monitoring began in 1995. Declining quality of the North Fen was recorded in the late 1980s and early 1990s. 69 As noted in Section 2, ground-water gradients were monitored at well nest locations in and upgradient of each fen. Water levels were monitored at other single-well installations. A staff gage was installed in the Trust Fund Pit to record water levels in the gravel pit lake. Present-day water table elevations were recorded in wells and borings, while evidence of past water levels was noted on the landscape and in wells and borings. Data from exploratory borings, well logs, and geophysical exploration were used to construct geologic cross sections. The cross sections reveal the subsurface along an approximate east to west trace (Figure 1 0.2). The North Fen is at the westem edge and the Trust Fund Pit at the eastem edge of the A-A' cross section (Figure 10.3), and the South Fen is at the westem edge ofthe B-B' cross section (Figure 1 0.4). The blue trace of the water table in Figure 10.4 shows that the water table intersects the ground surface along the upper edge of the South Fen, where it still provides upwelling groundwater supply to the whole calcareous fen. The water table in Figure 10.3 intersects the ground surface within the North Fen, possibly depriving the calcareous fen of part of its premining water supply. Monitoring was designed to investigate these conditions. Monitoring began at this gravel mine because DNR ecologists had noted that the North Fen was no longer as high quality of a calcareous fen as the South Fen. The major change on the landscape was upgradient gravel mining; thus, concem was expressed that mining had negatively affected the North Fen. 70 Figure 10.2. Locations of cross sections in the Felton pit area. 71 West East A A' FRS-2C 1020 OYIIclu08n stocl:plle 1010 North fen 1000 990 980 SG-- 970 - sand - sand (on<) gravel- 960 -) ~ 5 sand!- - 950 1ii ill jjj SG-- 940 '-' <') cobbles - 930 ~- aand((l-m sl~- ' 920 sand (c>vc) gravel • 910 Nnei~ (vi) 'aut- 900 890 880 1000 feet Figure 1 0.3. Geologic cross section along a trace between the North Fen and the Trust Fund Gravel Pit. 72 West 8 S&G sltybm South fen 591800 F-1 8' 1020 (f)-- 1010 1000 sand (m-e) gravel - 990 I 567353 I S&G clean gry '?. .... ~···-···-~~--··-···-··-·-·· SG.-- 980 S&GciBIIrtbm 970 peat-black 58 (vc) gravel - Cobbley S&G silly gr sand (f) - gray 960 - l sard (vc) gravel - liU - drlc gry 950 940 930 111 -drkjpy 920 910 900 1000 feet East I c 0 ~ ~ ill Figure 1 0.4. Geologic cross section along a trace from the South Fen to the height of the beach ridge south of the Trust Fund Pit. 73 Impacts on Ground-Water Flow Paths Mining in the pit has altered ground-water flow paths. The premining flow paths (Figure 10.5) were parallel, northwest-trending lines perpendicular to the beach ridge; simply put, ground water was flowing downhill. This flow pattern resulted in the creation of a seepage face at midslope where the water emerged at ground surface, keeping wet at all times the entire calcareous fen wetland. North Fan Soulh Fan 38 7 Ephemeral Stream Row Direcllon 32 5 I 1080 - 960 -- Equal Elevation Corr!Dur Figure 10.5. Ground-water flow paths (blue) prior to mining. Figure 10.6 shows the current condition of the South Fen. Mining created a radial flow pattern toward the pit along its south and west borders, resulting in a diversion of some water that would have flowed toward the North Fen and resulting in a decline in the amount of ground water available to the North Fen. 74 Groundwater Fluw Path - 960 -- Equal EleveCion Contour Figure 1 0.6. Ground-water flow paths (blue) during mining. Impacts on Water Table Elevations The decline in the water table elevation since the start of excavation below the water table is estimated to be approximately 15 feet. Note that this change has occurred without direct pumping from the pit for dewatering. The combination of the changes in flow paths and the decline in the water table has eliminated a portion of the ground-water basin of the North Fen. The groundwater basin of the South Fen has been affected to a lesser degree. The expression of this impact is seen in the hydrographs that show vertical ground-water gradients. An upward ground-water gradient underneath the calcareous fens provides the ground-water supply to the surface. Where 75 there is an upward gradient, the water level in a deep well will rise higher than the water level in a shallow well. The greater the difference in these water levels, the greater is the pressure that moves the ground water upward. At the North Fen (Figure 1 0.7), the water level in the deeper well ( 1 D) rises higher than the water level in the shallower well (1 S); thus, there is an upward gradient. The difference between the two water levels is about 0.5 ft. 970 969 968 965 964 963 Jun-94 ;-- i ~-- -~ fl l ... lfi ... ~ ... ~ I J ~ Oct-95 Mar-97 Jul-98 Dec-99 Date Figure 10.7. Vertical gradient at North Fen. u6.... .:.v..,:,-.:.. -+-18 #540953 4' - .-10 #540954 10' Apr.01 At the South Fen (Figure 1 0.8), the water level in the deeper well (20) rises about 1.5 ft higher than the water level in the shallower well (2S). The potential for ground-water upwelling is about three times greater at the upper edge of the South Fen (where these wells are located) than within the North Fen. 76 g c .52 982.5 982 981.5 981 -+-2S #567352 4' ·--\9- ·20 #567353 7.1' ~ 980.5 (!) w a; > (!) -I .... .s ~"' 980 979.5 979 978.5 978 Jun-94 Oct-95 Mar-97 Jul-98 Dec-99 Apr-01 Date Figure 1 0.8. Vertical gradient at South Fen. Impacts on Surface-Water Flow and Water Quality Changes in the surface-water basin direct runoff into the mine pits and from there into a ditch system. Increased flows into the ditch system have caused headward erosion and caused beavers to be attracted to the area. Beaver dam construction and blowouts have caused fluctuations in the water table and have also caused overland flows and sediment deposition into the wetland complex downgradient of the mine. The materials between the mine and the ditch are not known to be stable; thus, the possibility exists that the whole gravel mine lake could drain catastrophically. Conclusions Ground-water flow paths, water table elevations, ground-water gradients and both surface- and ground-water basins have been altered by mining below the water table. Excavation of the pit has caused water levels to decline near the mine and downgradient of the mine. The effect, as much as a 15-foot water level decline, is due to redirection of ground-water flow to the pond and then to the ditch system and is due to evaporation from the pond surface. The mine is located above a steep slope where watershed changes have set up the potential for subsurface piping and dike failure, which could lead to the loss of the gravel pit lake, complete drainage of the North Fen, and significant impacts on the South Fen. 77 Mining in the vicinity of calcareous fen wetlands must be undertaken only after evaluation of potential impacts on calcareous fens and planning to avoid those impacts. 78 79