United States Department of Agriculture Investigative Methods Forest Service Technology & for Controlling Development Program 7100 Engineering Groundwater Flow August 2003 0371-2801-MTDC to Underground Mine Workings United States Department of Agriculture Forest Service Investigative Methods Technology & for Controlling Development Program Groundwater Flow 7100 Engineering August 2003 0371-2801-MTDC to Underground Mine Workings Phyllis Hargrave and John Metesh, Montana Bureau of Mines and Geology Ken McBride, Bitterroot National Forest Steve Oravetz, Northern Region USDA Forest Service Technology and Development Program Missoula, MT 7E72G71—Acid Mine Drainage Study • August 2003 The Forest Service, United States Department of Agriculture (USDA), has developed this information for the guidance of its employees, its contractors, and its cooperating Federal and State agencies, and is not responsible for the interpretation or use of this information by anyone except its own employees. 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USDA is an equal opportunity provider and employer. i Contents Introduction ______________________________ 1 Discharge and Recharge Areas _________________ 18 Water Samples _____________________________ 19 Groundwater Flow to Underground Charter Oak Mine Site ________________________ 19 Mine Workings __________________________ 2 Introduction ________________________________ 19 History ____________________________________ 19 Mitigation Methods _______________________ 3 Geology ___________________________________ 19 Environmental Conditions _____________________ 20 Methods for Investigating Groundwater Underground Workings _______________________ 20 Source Areas ____________________________ 4 Mine Discharges ____________________________ 22 Hydrological and Geohydrological Methods _______ 4 Surface Water ______________________________ 24 Water Balance _______________________________ 4 Aerial Photography __________________________ 25 Age Dating of Springs _________________________ 4 Characterization and Mapping of Riparian Areas Tracer Tests _________________________________ 4 and Wetlands _____________________________ 25 Water Quality ________________________________ 4 Drilling _____________________________________ 4 Summary _______________________________ 28 Soils and Vegetation Methods ___________________ 5 Elkhorn Mine ________________________________ 27 Remote Sensing, Photo Interpretation, Charter Oak Mine ____________________________ 27 and Field Mapping __________________________ 5 Field Investigations for Soil and Vegetative Mapping _ 6 References ______________________________ 28 Geoscientific Methods _________________________ 8 Hydrology __________________________________ 28 Literature Review _____________________________ 8 Soil and Wetlands Interpretation ________________ 28 Aerial Photography in Relation to Fracture Aerial Photointerpretation _____________________ 28 and Structural Studies _______________________ 8 Elkhorn Mine ________________________________ 29 Field Mapping _______________________________ 9 Charter Oak Mine ____________________________ 29 Geophysics _________________________________ 9 Appendix A—Elkhorn Mine Riparian and Case Studies _____________________________ 10 Wetlands Characterization and Mapping ____________ 31 Elkhorn Mine and Mining District _______________ 10 Methods ____________________________________ 31 Introduction ________________________________ 10 Discharge and Recharge Areas _________________ 35 History _____________________________________ 11 Water Samples _____________________________ 37 Geology ____________________________________ 11 Environmental Conditions _____________________ 12 Appendix B—Elkhorn Mine Riparian Area Underground Workings _______________________ 13 and Wetlands Map Unit Descriptions _____________ 38 Mine Discharges ____________________________ 13 Map Unit 1F _________________________________ 38 Surface Water ______________________________ 15 Map Unit 2F _________________________________ 38 Regional Structure (Aerial Photographs) __________ 16 Map Unit 1H _________________________________ 39 Characterization and Mapping of Riparian Areas Map Unit 2H _________________________________ 40 and Wetlands _____________________________ 16 Map Unit 1FH _______________________________ 41 Office Procedures ___________________________ 17 Map Unit 3U _________________________________ 41 Field Investigations __________________________ 18 Map Unit W _________________________________ 41 Mapping Results ____________________________ 18 ii Introduction n 1998, an adit discharge water-qual- groundwater that may infiltrate mine work- acidic waters quickly as the water circu- ity and quantity monitoring project ings. Good-quality water is diverted from lates through the rock and mine workings. II began at the Elkhorn and Charter Oak the workings to prevent it from interacting A pyrite content of only a few percent by Mines in Montana. These mines were with pyrite and metal-bearing minerals. volume can produce acid for hundreds chosen as representative of hard-rock Infiltration controls may include grouting or thousands of years. Other metal-bearing mines with discharging adits (horizontal from outside the workings, soil or stream- minerals dissolve in the acidic water, entrances) throughout the United States. bed treatment to reduce infiltration capa- producing high concentrations of metals Three reports have been published de- city, storm water and runoff management, in the discharging waters. tailing the findings of this project: and recontouring or regrading natural recharge areas. This discussion explores Hargrave and others (2000) identified Adit Discharge Monitoring Summary methods for controlling groundwater flow about 336 discharging adits in Montana for the Elkhorn and Charter Oak into mine workings. Many of the methods based on work conducted by the Montana Mines, MT (0071–2858–MTDC) should be applied to investigations of all Bureau of Mines and Geology, the USDA three treatment categories: capture, Forest Service’s Northern Region, and the Using Recharge Control to Reduce treatment, and disposal; adit plugging or Montana office of the U.S. Department of Mine Adit Discharges: A Preliminary grouting; and controlling groundwater the Interior, Bureau of Land Management. Investigation (0071–2804–MTDC) recharge to the mine workings. Although the quality of the discharging water varied widely, nearly all discharges Treating Acid Mine Drainage From Numerous investigations of mining sites indicated elevated concentrations of Abandoned Mines in Remote Areas throughout the Western United States metals. The rate of discharge from the (9871–2821–MTDC) have identified thousands of discharging adits ranged from less than 1 gallon per mine adits. Many of these adits discharge minute to more than 100 gallons per This report, the fourth in the series, dis- poor-quality water that is created when minute. However, the vast majority of the cusses continued investigations at both groundwater circulates through the mine adits discharged water at a rate of 5 to 10 sites showing how additional data from workings and interacts with mineral-rich gallons per minute. This report will review conventional resources available to USDA rock exposed by mining. Many mines also methods of investigation for controlling Forest Service staff can be used to help contain abundant iron pyrite. Interaction groundwater flow to underground mine determine a specific recharge control of pyrite, oxygen, and water can produce workings. method for a particular mine discharge. Application of the various methods of controlling groundwater flow into mine workings requires knowledge of the source and pathways of the portion of the aquifer that is hydrologically connected to the mine. The following discussion is meant to provide an overview of the investigation. Information from several sources must be compiled to compose a conceptual model of the groundwater flow system near the mine. No one method will provide all the necessary information. The case-study portion of this report is divided into two sections: one for the Elkhorn Mine and the other for the Charter Oak Mine. Each section provides details of the methods that were used to investi- gate environmental problems at the mine. Controlling groundwater recharge to the workings requires identifying, character- The acid mine discharge at one of the adits at the Charter Oak Mine southeast of Elliston, MT, izing, and controlling surface water and in the Helena National Forest. 1 Groundwater Flow to Underground Mine Workings conomic deposits