Underground Hydrology of the Commodore Mine Complex and Implications for Source Control. Jeff T. Graves

Transcription

1 Underground Hydrology of the Commodore Mine Complex and Implications for Source Control Jeff T. Graves

2 Investigation Approach Purpose Determine location/s, quality and quantity of groundwater inflows into the mine complex. Develop conceptual groundwater models. Determine feasibility of source control implementation. Methodology Complete historical site research. Conduct geological literature search. Conduct physical underground investigation. Perform scientific studies: Dye tracer, isotopic studies, and pilot dewatering/pump test.

3 Setting Cond. AL_D CD_D CU_D FE_D MN_D ZN_D Site ph (us/cm) (ug/l) (ug/l) (ug/l) (ug/l) (ug/l) (ug/l) Nelson Adit Dissolved metals in Nelson Tunnel drainage (6/6/03). The Commodore Mine Complex is located in the San Juan Mountains of southern Colorado near the town of Creede. Site recently designated a Superfund Site. Water discharging from the drainage tunnel of the Commodore Mine complex, the Nelson Tunnel, contaminates three miles of Willow Creek and eventually the Rio Grande River. Nelson Tunnel is the single largest metals source in the watershed. Discharge from the portal averages over 350 gallons per minute.

4 Geology of the Creede Mining District. Reprinted from Steven and Eaton, 1975 Creede District faults. Reprinted from Bethke and Rye, 1979 Geology Creede Mining District occupies a geologically complex region of Tertiary aged volcanic activity. Collapse and resurgence of several calderas resulted in extensive fracturing and distentional faulting that provided pathways for migration of ore forming solutions. Ore mineralization consisted of native silver and silver chlorides near the surface with base metal sulphides at depth (sphalerite, galena and chalcopyrite.) Regional groundwater flow is generally confined to preferential paths including faults and fractures due to the low permeability of the volcanic units. Vertical projection of the Amethyst Lode. Reprinted from Emmons and Larson, 1923

5 Geology Structural Progression Dip Slip Major Graben Faults Opens horizontal pathways. Strike Slip Ladder Faults and Fracture Swarms Opens vertical pathways. Dip and Strike Slip Closes some pathways. Provides some inter-connectivity. Ore formation predominates on ladder faults and extensional features. Strike slip features appear to be predominant open pathways.

6 History During the 1890 s extensive mining of silver ores took place along the Amethyst Vein. In the late 1890 s competing tunnel companies drove the Nelson Tunnel and Commodore 5 Level Tunnel to provide drainage and cheaper haulage of ore. History of water at site: Water encountered at around 200 ft below surface during shaft development. Nearly 19 cfs of discharge was document during development of the Nelson Tunnel at the base of the Amethyst and Last Chance Shafts. Dewatering at adjacent Bulldog mine reduced flow at Nelson Portal to less than 45 gpm. Creede in (Western History/Genealogy Department, Denver Public Library)

7 Commodore Mine Complex Over 8 separate mines developed workings along the Amethyst vein, mostly through shafts. Workings extend over three miles horizontally and 1400 feet vertically.

8 Underground Investigations Underground investigations conducted through the Commodore 5 level tunnel since the Nelson Tunnel was collapsed at the portal. Extensive rehabilitation and safety work performed throughout the mine. Surveying was conducted to establish vertical and horizontal control. Established locations for water quality sampling. Plan to conduct detailed underground joint and fracture mapping.

9 Underground Investigations

10 Underground Investigations Underground investigations indicated the following: The Nelson Tunnel is the main water conduit within the mine. Nelson Tunnel flooded throughout most of the Commodore Mine Complex by at least three separate mine pools. Less than 10% of the discharge at the Nelson Portal is entering the mine from the upper workings. Nearly 90% of the flow and metals load originates from upper mine pool. Mine pool elevation fluctuations have some seasonal component possibly due to piston flow. Mine water temperatures 19 C, surface water temperature averages 4 C. Northern most section of mine is extremely dry.

11 Underground Investigations

12 Dye Trace Studies Tracer Studies 2001 Florescent dye added at Berkshire Shaft travelled through the upper pool to the portal (7300 ft) in 4½ days Mass loading tracer study. Three different salt tracers added at different locations in upper and lower mine pools. Plan additional tracer study to bracket upper mine pool.

13 Preliminary data courtesy of Dr. Mark Williams and Tony Krupicka, CU-Boulder. Tracer Studies Lithium Lithium BCH Lithium PORT 100 9/20/06 0:00 9/30/06 0:00 10/10/06 0: Sodium NN Sodium BCH Sodium PORT /20/06 0:00 9/30/06 0:00 10/10/06 0:00 10/20/06 0: Potassium NN Potassium BCH Potassium PORT /20/06 0:00 9/30/06 0:00 10/10/06 0:00 10/20/06 0:00

14 Isotope Studies Isotope Analysis 2001 Tritium sampled at various locations. Mine waters have pre-bomb (1950 s s) component Stable isotopes (δ 18 O and deuterium) of water, tritium, and δ 14 C on the dissolved inorganic carbon (DIC) content were sampled at various locations. Tritium indicates mine water average age older than 60 years. δ 14 C and DIC indicate median age of approximately 10,000 years. Stable water isotopes suggest a well mixed reservoir. Preliminary data courtesy of Dr. Mark Williams and Tony Krupicka, CU-Boulder.

15 Pilot Dewatering/Pump Test Temperature (C) Elevation and Temperature Data at Berkshire Shaft Temperature Mine Pool Elevation /15/2007 9:00 2/15/ :00 2/15/ :00 2/15/ :00 2/16/2007 1:00 2/16/2007 5:00 2/16/2007 9:00 Date Mine Pool Elevation (ft) Confirmed reasonable connectivity in the upper mine pool. Estimated 19.4 million gallons of water impounded in upper mine pool. No significant chemical change before and after pump test. Minor temperature change during pumping.

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17 Regional Conceptual Groundwater Model Regional flow paths appear confined to large structures. Likely groundwater recharge in upper Rat Creek watershed. Likely path to mine stair step along grabens and extensional faults. Significant up-welling component possible. Regional groundwater enters Commodore Mine complex at structure convergence. Specific pathway feeding mine discharge remains unknown. Modified from Bethke and Rye, 1979

18 Is it currently feasible? No because: Inflow pathways unknown. Source Control Quality of water entering mine is unknown. Extensive flooding in mine has made refinement of inflow paths difficult. Discovery of discreet flow paths will be required for successful source control implementation. Next step may be to attempt mine dewatering to investigate inflows. Potential source control options: If water enters mine clean, collect and pipe to portal. If water enters contaminated, grout pathways if discreet locations. Maintain regional water table below Nelson Tunnel inflow point through pumping or drain holes. Source control may not be feasible.

19 Conclusions Conceptual groundwater models successfully developed through investigation approach. Model refinement directs further study. Completion of investigation in hopes of source control implementation may prove too costly. Acknowledgements: Mike Wireman, USEPA Mark Williams, CU Boulder Tony Krupicka, Masters Student, CU Jim Herron, CDRMS (retired) Members of the Willow Creek Reclamation Committee (WCRC) Commodore Levels 3, 4 and 5.