2021-12-22_PERMIT FILE - C1981019A

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RULE 2 PERMITS by a mass -wasting flood event, and the water table which it intersected was substantially lowered, as evidenced by the new wells MW -05-03A and MW -05-03B that were installed to bedrock at the confluence of the unnamed drainage leading out of the box -cut area and Wilson Creek (near historic A-5). Both wells were dry at the time of construction, and MW -05-03A was permanently closed. MW -05-03B has about '/2 foot of water at the alluvial -bedrock contact. Transmissivity of the alluvium ranged from 1,935 ftZ/day to 10,909 ft' /day. Corresponding hydraulic conductivity ranged from 29.7 ft/day to 175 ft/day. Groundwater Quality — Baseline monitoring was conducted in 1984-85 of alluvial aquifers and springs, in 1996-1997 of the bedrock and alluvial aquifers, in 1999 -2000 of the bedrock and alluvial aquifers, and during 2005-2006 of the alluvial aquifers and seeps and springs in the South Taylor/Lower Wilson permit revision area (tables 2.04.7-28 through -31). The Colorado Mined Land Reclamation Division's Guidelines for the Collection of Baseline Water Quality Data were utilized during the baseline monitoring. Water quality sampling and laboratory analyses were conducted in accordance with Rule 2.03.3(4). The depth to water in each of the wells and field measurements for pH, electrical conductivity, and temperature were monitored monthly. Quarterly monitoring included a full suite of analyses on each well. Well samples were filtered in the field and preserved for analysis of dissolved metal concentrations. Figures 2.04.7-12 through 2.04.7-18 present hydrographs for representative bedrock and alluvial monitoring wells from the data collected during baseline monitoring in 1996/97. Bedrock water quality as measured by specific conductance is also illustrated to show seasonal variability with respect to water levels in the wells. The Trout Creek Sandstone wells exhibit greater variability in water levels than do the Williams Fork Formation wells. The lowest water levels in the bedrock wells typically occur in the winter, whereas the highest water levels generally occur in the fall. Because the bedrock aquifers are under confined conditions, recharge from spring runoff and snowmelt is not readily apparent in these figures. The alluvial wells exhibit a greater response in water levels in the spring, corresponding to an increase in recharge from infiltration of runoff and snowmelt. The rise in water levels in these wells was most significant in well MW -95-02, which is drilled into the alluvium of Wilson Creek to a depth of about 38 feet. Groundwater samples were collected in the permit area as part of baseline monitoring studies conducted by Colowyo. Groundwater data from 1984/1985, 1996/1997, 1999/2000, and 2005-2006 are presented in CDM (1985a), and in tables 2.04.7-28 and 2.04.7-29. These tables include those constituents identified in Rule 2.04.7(1)(a)(v), except for total iron and manganese, which were analyzed and reported as dissolved concentrations. Trilinear diagrams depicting the mean groundwater quality for wells monitored both for this permit revision and for general Colowyo operations are presented in figures 2.04.7-19 through 2.04.7- 21. The bedrock water quality is principally bicarbonate, with relatively equal proportions of calcium, magnesium, and sodium. Groundwater from two bedrock wells, W-95-15 and W-95-02, is principally sodium bicarbonate type water. Well W-95-02, completed in the Trout Creek Sandstone, also exhibited artesian flow and warmer temperatures. The average TDS content of the bedrock aquifer ranged from 702 mg/L in well W-95-02, to 1,064 mg/L in well UL -95-45. For comparison purposes, the USEPA secondary drinking water standard for TDS is 500 mg/L. As shown in Table 2.04.7-28, dissolved iron and manganese concentrations in the bedrock aquifers also often exceed the USEPA secondary drinking water standards of 0.3 and 0.05 mg/L, respectively. The generally poor quality of the bedrock aquifers typically limits their use for domestic water supplies. South Taylor/Lower Wilson — Rule 2, Page 22 Revision Date: 11/18/16 Revision No.: MR -163