2013-10-02_PERMIT FILE - C1981019A (3)

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RULE 2 PERMITS General Groundwater Quantity -Groundwater monitoring in the general and South Taylor/Lower Wilson permit azea has been conducted since 1983. Monitoring wells have been established in [he alluvium, the Williams Fork Formation interburden and coal, and the Trout Creek Sandstone. A summary of the bedrock and shallow monitoring wells relevant to the permit revision is presented in tables 2.04.7-25 through 2.04.7-29 and are shown on Map 10A. Locations of all known wells and test boreholes within the permit area and adjacent area are illustrated on Map 11A. Previous studies by CDM (1985a) and Dennis (2001, 2006) determined the hydraulic characteristics of the bedrock aquifers in the Williams Fork Formation and the Trout Creek Sandstone. The results of these studies are presented in Table 2.04.7-26 and discussed in detail below. The Trout Creek Sandstone is a moderately permeable confined aquifer and the Williams Fork Formation is mostly dry with a few, low permeability, discontinuous, and confined aquifers of limited extent. Alluvial aquifers have moderate to high permeability where encountered, with a wide range of hydraulic values encountered. There is little groundwater in the alluvium along Wilson Creek immediately below the Lower Wilson affected area. There is groundwater in the alluvium along the West Fork and main stems of Good Spring Creek below the South Taylor affected area. General Groundwater Quality -Bedrock water quality (Williams Fork Formation and Trout Creek Sandstone) in the general area was determined by previous investigations (CDM, 1985a; Dennis, 2001; Colowyo 1992). Data from these investigations indicate that the principal water type in the Williams Fork Formation is a calcium- or sodium-bicarbonate type water, containing only minor concentrations of other major ions. The concentration of total dissolved solids (TDS) is low to moderate, ranging from 440 milligrams per liter (mg/L) to 640 mg/L (CDM, 1985a). The CDM investigation revealed that the water type of the Trout Creek Sandstone is more variable than that of the Williams Fork Formation. TDS concentrations ranged from 600 mg/L to 710 mg/L, and the water type ranged from asodium-sulfate, sodium-bicarbonate type, to a predominantly mixed cation-bicarbonate type water with equal percentages of calcium, magnesium, and sodium (CDM, 1985a). The water quality of the bedrock during [he 1985 investigation did not exhibit significant seasonal variability. The water quality of the alluvium in the general area was also investigated by CDM in 1985. These investigations revealed two distinct trends in water quality: a temporal uniformity in water type, in the relative percentages of major cations and anions, and general variability in the water quality from well to well. The water type is predominantly amagnesium-sulfate type, with moderate to high concentrations of TDS, ranging from 645 mg/L to 3,780 mg/L (CDM, 1985a). In contrast to the water quality of the bedrock, the alluvial water quality showed significant seasonal variation in the majority of [he wells sampled, with TDS concentrations increasing in the spring. General Groundwater Use -Groundwater withdrawals in the Lower Yampa River basin totaled nearly one million gallons per day (mgd) in 1995 (USGS 1995). Groundwater consumption in the basin is predominantly associated with irrigation use. About 52 percent of the groundwater withdrawals (0.5 mgd) are used for irrigation. Livestock and mining use account for the remaining groundwater withdrawals. South Taylor/L.ower Wilson -Rule 2, Page 38 Revision Date: 11/30/06 Revision No.: PR-02