2013-04-18_PERMIT FILE - C2009087 (6)

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Total dissolved solids (TDS) concentrations vary inversely with discharge, and also increase in the downstream direction. Iorns et al. (1965b) present data that portrays the relation between water • discharge and chemical quality of water for the Yampa River near Maybell, Colorado. For a weighted average discharge of 108 cfs, TDS averages 389 mg/L, while for a weighted average discharge of 9,750 cfs, TDS averages only 98 mg/L. The weighted average TDS concentration at this site is reported at 140 mg/L, with an average TDS discharge of 699 tons per day. Much further upstream at Steamboat Springs, Colorado, the Yampa River is reported to have a weighted average TDS concentration of only 74 mg/L, with an average TDS discharge of only 94 tons per day. Iorns et al. (1965b) also includes water chemistry data for evaluating the suitability of water for irrigation collected at low, medium, and high flows at three progressively downstream sites along the Yampa River: 1) Steamboat Springs; 2) Craig; and 3) near Maybell. For all three sites over the range of flows sampled, the water in the Yampa River is classified as either C1 -S1 or C2 -S1, which categorizes the Yampa River as exhibiting low to medium salinity hazard, and low sodium (alkali) hazard. This categorization makes water in the Yampa River well suited for irrigation. Based on data presented in Iorns et al. (1965b), over 51,000 acres were irrigated in the Yampa River Basin above Maybell, Colorado in 1957. At that time, the total annual TDS discharge was estimated at 218,800 tons, and irrigation accounted for 13.6 percent (29,800 tons) of the total annual TDS discharge. Regional Groundwater Hydrology Two relatively thick, laterally continuous sandstone units which comprise regional aquifers are present within the general area: the Trout Creek Sandstone Member at the top of the Iles Formation • and the Twentymile Sandstone Member of the upper Williams Fork Formation. Separating these regional aquifers are confining units comprised of marine shale of the middle member and local aquifers of the lower member of the Williams Fork Formation. Thus, the hydrostratigraphic sequence includes, in descending order: Upper confining unit — Lewis Shale Regional aquifer — Twentymile aquifer Confining unit — middle member of Williams Fork Formation Local aquifer — Basal Williams Fork aquifer (coal seams and sandstones of lower member) Leaky confining layer — shale beds of lower member of Williams Fork Formation Regional aquifer — Trout Creek aquifer The Twentymile aquifer is the upper of the two regional aquifers. Robson and Stewart (1990) define the aquifer as extending from the base of the Lewis Shale to the top of the middle member of the Williams Fork Formation. Based on hydrogeologic information collected at the Twentymile, Seneca, and Yoast Mines, the aquifer extent beyond the Twentymile Sandstone Member is limited to the immediately overlying and underlying sediments and more distal sediments are not in hydrologic connection with the Twentymile Sandstone. The Twentymile aquifer outcrops in a generally north- trending strip through the central part of the PSCM permit area and in a northwest- trending strip through the southwest part of the PSCM permit area. Where the Lewis shale is present above the Twentymile aquifer, it forms an overlying confining layer. • The marine shale of the middle member of the Williams Fork Formation forms an underlying regional confining layer to the Twentymile aquifer and separates it hydraulically from the lower member and also from the Trout Creek aquifer. PSCM Permit App. 2.04 -21 6/15/09