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Grassy Creek Drainage. Three monitoring sites lie within the Grassy Creek drainage: <br /> Surfac <br />Water SW-52-1 <br />Sit <br />t <br />l <br />d G <br />k <br />N <br />Di <br />C <br />h i <br /> e e <br />, <br />oca <br />e <br />upstream on rassy <br />ree <br />; <br />PDES <br />sc <br />arge Po <br />nt 003, <br /> also known as the Shop Reservoir or the Northwest Impoundment, located just no rth of the <br /> mine shop area ; and Surface Water Site SW-52-2, located approximately 0.1 mile downstream <br /> of where NPDES 003 discharges into Grassy Creek. <br />Surface Water Site SW-S2-1. This site monitors the upper reaches of Grassy Creek which <br />are undisturbed by mining of the Seneca l] Mine. Review of the hydrogra ph of monthly flow <br />readings shows low runoff in 1991 similar to 1987 to 1990 and significantly less than 1982 <br />through 1986. <br />Review of Table 16 (Appendix A) shoos the eater at this site to be a CaMg-SO4HC03 type. <br />Increasing sulfate concentrations and slightly increasing TDS values are most likely due <br />to background conditions in the watershed above this site and the decreased runoff the <br />Last five years. <br />NPDES Discharge Point 003. Hydro9 ra phs presented in Appendix E shoo that flow has <br />increased a[ [his site since September 18, 1984, when operation of the truck cash <br />facilities began. Water is siphoned from the Wadge Impoundment (NPDES 002) into a holding <br />pond. Ove rflou from the holding pond is diverted into the shop ditch which floes into <br />NPDES 003. Water is pumped from the holding pond to the wash rack, drains into a settling <br />pond, and then into the same shop ditch. This operation has increased the flow at NPDES <br />003 chile decreasing the floe at NPDES 002. Tuo other events that affected the long-term <br />hydrograph include the diversion away from NPDES 003 of runoff from a fly ash pi[ operated <br />by Colorado-Ute Electric Association in October, 1980, and the diversion away from NPDES <br />003 of runoff conTained in the Northwest Diversion Ditch (see the Seneca II Permit <br />Application, Exhibit 7-7, Surface water Hydrology Map and also the 1982 Seneca II AHR) in <br />April, 1982. Finally, discharge volumes ai the 003 site have been effected by the <br />reduction in diameter of the discharge culvert (36 inches to 6 inches) which occurred on <br />uuly 19, 1988. comparison of this years' floes-to prior years is not made since the <br />operation of the truck cash system tends to influence the natural floe Conditions at Pond <br />003. <br />Review of Table 15 (Appendix A) shoos a dramatic shift in water quality from 1985 through <br />1991. Prior to the truck wash operation, the average annual TDS of the flows at NPDES 003 <br />'~ ranged from 1,225 to 1,578 mg/l. After the truck wash operation, the TDS range increased <br />17 <br />