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Creek were made using a mass-balance approach. Plots of TDS data <br />collected at the Seneca II Mine are presented in Appendix 7-4A. • <br />Grassy Cree}: Dre.inage. Increasing TDS concentrations have been <br />observed in most, if not all, Grassy Creek and Little Grassy <br />Creek surface water monitoring sites. At Site 2, average TDS <br />concentrations have increased approximately 1100 mg/1 between the <br />1980 and 1991 water years. Recent Seneca II Annual Hydrology <br />Reports have noted that decreased snowmelt runoff and decreased <br />precipitation, when compared to earlier water years, have reduced <br />streamflows, causing elevated TDS concentrations. To some <br />extent, the increased TDS values observed at the Seneca II Mine <br />can be attributed to natural conditions, and this is supported by <br />the TDS increases seen at "upstream" monitoring sites unaffected <br />by mining activities (i.e., Sites 1 and 5). <br />There is little doubt that mine activities have caused at least a <br />portion of the TDS increase in Grassy Creek as measured at Site <br />2. It has been observed in recent AHRS that the construction of <br />the truck wash system (September 18, 1984) at the mine shop area <br />has significantly affected the water quality at NPDES 003. <br />Before the truck wash came into use, the average TDS value at <br />NPDES 003 was 1433 mg/1. Since operating the truck wash, the <br />average TDS concentration at NPDES 003 through the 1991 water <br />year is 2661 mg/l, and this has caused corresponding TDS <br />increases a. SW-S2-2 or. Grassy Creek. PHC analyses conducted in <br />1986 estimated that TDS maximum levels in Grassy Creek (as <br />measured at SW-S2-2) would be 1233, 1908, 2168, and 2341 mg/1 <br />during the low flow/irrigation months of June, July, August, and <br />Septen~er, respectively. The majority of the data used to <br />calculate the TDS maximums were collected prior to construction <br />of the truck wash system during a period of above-average <br />precipitation (see "Precipitation" discussion above), so maximum <br />TDS estimates were calculated using relatively low TDS data. <br />Consequently, TDS values that exceed the monthly maximums listed <br />above have been recorded at SW-S2-2, TDS concentrations of 1752 • <br />and 1994 mg/1 have been measured <br />7-325 Revised 02/05/97 <br />