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• There appears to be some seasonality to the overburden water chemistry at the Nucla East mining area. In comparing spring versus fall chemistry, alkalinity, bicarbonate, chloride, potassium, sodium, sulfate and TDS levels tend to decrease in the fall. In comparing fall versus winter chemistry, alkalinity, bicarbonate, chloride, potassium and sodium levels increase and pH decreases in the winter. In comparing summer versus winter chemistry, alkalinity, bicarbonate, chloride and sodium levels increase and calcium levels decrease in the winter. In addition to chemical seasonality, there are some overburden chemical'. trends during the irrigation season with time. Well CW-N18 shows a decrease in calcium and an increase in sodium, bicarbonate, sulfate and TDS levels. A summary of the ranges in concentration for trace elements measured in the overburden wells is presented in Table 7-8. Well GW-N18 water exceeds the CWOCC agricultural standards for fluoride and manganese and the water at Wells CW-N15 and CW-N21 exceed the manganese standard. In comparison to livestock drinking water standards (Table 7-9), Well GW-N15 water exceeds the magnesium, sulfate and TDS standards and Well GW-N18 water exceeds the fluoride standard. Well CW-N 21 water is suitable for use as livestock • drinking water and water at well GW-N18P1 is suitable for both agricultural and livestock use. Measurable trace element concentrations in the overburden aquifer include aluminum, arsenic, boron. cadmium, cobalt, fluoride, iron, lithium, manganese, nitrate, selenium, vanadium, silver and zinc. The highest concentrations of cadmium, nitrate, selenium and silver were measured in the overburden aquifer at the Nucla East mining area. Three wells (GW-N14, GW-N 17P1 and GW-N20) monitor the Dakota coal aquifer water quality in the Nucla East mining area. Statistical summaries of the water analyses performed on water sampled from these wells are presented in Tables 7-21 through 7-23. Trilinear diagrams representing analyses performed during periods of irrigation and no irrigation are presented in Appendix 7-3. The intlividual water quality analyses for the Dakota coal aquifer wells in the Nucla East mining area are presented in Appendix 7-4. The well locations are shown on Exhibit 7-1. The three Dakota coal monitoring wells exhibit twa water types during periods of irrigation. Wells GW-N 14 and GW-N30 have magnesium calcium sulfate water types and Well GW-N17P1 has a sodium sulfate water type. During the non-irrigation periods, Well CW-N20 . changes to a calcium magnesium sulfate water type. The only significant TDS change when 7-61 Revised 04/11/88