Laserfiche WebLink
• <br />• <br />during periods of no irrigation. Mean TDS levels for the three Dakota coal wells (GW-N14, GW- <br />N 17P and GW-N20) during periods of irrigation and no irrigation were 4,323 and 4,366 mg/I, 2,740 <br />and 1,798 mg/I, and 2,198 and 2,251 mg/I, respectively. <br />During periods of no irrigation, Well GW-N14 showed a decrease in magnesium, but increases in <br />calcium, bicarbonate and sulfate thataccounted forthe small increase in mean TDS. TDS actually <br />increased when irrigation was discontinued. The seemingly large decreases in calcium and <br />magnesium were more than offset by the increases in sodium, bicarbonate and chloride. The <br />sodium increase was principally responsible for the increase in TDS because it occured at <br />milliequivalentlevels thatwere afactor of 10 times greaterthan the calcium and magnesium levels. <br />The high sodium values and very alkaline pH level in Well GW-N17P1 strongly suggested thatthe <br />well completion zone was partially open to high sodium overburden and interburden units as well <br />as the coal. Well GW-N20 showed a significant decrease in TDS during non-irrigation periods. <br />Calcium, magnesium, sodium and sulfate levels all decreased when irrigation was discontinued. <br />Only bicarbonate increased slightly, but at very low milliequivalent levels. <br />The pH level for Wells GW-N14 and GW-N20 ranged from 4.7 to 7.6 while Well GW-N 17PI ranged <br />from 8.2 to 9.8. As was previously mentioned, these very alkaline pH's were atypical ofcoal aquifer <br />levels and were believed to beduetothewellcompletion. Mean pHvaluesforWellsGW-N14,GW- <br />N20 and GW-N17P1 were 4.95, 5.03 and 8.45, respectively. <br />There was some suggestion of a limited amount of seasonal variabilityto the Dakota coal chemistry <br />at the New Horizon 2 mining area. In comparing spring versus fall chemistry, magnesium, sulfate <br />and TDS levels decreased and sodium and potassium levels increased in the fall. A comparison <br />ofsummerversusfall chemistry suggested thatbicarbonate, chloride and pH levels increased and <br />magnesium levels decreased in the winter. <br />Chemical trends over time are also evident in the Dakota coal aquifer. Well GW-N14 showed a <br />decreasing trend in calcium levels during irrigation periods. Well GW-N17P1 showed a decreasing <br />trend in calcium, magnesium and bicarbonate levels and an increasing trend in sodium and chloride <br />levels during periods of irrigation. Irrigation period trends for well GW-N20 included decreasing <br />calcium, magnesium, sodium, sulfate and TDS levels and increasing chloride levels. <br />• (REVISED 6/28101) 2,04.7-1 rJ <br />