1988-04-11_PERMIT FILE - C1981008A (9)

Home Browse Search

comparing irrigation versus non-irrigation periods is at well GW-N20. The TDS level decreases 672 mg/l during periods of no irrigation. Slight TDS increases occur at Wells GW-N14 and GW-N17P1 during periods of no irrigation. Mean TDS levels for the three Dakota coal wells (GW-N14, GW-N17P1 and GW-N20) during periods of irrigation and no irrigation are 4,323 and 4,366 mg/l, 2,740 and 1,798 mg/l , and 2,198 and 2,251 mg/l, respectively. Schoeller diagrams, showing mean milliequivalent values for the Dakota coal wells for the two above-referenced time periods, are presented on Figures 7-12 through 7-14. During periods of no irrigation, Well GW-N14 shows a decrease in magnesium, but increases in calcium, bicarbonate and sulfate that account for the small increase in mean TDS. On first appearance, one would conclude that Figure 7-13 is showing an accounting of a decrease in TDS during periods of no irrigation. TDS actually increases when irrigation is discontinued. The seemingly large decreases in calcium and magnesium are more than offset by the increases in sodium, bicarbonate and chloride. The sodium increase is principally responsible for the increase in TDS because it occurs at milliequivalent levels that are a factor of 10 times greater than the calcium and magnesium levels. The Schoeller plot for Well GW-N17P1 looks atypical for coal aquifer chemistry. The high sodium values and very alkaline pH level strongly suggest that the well completion zone is partially open to high sodium overburden and interburden units as well as the coal . Well GW-N20 shows a significant decrease in TDS during non-irrigation periods. Calcium, magnesium, sodium and sulfate levels all decrease when irrigation is discontinued. Only bicarbonate increases slightly, but at very low milliequivalent levels. The pH level for Wells GW-N14 and GW-N20 range from 4.7 to 7.6 while Well GW-N17P1 ranges from 8.2 to 9.8. As was previously mentioned, these very alkaline pH's are atypical of coal aquifer levels and are believed to be due to the well completion. Mean pH values for Wells GW-N14, GW-N20 and GW-N17P1 are 4.95, 5.03 and 8.45, respectively. There is some suggestion of a limited amount of seasonal variability to the Dakota coal chemistry at the Nucla East mining area. In comparing spring versus fall chemistry, magnesium, sulfate and TDS levels decrease and sodium and potassium levels increase in the fall. A comparison of summer versus fall chemistry suggests that bicarbonate, chloride and pH levels increase and magnesium levels decrease in the winter. Chemical trends over time are also evident in the Dakota coal aquifer. Well GW-N14 shows a decreasing trend in calcium levels during irrigation periods. Well GW-N17P1 shows a 7-65 Revised 04/11/88