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Irrigation also affects the groundwater system. Hydrographs of water levels in wells <br />• show inconsistent cyclical changes as a result of increased recharge during the irrigation <br />season. Seasonal changes as high as 18 feet have occurred. Dilution effects also occur, <br />but are not as great as with Calamity Draw. Longer term data are available around the <br />Nucla Mine than at Nucla East, so CW-N9 was chosen to illustrate this effect. This <br />monitoring well is north of the mine, lying in an irrigated area between the irrigation <br />and return ditches. Figure 17-16 is a hydrograph for this well; the TDS concentrations <br />for water samples are also shown. The highest measured TDS concentration (4,708 mg/1) <br />occurred when the depth to water was the greatest, just after the start of the irrigation <br />season. The lowest measured concentrations (3,094 mg/1) occurred at the end of the <br />irrigation season. Note that dilution decreases the concentrations to about 2/3 of the <br />maximum, compared with 1/3 in Calamity Draw. <br />The changes in TDS in the CW-N9 samples do not necessarily reflect changes in the <br />chemistry of the water in the overburden, but instead, may result from the construction of <br />the well and the seasonal influx of water into the top of the saturated zone. The well is <br />constructed with a screen interval extending from 45 to 72 ft below land surface, and with <br /> a gravel pack extending from above the screen to 72 ft. The depth to water ranges from 32 <br />• 48 f <br />b <br />l <br />l <br />f <br />d n se <br />in <br />irri <br />ati <br />h <br />d <br />th <br />Wh the lower <br />ason <br /> <br />to <br />t <br />e <br />o <br />w <br />an <br />sur <br />ace. <br />g <br />g <br />en rec <br />arge occurs <br />ur <br />e <br />o - <br />, <br /> TDS recharge water will be present above the higher TDS water. When the hell is pumped <br /> for sampling, both waters are drawn into the well, and mixed. The degree of mixing that <br />occurs in the overburden may be small. The interbedded nature of the sandstones and <br />shales in the overburden will restrict the amount of vertical movement that can occur and <br />the degree of mixing that would result. <br />The major effect of the irrigation season on the ground-water system is to increase water <br />levels in the overburden, coal, and underburden. The amplitude of the change in water <br />levels decreases downward. For example, the seasonal change in CW-N21, CW-N20, and CW-N19 <br />(overburden, coal, and underburden) is approximately 23, 14, and 5 ft, respectively. <br />Littie mixing of water in the overburden is occurring, except in the upper few feet; the <br />changes in water quality in the overburden wells probably result from mixing in the well <br />during sampling. <br />During the irrigation season, the surface waters exhibit a decrease in concentration, but <br />. an increase in the TDS flux (as represented by sulfate flux). In the Nucla East area, the <br />sulfate flux increases as the water moves downstream, and it appears that ground water <br />provides an important part of this increase. <br />17-55 Revised 04/11!88 <br />