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r~ <br />u <br />Irri gati on also affects the ground-water system. Hydrographs of water levels in wolls <br />show ineonsi stent cyclical changes as a result of increased recharge during the irrigation <br />season. Seasonal chafgea es high as 18 feet have occurred. Dilution effects also occur, <br />but are not as great as with Calamity Draw, Longer term date are available around the <br />Nucla Mine than at Huela Eaat, sa CW-N9 nes chosen to illustrate this effect. This <br />monltori ng wall is north of the mine, lying in an irrigated area between the irrigation <br />and return ditches. Figure 77-16 1a a hydrograph for this welly the TOS eoneantrati ann <br />for water samples are also shown. The hi gheaC measured TDS concantrati on (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 concentrati one L3 ,094 mg/1) occurrnd at the end of the <br />i rri gaff on season. Note that dilution decreases the eoneantrati one to about 2/3 of the <br />maximum, comps red with 1/3 in Calamity Draw. <br />7ha changes in TDS in the 0W-N9 samples do rwt nacessarily~ reflect charges .in the <br />Chemistry of the water in the overburden, but instead, rosy result iron the construetf an of <br />the well and the seasonal influx of rater into the top o/ the saturated zone. Ths wall is <br />constructed with a screen interval extending from 45 to 72 Tt hsl ow Tand surface, and with <br />a gravel pack extending from above the screen to 72 ft. The depth to water ranges frm 32 <br />• _. to 48 ft below land surface. When recharge ocwra during the irri gait on season, tho lower <br />TDS recharge water will be present above the higher TDS water. When the well is pumped <br />for sampling, both waters are drawn into the well, and mixed. The degree of ni:i ng that <br />occurs in the overburden may be small. The interbedded nature of the sandstones and <br />ah el es in the overburden will res met the amount of vertical movement that can occur and <br />the degree of nixing that would result. <br />The major effect of the irrigation seanon on the ground-water system is to increase xater <br />levels in the overburden, coal, antl underburden. The amplitude of the change Sn water <br />levels decree sea dowmrard. For exampl a, the seasonal charge in GN-N21, CW-N20, and (Tf-N19 <br />(overburden, coal, and underDUrden) {s approximately 23, 14, and 5 ft, respectively, <br />Little mixing of water Sn the overburden is occurring, exwpC in the upper few feet; the <br />changes to water queltty in the overburden malls probably result from mi zi ng in tfie hall <br />during sampling. <br />During the irrigation season, the surface waters exhibit a decrease in concentration, Dut <br />an increase in Che TDS flux (as represented by sulfate flux). In the Nucle Eaat area, the <br />) <br />~~~ ~ sulfate flux Sncreasea 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 />11 <br />J <br />REVISED MARCH 2006 Attachment 2.05.6(3x2-65 <br />