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~_ <br />• The Effects of Irrigation on the <br />is presently irrigated with water Cround and Su rfaee Water Chemistry. <br />from the San Miguel River, via the The Nucla East area <br />Highline Canal, which <br /> is locally called the Second Pa rk Irrigation Ditch. This water is of high quality, <br /> indicated by analyses of samples collected from monitoring sites SW-N105, and SW-N106. <br /> The TDS of this water ranges from approximately 115 to 275 mg/1. <br />The water quality in Calamity Draw has been measured at sites SW-N101 and SW-N103. Table <br />17-12 summarizes TDS data for these sites, for irrigation and non-irrigation seasons. <br />Site SW-N101 is upstream of the proposed area to be mined, while SW-N103 is downstream. <br />The TDS concentrations during the non-irrigation, base-flow season are similar to those <br />measured in ground water samples. An increase in TDS at the downstream sites is apparent. <br />During the irrigation season, dilution with San Miguel River Water decreases <br />concentrations to approximately one-third of concentrations during the non-irrigation <br />season. <br />Table 17-13 presents calculated fluxes of sulfate (concentration times stream flow) past <br />the monitoring stations (SW-N101 and SW-N103), the change in sulfate flux between the two <br />• stations, and estimated combined ground water discharge and surface runoff (calculated by <br />subtracting the measured flow at SW-N101 and SW-N102 (on an upstream tributary to Calamity <br />Draw) from the measured flaw at SW-N103). The irrigation season ran from approximately <br />May through September. Ungauged influx into Calamity Draw increases sharply (from 1.14 to <br />7.73 cf s) with the beginning of irrigation in May 1987. (The discharge measurement at <br />SW-N101 for June 24 is inconsistent with other data, so that the calculated fluxes are <br />questionable). With cessation of irrigation, the ungauged influx decreases over a period <br />of several months. Irrigation causes sulfate concentrations in the stream to decrease, <br />while at the same time the flux of sulfate increases. The decrease in concentration is <br />offset by the increase in flow. Within the stretch between SW-N101 and SW-N103, the <br />stream picks up between 6,000 and 33,000 (or higher) kg 504 per day, depending on the <br />season. This increase in 504 flux is partially due to ground water. For the July 16, <br />1986 sampling, the ungauged inflow had a concentration of about 900 to 950 mg/1 <br />(calculated by dividing the increase in sulfate flux by the increase in streamflow). This <br />concentration is greater than the sulfate concentration in the irrigation water, <br />indicating a ground water component. A similar analysis for April, 1987 indicates an <br />influx sulfate concentration of about 2,400 mg/1. Sulfate concentrations in samples of <br />. overburden ground water near Calamity Draw range from 875 mg/1 (end of irrigation season) <br />to 2,650 mg/1 (and of non-irrigation season). <br />17-52 Revised 04/11/88 <br />