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Some conclusions may be drawn from Table 3. Pre-mining TDS concentrations in <br />Calamity and Tuttle Draws vary from 1200 to 3500 mg/1 and are high enough to <br />decrease productivity of moderately salt tolerant plants. TDS concentrations <br />have historically increased downstream of the Nucla Mine site in Tuttle Draw. <br />However, TDS concentrations downstream from the Nucla mine have not increased <br />enough to significantly affect crop yields when the water is used for <br />irrigation. On the other hand, the quality of water in the San Miguel River <br />is such that the water can typically be used to irrigate plants which are <br />moderately sensitive to salt. <br />Table 4 lists predicted impacts of mining on TDS concentrations which were <br />calculated for the San Miguel River below the northern most tributary in this <br />basin, Tuttle Draw, using weighted averages of discharge and TDS from six <br />sites. The prediction of TDS was calculated by adding the products of <br />discharge and TDS, and dividing by the sum of the discharge as in the <br />following equation: <br />(Qu Cu) + (Qd Cd) = Cp <br />u+ <br />Q - Discharge (cfs) <br />C - Concentration of Total Dissolved Solids (mg/1) <br />u - upstream <br />d - downstream <br />p - predicted <br />While TDS concentrations are typically higher during the non-irrigation season <br />in Tuttle and Calamity Draws (Table 3), predicted values along the San Miguel <br />River show the opposite effect. This results from limited spoils discharge <br />during the winter. <br />Impacts from active surface mining operations were minimal during periods of <br />mean flow or greater. As expected, when flows increase, TDS values decrease <br />and impacts from a salt-generating operation are minimized. However, <br />post-mining TDS concentrations from spoils discharge increased 9.2% based on <br />flows during the 1987 irrigation season. TDS concentrations were still well <br />within acceptable levels for irrigation of salt-sensitive plants. <br />During low flow periods, baseline TDS concentrations for Calamity and Tuttle <br />Draws and the San Miguel River far exceed acceptable levels for irrigation <br />with 1962 mg/1 TDS predicted during the irrigation season. Operational and <br />post-mining TDS concentrations increase up to 14% from baseline. During low <br />flows there may be no water in Tuttle and Calamity Draws and therefore no <br />impact from the mines. <br />-24- <br />