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Infrared photography and field investigations indicated that the Fish Creek alluvial <br />body was, in places, flood irrigated, and was generally sub-irrigated, potentially <br />farmable land. Based on these investigations, the Division has determined that the Fish <br />Creek alluvial body met the criteria of an AVE This AVF extends the length of Fish <br />Creek to its confluence with Trout Creek and along the length of Trout Creek to its <br />confluence with the Yampa River. <br />Effect of Mining on Identified Alluvial Valley Floors <br />Water quality data were supplied in the renewal application from monitoring of surface <br />water stations on Cow Camp and Bond Creeks, tributaries to Fish Creek. Mining in <br />the Fish Creek drainage basin began in 1982. Since that time increases in both flow <br />and total dissolved solids (TDS) concentrations have been observed. Whereas flow in <br />the channel immediately below the mine, Cow Camp Creek, was ephemeral prior to <br />mining, perennial flow is now observed at the permit boundary. This effect is <br />attributable to spoil aquifer discharge, as well as an increase in runoff from mined <br />lands, and a decrease in evapotranspiration due to removal of vegetation from mining <br />areas. Recent observations have indicated, however, that stream flow is not maintained <br />in the channels of the two tributaries during the months of August and September near <br />their confluences with Fish Creek. The surface effects of water discharged from the <br />mine on the downstream AVF locations is restricted to the months of June and July. <br />Subsurface effects are expected to predominate in August and September. <br />Water quality constituents of concern to the AVF identified on Fish Creek include trace <br />metals, sulfate, and TDS. An evaluation of concentration levels present in the mine <br />discharge indicate TDS to be the constituent most likely to place a restriction on the <br />quality of irrigation water. Rule 2.06.8(5)(b) identifies material damage as increases in <br />electroconductivity to levels above a threshold value at which crop yields decrease. <br />The relationship between electroconductivity and TDS level has been demonstrated by <br />Seneca Coal Company, in the permit application. <br />The operator has submitted an analysis based on a mass-balance loading of TDS in Fish <br />Creek. Concentrations of TDS in mine effluent were the maximum values from Spoils <br />Spring # 1 in the Cow Camp Creek drainage. Mean flow values for June and July <br />were used for Cow Camp Creek, Bond Creek, and an upstream location on Fish Creek <br />in the calculations. Results indicated significant dilution would take place and <br />maximum expected TDS concentrations of 607 mg/l were expected in Fish Creek <br />below the mine. <br />The maximum TDS level projected would correspond to a conductivity of less than 1 <br />mmho/cm. A conservative estimate of the relationship between irrigation water salinity <br />and root zone soil salinity, as referenced in USDA Handbook 60, is 1 to 1.5, <br />respectively. Thus, irrigation water applied with a specific conductance of 1 mmho/cm <br />would result in root zone soil water with a specific conductance of 1.5 mmho/cm. No <br />decrease in production of any of the species identified in data submitted by the operator <br />29