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For the purposes of this CHIS, it was determined that no other existing or <br />proposed coal mining operations exist near the Nucla Mine. As such, the <br />probable hydrologic consequences which are expected to result from the <br />operation over the life of mine comprise the cumulative hydrologic impacts, <br />The general area considered in this CHIS corresponds to the surface and ground <br />water basin boundaries. The surface water basin is controlled by topographic <br />and drainage limitations of the watershed in which the mine is located. This <br />basin is defined as that contributing runoff to the ponds along Tuttle Draw. <br />The receiving stream with a potential for impact is Tuttle Draw and the San <br />Miguel River below the confluence with Tuttle Draw. Similarly, the ground <br />water basin is that defined as the aquifers of the upper Dakota Sandstone <br />downgradient of the mining operations at the Nucla Mine. <br />B. Surface Water Assessment <br />The probable hydrologic consequences of mining on the surface water system <br />have been identified in the permit application by the applicant. The primary <br />effect noted from water quality monitoring is increased total dissolved solids <br />(TDS) concentration between upstream and downstream surface water stations at <br />the mine. This effect, however, has been attributed to irrigation return flow <br />rather than any direct mining-related cause. <br />Streamfl ow in Tuttle Draw increases between upstream and downstream monitoring <br />sites, Part of this increase is due to the increase in contributing drainage <br />area between stations. The primary source of this additional water is <br />irrigation return flow from the Lower Second Park Irrigation Ditch which <br />traverses the area near the mine at an elevation above the level of the pits. <br />The lands north and northwest Supgradient) of the permit area are intensely <br />flood irrigated with water from the San Miguel River. Irrigation return flows <br />are intercepted by the mine pits and eventually arise as springs and seeps or <br />as alluvial ground water contributing to baseflow in Tuttle Draw: <br />This additional source of streamflow is significant in two ways. The <br />irrigation return flow can act to dilute the mine-affected waters as well as <br />the natural streamflow in Tuttle Draw. Secondly, the irrigation return flow <br />serves as an artificial source of water that becomes affected as it passes <br />through the mine pits and spoils. <br />The irrigation return flow is high in total dissolved solids. Monitoring data <br />collected recently from Spoils Spring No. 1 indicated a mean TD5 concentration <br />of 3780 mg/1 with a maximum value of 4190 mg/l. Flow from the spoils spring <br />is difficult to quantify but an estimate of about 10 gallons per minute is <br />considered representative at this location. The subsurface component of the <br />irrigation return flow reaches the streams during the period of low flow <br />during the winter. <br />The practice of irrigation is a historic use of the water and predates the <br />mining at the Nucla Mine: The baseline water quality data indicates elevated <br />TDS concentrations existed prior to recent mine related disturbances. The <br />volume of irrigation water flowing through the mine increases the potential <br />for leaching of spoil material. Although it is recognized that this condition <br />exists, it is difficult to quantitatively determine the precise magnitude of <br />-16- <br />