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Alluvial deposits along the Purgatoire River are complex. The headwaters of the <br /> river are underlain by volcanic rocks and sediments. As it flows to the east, the river cuts <br /> its way through many different geologic formations including Pleistocene glacial-fluvial <br /> sediments, foothills belt of Paleozoic-Mesozoic strata, terrestrial sediments of the <br /> Tertiary, and marine deposits of the Pierre Shale. Channel shape and structure of the <br /> Purgatoire also vary within each of these formations as does the water quality. A natural <br /> deterioration of water quality can be expected as the stream flows out of the igneous <br /> terrains, through sodic sediments, and into marine sediments. Alluvial deposits in the <br /> area of the mines range from their current location to older terraces of forty to fifty feet <br /> above the Purgatoire. Buried channels filled with alluvium incised into the underlying <br /> Raton Formation have also been found. These alluvial deposits support typical <br /> hydrophytic vegetation characteristic of floodplains and contain ground water <br /> hydrologically connected to the Purgatoire River. <br /> 3.1 Surface Water <br /> Surface water availability is directly related to precipitation received in the <br /> drainage. The climate summary, as described in the 1981 mine permit application, <br /> indicates total calculated precipitation for the mines should be 16.92 inches. As <br /> previously mentioned, average precipitation was recorded at the mine weather stations <br /> and average snowfall was recorded in the upper elevations of the Purgatoire drainage <br /> which indicates that 1985 was an average year for runoff and stream flow. <br /> The Purgatoire River valley ranges in width from a few hundred to a few thousand <br /> feet. Use of the river in the area of the mines is limited to stock watering, flood <br /> irrigation on bottomland terraces, water supply for the New Elk Mine and preparation <br /> plant, and industrial supply for mining at both mines. Appendix 2 presents water uses for <br /> mine operations which could be as much as 35,000 gallons per day. It is also estimated <br /> that approximately 6000 ac-ft per year are diverted from the upper Purgatoire River <br /> Valley for irrigation use. <br /> -9- <br />