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the area may be chazacterized as stream deposits including channel-point baz fades, <br />• floodplain deposits and swamp deposits. <br />Alluvial deposits along the Purgatoire River are complex. The headwaters of the river aze <br />underlain by volcanic rocks and sediments. As it flows to the east, the river cuts its way <br />through many different geologic formations including Pleistocene glacial-fluvial sediments, <br />foothills belt of Paleozoic-Mesozoic strata, terrestrial sediments of the Tertiary, and marine <br />deposits of the Pierre Shale. Channel shape and structure of the Purgatoire vary within each <br />of these formations as does water quality. Alluvial deposits in the azea of the mine range <br />from their current location to older terraces of forty to fifty feet above the Purgatoire. Buried <br />channels which aze incised into the underlying Raton Formation have also been found. <br />Purgatoire River alluvial deposits support typical hydrophilic vegetation chazacteristic of <br />floodplains and contain groundwater hydrologically connected to the Purgatoire River. <br />Soils are generally shallow and coarse-grained. They are derived from the underlying <br />sandstones and shales. Bedrock exposures aze common and scattered throughout the area. <br />The valley bottoms aze characterized by alluvial and colluvial deposits which aze also coazse- <br />grained and deeper than soils of the mountain slopes. <br />Vegetation vanes from riparian and blue grama complexes in the valley bottoms to pinion- <br />juniper, oak brush, and pine cover on the side slopes. Vegetative cover ranges from 100 <br />percent in the valley bottoms to none on the steeper side slope rock outcrop areas. <br />• The nearest weather station to the mine site is located at the Trinidad Airport (elevation 5,746 <br />feet) approximately 30 miles east. Over a recording period from 1961 to 1990, total annual <br />precipitation averaged 13.5 inches. The majority of this precipitation (65 percent) occurs <br />from May to September. July usually has the most rainfall. The mean annual temperature is <br />51.8°F. January is the coldest month with a mean temperature of 32.9, and July is the <br />warmest month, showing a mean of 60.2°F (Owenby and Ezell, 1992). The project azea is <br />approximately 1,650 feet higher in elevation than the Trinidad Airport and can be considered <br />to have 5°F lower mean temperatures and higher average precipitation. <br />Surface water availability is directly related to precipitation received in the drainage. The <br />climate summary, as described in the mine permit document, projects a mean annual <br />precipitafion neaz the mine of 16.92 inches. <br />The United States Geological Survey (USGS) monitored the flow of the Middle Fork of the <br />Purgatoire at Stonewall (07124050), approximately four miles upstream from the New Elk <br />Mine, from May 1978 to September 1981. The Purgatoire River at Madrid (07124200), <br />approximately 18 miles downstream from the New Elk Mine, has been monitored since <br />1972. The size of the drainage azea above the Stonewall station is 52.1 square miles (miZ). <br />There aze some diversions for irrigation above the station. Recorded discharges during the <br />three-and-a-half yeaz period range from 2.8 cfs to 522 cfs (LJSGS, Water Resources Division, <br />1982). The size of the drainage above the Madrid station is 505 mil. There aze diversions for <br />irrigation for about 6,000 acres upstream of the Madrid station. The average annual <br />. dischazge between 1972 and 1995 at Madrid was 71.4 cfs and the average annual runoff was <br />3 <br />