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the area may be characterized as stream deposits including channel-point bar facies, floodplain <br />• deposits and swamp deposits. <br />Alluvial deposits along the Purgatoire River are complex. The headwaters of the river are <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 of <br />these formations as does water quality. Alluvial deposits in the area of the mine range from <br />their current location to older terraces of forty to fifty feet above the Purgatoire. Buried <br />channels which are incised into the underlying Raton Formation have also been found. <br />Purgatoire River alluvial deposits support typical hydrophilic vegetation characteristic 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 are common and scattered throughout the area. The <br />valley bottoms are characterized by alluvial and colluvial deposits which are also coarse- <br />grained and deeper than soils of the mountain slopes. <br />Vegetation varies 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 from <br />May to September. July usually has the most rainfall. The mean annual temperature is 51.8°F. <br />January is the coldest month with a mean temperature of 32.90P, and July is the warmest <br />month, showing a mean of 60.2°F (Owenby and Ezell, 1992). The project area 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 />precipitation near 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 1972. <br />The size of the drainage area above the Stonewall station is 52.1 square miles (mi2). There are <br />some diversions for irrigation above the station. Recorded discharges during the three-and-a- <br />half year period range from 2.8 cfs to 522 cfs (USGS, Water Resources Division, 1982). The <br />size of the drainage above the Madrid station is 505 mil. There are diversions for irrigation for <br />about 6,000 acres upstream of the Madrid station. The average annual discharge between 1972 <br />and 1995 at Madrid was 71.4 cfs and the average annual runoff was 51,740 acre-feet <br />• 3