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ARCADIS <br />The New Elk Mine is located at the Middle Fork of the Purgatoire River in Las Animas County, Colorado, <br />approximately 1.5 miles upstream of the confluence of the North Fork, and on stream segment number <br />COARLA05a of the Lower Arkansas River. Coal mining has historically been conducted in an area called <br />the Picketwire Valley, underneath as well as north and south of the Purgatoire River. Following temporary <br />mine closure and cessation of dewatering in 1988, lower areas of the mine gradually flooded. In 2008, <br />dewatering activities were resumed (NECC 2012). The purpose of the geochemical analyses and <br />modeling presented in this memo was to assess if mine dewatering activities have influenced or may be <br />expected to influence surface water quality in the Purgatoire River. <br />Relevant geologic units in the area of the mine include recent alluvial deposits and the Raton Formation. <br />Recent alluvium is deposited in channels that cut various geologic formations along the Purgatoire River, <br />including the Raton Formation. Groundwater in recent alluvium is hydraulically connected with the <br />Purgatoire River. The Raton Formation is characterized by very fine- to medium- grained sandstone <br />interbedded with siltstone and shale, and is the formation in which mined coal is located (NECC 2012). <br />Surface water availability is related to precipitation in the drainage area, mean annual precipitation near <br />the mine is estimated at approximately 16.92 inches. Discharge rates in the Purgatoire River have been <br />historically measured at stations upstream and downstream of the mine. Between 1978 and 1981, <br />discharge rates between 2.8 and 522 cubic feet per second (cfs) were measured upstream of the mine at <br />United States Geologic Survey (USGS) station Stonewall (07124050). Between 1972 and 2011 the <br />average annual discharge at downstream USGS station Madrid (07124200) was 69.5 cfs. Water quality of <br />the Purgatoire River is controlled by the quantity of flow and the materials through which the water flows. <br />At lower flow rates and further downstream higher electrical conductivity (EC) measurements are reported <br />(NECC 2012; Tetra Tech Inc. [Tetra Tech] 2012). <br />Because water in the COARLA05a segment of the Lower Arkansas River is designated for beneficial use <br />for agriculture, and as a result of high ionic strength of mine waters, and particularly the high proportion of <br />sodium relative to calcium and magnesium (Section 3), discharge of mine waters requires consideration <br />of the defined EC and sodium adsorption ratio (SAR) criteria protective of the most salt sensitive crop that <br />may be irrigated with water from the Purgatoire River downstream of the mine — alfalfa (Water Quality <br />Control Board [WQCB] 2008, Tetra Tech 2012). These considerations have been addressed with a simple <br />geochemical mixing model, as discussed in Section 5. <br />2. Summary of Site Data Used Analysis <br />Surface and groundwater monitoring programs have been conducted at the mine since February 1984. <br />The monitoring program at the mine has historically included sampling of surface water at the Purgatoire <br />River, ponds and alluvial wells, springs and seeps, mine water, bedrock wells, and discharge points. For <br />the purpose of this assessment, historical monitoring data from two surface water stations, four alluvial <br />Page <br />c WsersFo t=VsonlaWaWbcafirmaosmtwix mtgff poray inWmt files�cun6antoulbeM&AtuYwm& d= 218 <br />