2009-05-19_REVISION - M1981021 (36)

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9.0 Surface Water Neither perennial nor intermittent surface water bodies are present in the direct vicinity of the Sunday Mines Group. Surface water in the affected areas of the mines is limited to stormwater, which is managed in accordance with an existing stormwater discharge permit. This section provides general information regarding perennial and intermittent surface water in the vicinity of the Sunday Mines Group, and detailed information regarding management of stormwater at the mines. 9.1 Perennial and Intermittent Surface Water The affected areas of the mines are located on the south side of Big Gypsum Valley approximately 1/2 mile from an ephemeral drainage called Big Gypsum Creek, which drains into the Dolores River several miles downstream of the affected areas. Surface water is generally not present at the Sunday Mines Group, except during periods of snow melt or in response to episodic precipitation events. Storm water retention ponds are present at the mines, which are designed to retain all storm water generated by a 100 year precipitation event at the mines. The Dolores River is the major surface water feature in the area of the Sunday Mines Group. The river extends for 183 miles from an area northeast of Rico, Colorado, to the Colorado River at the town of Cisco, Colorado. The Dolores River Basin is . approximately 96 miles long and 34 miles wide, encompassing an area of 6,222 square miles. Most of the flow in the river originates on the western slopes of the La Plata • and Rico mountains, with additional contributions from snowmelt and springs of higher mesas and mountains. The Dolores River also receives inflows from groundwater, which are estimated to be approximately 3 liters per second per kilometer in the portion of the river extending from the town of Dolores to Cisco (Weir et al. 1983). Big Gypsum Valley is located within the central portion of the Dolores River Basin, in an area that is among the most and portions of the basin based on analyses presented by Weir et al. (1983). An ephemeral drainage is present within Big Gypsum Valley approximately 1/2 mile north of the Sunday Mines Group. Surface water was not observed in this drainage during site visits in June, August and December, 2008, but the geomorphology of the drainage suggests that ephemeral flows do occur occasionally in response to snow melt or large precipitation events. Intermittent flows may also be present in the lower portion of the drainage. 9.2 Design Specifications for Stormwater Management Facilities A stormwater drainage report for Topaz Mine was developed by Tetra Tech EM Inc. in 2007 (Tetra Tech 2007). CDM reviewed the Tetra Tech drainage report and provided recommendations to improve the Topaz Mine drainage facilities designs. C7 9-1 T:\84988-Denison Mines\Task Order 4 - EPP Sunday Mines GroupTINAL EPPTINAL - Environmental Protection Plan Sunday Mines.doc