2012-04-03_GENERAL DOCUMENTS - C1991078

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form several decades in the future. Disregarding evaporation en route, an 11 -gpm spoil spring would possibly increase the TDS in Naturita Creek surface water by 22 mg/1 at low flow, raising it from 1700 mg/1 to 1722 mg/l. The small magnitude of this impact, and its projected occurrence in the distant future, do not warrant establishment of an alluvial ground water point of compliance for Hamilton. Hydrologic Balance: - Surface Water The Hamilton Mine is located in the San Miguel River drainage basin approximately three quarters of the distance between the rivers source in the San Juan Mountains and its confluence with the Dolores River. The San Miguel River drains an area of 1,080 square miles. The proposed permit area constitutes 0.1 percent of the San Miguel River drainage. The area disturbed by mining activities will be 0.04 percent of the same drainage basin. It traverses the interior portions of a basin within the Uncompahgre Plateau. This basin trends southeast to northwest and is chiefly underlain by Dakota and Burro Canyon Sandstones, the Morrison Formation and Mancos Shale. The source area is primarily composed of tertiary volcanic intrusives. The Morrison Formation and Mancos Shale have the greatest potential for influencing San Miguel river water chemistry. Waters draining from this area are of a calcium bicarbonate - sulfate type during high flow periods. These waters contain less bicarbonate during low flows while the calcium and sulfate concentrations increase. This concentration increase is caused by less dilution from snowmelt runoff. In addition, approximately 15,500 acres of irrigated land lies between Placerville (near the headwaters to the southeast) and Naturita, Colorado. Naturita is approximately 11/2 miles downstream of the proposed Hamilton Mine permit area. It is estimated that 2.8 tons of total dissolved solids (TDS) per acre are contributed to the waters annually from this area. This results in increased levels of magnesium, potassium, sulfate and chloride. Water quality samples taken from the San Miguel River at Naturita have a weighted average of 316 milligrams per liter (mg/1) TDS. Specific conductivity varies between 318 and 730 millimhos (mmhos). The mean sodium adsorption ratio (SAR) is about 0.7 according to U. S.G. S data from samples taken at Naturita. Discharge records indicate that flow in this river exceeds 60 cubic feet per second (cfs) 90% of the year, while flows exceed 1,000 cfs about 10% of the year. The mean discharge of the river is 351 cfs. San Miguel River waters are suitable for domestic use except during low flow periods when sulfate levels exceed EPA domestic use standards. Naturita Creek originates near the northern edge of the San Juan Range, flowing northwest approximately 30 miles to its confluence with the San Miguel River upstream of Naturita. Naturita Creek drains approximately 203 square miles. Naturita Creek and its tributaries traverse mostly Upper Cretaceous Dakota and Lower Cretaceous Burro Canyon Sandstones which have interfingered shale, mudstone, and Dakota coal beds. There are three diversion systems within the Naturita Creek headwater area to the southeast. All return flows from these diversions enter Naturita Creek upstream from the proposed permit area. Approximately 9 square miles of the Naturita Creek drainage basin surface flow may be affected due to the proposed mining activities at the Hamilton Mine. An additional 8 square miles of surface flow along Bramiers Draw may also be affected. Bramiers Draw empties directly into the San Miguel River. Surface runoff from the proposed permit area flows north into Naturita Creek along Section 33 Creek. Runoff also flows west to Bramiers Draw which terminates in the San Miguel River less than one quarter mile below its confluence with Naturita Creek. Stream flow across and adjacent to the proposed permit area is ephemeral, resulting from snowmelt and storm activity only. Springs in the area which may have April 3, 2012 RN4 7