2010-10-01_REVISION - C1981008 (11)

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chemistry. The San Miguel River drains an area of 1,080 square miles. In addition, approximately 15,500 acres of irrigated land lies between Placerville and Naturita, Colorado. 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, Colorado, have a weighted average of 316 milligrams per liter (mg/1) TDS. Specific conductance varies between 318 and 730 millimhos (mmhos). The mean sodium adsorption ratio (SAR) is about 0.7 at Naturita. Water curves indicate that, 90 percent of the time flows in this river exceeds 60 cubic feet per second (cfs), while flows exceed 1000 cfs about 10 percent of the time. The mean discharge of the river is 351 cfs. San Miguel River waters are suitable for domestic usage except at low flow periods when sulfate levels are high. The Colorado Cooperative Ditch Company diverts as much as 145 cfs of water from the San Mim,Pl River approximately ten miles east of the town of Nilrla The main diversion ditch then flows westerly passing just north of the town of Nucla and then flows northwestward passing north of the New Horizon Mine 1. Diversion ditches distribute water to mainly agricultural users in the area. Since the completion of the irrigation system for this area in about 1910, these areas have been intensively reworked for irrigated agriculture. Additional uses include domestic, municipal and stock pond consumption. The New Horizon Mine 2 permit block is located just west of the town of Nucla in the mildly sloping upland area between Tuttle Draw and Calamity Draw, approximately in the middle of First Park. In the Mine 2 mining area, the surface water system is characterized by a "disjointed" tributary drainage system almost completely controlled by return flow ditches of the local irrigation network. The West Lateral irrigation ditch is part of the Colorado Co-operative Company's main ditch system that originates on the San Miguel River just upstream of Cottonwood Canyon (about 12 miles upstream of Naturita). Construction of the main ditch commenced in 1896, and water began flowing to the First Park area via the ditch in the spring of 1904 (Mercer, 1967). The main ditch, as well as the main laterals, is basically an unlined open channel excavated in shallow soils and sandstone. Where the ditch crosses drainages, pipes and aqueducts have been constructed to carry the flow. The main ditch has an average gravity run of four feet per mile from the San Miguel River to the First Park area (Mercer, 1967). For a more historical discussion on the Colorado Co-operative Company's development of the regional ditch system, the reader should refer to Section 2.04.3, Site Description and Land Use Information. The West Lateral splits off the main ditch near the northern boundary of the Nucla Townsite. The West Lateral follows a ridge-top course westward from Nucla through the New Horizon Mine 2 area to the main north-south county road (2700 Road), then winds south and southwest until the conveyed irrigation water is used by the various shareholders located along the ditch's course. A total of 18 diversions were identified on this lateral; many consist of wooden "splitter boxes" that divert a consistent proportion of the irrigation water from the lateral to fields via small ditches and channels. Some diversions simply consist of pipes and hoses that also route irrigation water to fields which are slightly lower in elevation. 11