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the increase. A comparison of 1979 conditions to those monitored during 1986 indicate no significant change in TDS concentrations. A comparison of annual means at the downstream station indicates that a mean TDS concentration of 2140 mg/1 is a representative value which has remained stable over a period of years. An evaluation of the cumulative effect of the discharge from the mine on the San Miguel River was performed by the Division. This analysis considered loading of TDS from the mine into the river through a mass-balance technique. Water quality data used in this CHIS was obtained from two sources. Streamflow and TDS for the San Miguel River were obtained from the U.S. Geological Survey database known as WATSTORE. Information on mine effluent was taken from Volume 5, Exhibit 7-10 of the permit application. The approach taken in the surface water analysis was to load the maximum concentrations of TDS in mine effluent into low-flow conditions of the San Miguel River. The lowest Streamflow of record on the San Miguel River was in 1977. At the San Miguel River at Naturita (USGS No. 09175900'} Streamflow completely ceased during August at this location approximately four miles above Tuttle Draw. A minimum of 9.4 cfs was recorded at the San Miguel River at Uravan (USGS No. 09177000) located about 10 miles downstream from Tuttle Draw. The mean Streamflow at this station is 322 cfs. Four tributaries, including Tuttle Draw, contribute flow to this reach of the San Miguel River between upstream and downstream stations. As some amount of Streamflow would likely occur in the river immediately above Tuttle Draw, a value of 9.4 cfs was considered representative of low-flow conditions of the San Miguel River and was chosen for use in this analysis. Water quality data were collected at the Naturita location from March 1977 to June 1978. No analyses of TDS were performed, however, measurements of specific conductance were made. A mean value of 5000 umhos/cm was calculated from the 11 measurements made during this period. Values ranged from a maximum of 8000 nmhos/cm in March 1977 to 1150 umhos/cm during high flow conditions in May, 1978. Ina report entitled: "Summary of Water Quality Data for Selected Streams in Colorado," by Michael W. Gados, Open File Report 80-682, U. S. Geological Survey, 1980, regression equations are presented for estimating TDS concentrations based on conductivity data. A linear regression equation was developed for the Uravan station: TDS (mg/1) = 0.72 Ec (umhos/cm) + 2.26 This equation had a correlation coefficient of 0.97, and was considered acceptable for use in this analysis. Substitution of the mean value of 5000 umhos/cm into the equation resulted in a predicted TDS of 3600 mg/1 for the San Miguel River. In order to estimate the load from Nucla Mine effluent, the furthest downstream station (SW-N3} was used because it represents the sum total of all surface water discharged from the mine at this location on Tuttle Draw. This station is located about 2.5 miles above the confluence of Tuttle Draw with -17-