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use, because there is no known beneficial use of ground water in the area of the mine. The mine water being discharged at present to North Thompson Creek has ionic characteristics that differ from the natural stream levels. The discharge is predicted to shift the ion balance in North Thompson Creek from its present calcium- bicarbonate type to a calcium-sulfate type. The magnitude of this shift, however, cannot be quantified, because the receiving stream will tend to dilute the effect. Assuming a mean annual low flow of 2.36 cfs and a maximum discharge of 0.08 cfs from the mine (three percent of total flow), the dilution ratio would be 33 to 1. This shows that a near or complete shift in the ion balance, even during low flows, is unlikely. No standards have been set for these ionic changes. However, it is known that sudden changes of high magnitude can affect fish and other aquatic life. In this case, however, no significant changes are anticipated. Past monitoring data at the mine shows only a slight increase in sulfate levels downstream from the mine. The mine has the potential to increase total dissolved solids content of the North Thompson Creek. To evaluate this impact, the Division considered the additional total salt load on Thompson Creek. Using the highest average annual mine discharge over the last five year of 0.08 cfs and the five-year average TDS concentrations from Mine Nos. 1 and 3, a conservative estimate of salt loading to Thompson Creek from the mine discharge would be 0.36 tons per day or 132 tons per year. This is less than the allowable discharge of one ton per day established by the Colorado Department of Health and compares to an average normal salt load of 5823 tons/year in the total Thompson Creek drainage. The additional contribution by the mine on Thompson Creek is 2.3 percent; the contribution to North Thompson Creek alone is 5.7 percent. Both of these values are low and will not result in salinity increases above any standards. Furthermore, the above estimate of salt loading from the mine discharges incorporated TDS levels averaged over the last five years, but TDS levels have actually decreased over the years, so that an estimate based on current TDS levels would show even less impact. The salt load on Thompson Creek during a mean monthly low flow might be more critical. This can be calculated assuming an average monthly low flow of 4.4 cfs (10 years of record), an average normal salt load of 2200 mg/1 and 928 mg/I for Mines No. 1 and No. 3, respectively, and a mine discharge of 0.08 cfs. Using these values, the mine will increase salt loads by 11 tons/month over an estimated low flow value of 57 tons/month. This is a 19 percent increase during this low flow period. Although this increase appears significant, a 19 percent increase in baseline total dissolved solids will _Ig_