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The underground mine workings generate water that may, in part, be tributary to the North and Middle Thompson Creeks. Over the last five years, the highest average annual combined discharge from Mine No. 1 and No. 3 was 0.12 cfs. This occurred in 1997. Assuming all mine water is derived from North Thompson Creek, stream depletions are expected to be minimal, resulting only from evaporation and infiltration, because this water is discharged back in[o the North Thompson Creek. On the other hand, if we assume that all of the mine discharge is derived from ground water (a worst-case approximation), an average mine discharge of 0.12 cfs would deplete the aquifer at a rate of approximately 87 acre-feet per year. Although it is not likely that all of the mine discharge water originates in the aquifers, even this maximum discharge would not significan[ly affect ground water 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 acalcium-sulfate type. The magnitude of this shift, howevec, 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 0.12 cfs from the mine (five percent of total flow), the dilution ratio would be 20 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. The highest average annual salt loading of Thompson Creek by Mine No. 1 and Mine No. 3 discharge in the last five years occurred in 1997, at .445 tons/day, or 162 tons/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 2.8 percent; the contribution to North Thompson Creek alone is 6.9 percent. Both of these values are low and will not result in salinity increases above any standards. The salt load on Thompson Creek during a mean monthly low flow might be l9