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• -26- Existing surface water monitoring data taken at .the mine over the past two years Indicates that suspended sediment levels downstream of the mine are equal or lower than the upstream monitoring site about 50X of the time. Th15 shows, in part, that the sediment control system is functioning. The sediment ponds themselves have the potential of affecting water quantity. Ponds may impound water that otherwise would 6e available to the receiving stream. The total amount of water to be impounded at the mine area 1s only 5.6 acre-feet. This amount is 0.0005X of the total flow in Thompson Creek (12,100 acre-feet/year). However, the Division still requires the operator to dewater the ponds within 72 hours as a mitigation measure to water rights holders downstream. The underground mine workings generate water that may, 1n part, be tributary to the North and Middle Thompson Creeks. Part of this water is consumed by the operation and part is discharged back into the North Thompson Creek. This aspect of the operation may cause several potential surface water impacts. Assuming all tnfiows in the mine ire from stream flews (a conservative assumption), the amount •f water 1eNletel from North and Middle Thompson Creeks from Mines No. 1 and N~. 3 combined is 1.3 cfs. This is, at present, a worse-case appr~xtmati~n. Future m~nttrrring shaul~ show the depletion to be less than predicted. Assuming all mine inflows came solely from North Thompson Creek and assuming all water is c~nsume~ (a c~nservattve approach), depletions would amount to 217 acre-feet/year, •r 1.3 cfs average discharge (see Table 4). This is •nly a mean annual reduction of less than 1X of flow. However, the effect of this reduction is more severe during low flows. The one-day low flew •f record (13 years of data) •n the Lower Thompson Creek is 1.5 cfs (9!1!54). Far this extreme, a 21X reduction of flow will be experienced. Insufficient data 1s available to establish the frequency of this event. The average daily year low flow of record is 2.36 cfs. For this flow, a 13X reduction would be experienced. This short-lived depletion is not predicted to cause any material damage to the stream. '~ 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, since the receiving stream will tend to dilute the effect. Assuming a mean annual low flow of 2.36 cfs and a maximum discharge of 0.3 cfs from the mine (13X of total flow), the dilution ratio would be 8 to 1. 7hfs shows that a near or complete shift to the tan balance, even during low flows, in 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 1n sulfate levels downstream from the mine.