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-2s- Existing surface water monitoriny 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 50% of the time. This 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 be available to the receiviny stream. The total amount of water to be impounded at the mine area is only 5.6 acre-feet. This amount is 0.0005% 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 underyround mine workings generate water that may, in part, be tributary to the North and f4iddle 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 inflows in the mine are from stream flows (a conservative assumption), the amount of water depleted from North and Middle Thompson Creeks from Mines No. 1 and No. 3 combined is 0.3 cfs. This is, at present, a worse-case approximation. Future monitoring should show the depletion to be less than predicted. Assuming all mine inflows come solely from North Thompson Creek and assuming all water is consumed (a conservative approach), depletions would amount to 217 acre-feet/year, or U.3 cfs averaye discharge (see Table 4). This is only a mean annual reduction of less than 1% of flow. However, the effect of this reduction is more severe during low flows. The one-day low flow of record (13 years of data) on the Lower Thompson Creek is 1.5 cfs (9/]/54). For this extreme, a 20% reduction of flow will be experienced. Insufficient data is available to establish the frequency of this event. The average daily year low flow of record is 2.36 cfs. For this flow, a 13% 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 acalcium-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 (13% of total flow), the dilution ratio would be 8 to 1. This shows that a near or complete shift in the ion 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 in sulfate levels downstream from the mine.