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-31- The projected total dissolved solids concentration resulting from the long-term effects of mining would sometimes exceed a total dissolved solids concentration of 1000 mg/1. Water having TDS concentrations greater than 1000 mg/1 may have adverse effects on many crops (EPA, 1975, p. 208). An examination of the long-term scenario used in the model show that most of this increase is the result of the projections used for the Middle Creek underground mine. The Middle Creek Mine is currently inactive but the mine continues to impact due to discharge through the portals. Model results for lower Fish Creek are also summarized in Table 2. The short-term effects show a slight decrease in stream flow resulting from the Foidel Creek Mine intercepting ground water currently discharging to the stream system. Both total dissolved solids and total salt loading would increase as a result of short-term and long-term effects. Again, the long-term effects of mining have a greater influence on receiving streams. The long-term effects of mining may limit the use of water during the low flow months of dry years. During these months, total dissolved solids concentrations could limit the use of this water for flood irrigation. During the seventy-two month period projected by the U.S. Geological Survey water quality model, the total dissolved solids concentrations resulting from the long-term effects of mining average a total dissolved solids concentration of approximately 1,000 mg/1 in six months, and significantly exceed the 1,000 mg/1 total dissolved solids concentration in only four months. In the lower Trout Creek drainage (Table 2), mining would again cause increases in flow, total dissolved solids and total monthly loading. However, Trout Creek is a perennial stream whose natural flows would greatly dilute the effects of mining. Although long-term effects are more significant than short-term effects, neither the short-term effects, nor the long-term effects would increase total dissolved solids concentrations to the level that would limit water use. Surface waters in lower Trout Creek are used both for flood irrigation and domestic use. The maximum predicted total dissolved solids concentrations are well below a threshold concentration of 1000 mg/1. The threshold concentration for domestic water use is less distinct. The EPA has suggested a criteria of 250 mg/1 for chlorides and sulfates in domestic water supplies (Quality Criteria for Water, U.S. EPA, 1976). The water in lower Trout Creek would be expected to have bicarbonate and sulfate as the dominant anions. Sulfates in the 1000 to 1500 mg/1 frequently have laxative effects, but concentrations as low as 200 to 500 mg/1 may affect some individuals. Total dissolved solids concentrations as high as 518 mg/1 were projected to result from the long-term effects of mining. However, these total concentrations would be considerably less than the total. Water in the range predicted would still be considered "good" in terms of palatability. Affected surface waters in lower Trout Creek would remain suitable for both flood irrigation and domestic use.