Laserfiche WebLink
Effects of Mining on Surface Water <br />Changes in the quantity of water to the prevailing hydrologic balance are <br />primarily due to the location and timing of flows. It is difficult to define <br />any changes due to mining in the Grassy Creek basin because baseline data <br />collected in 1980 include the influence of 12 years of previous mining <br />activity at Seneca II Mine. The Fish Creek drainage, however, does indicate a <br />change in the ground and surface water systems. Mining has increased the <br />amount of ground water available to be discharged into the surface water <br />system, due to discharging spoil aquifers. Cow Camp Creek now exhibits <br />perennial stream flow as a result of mining in this drainage whereas prior to <br />mining the creek emphemeral stream flow conditions existed. <br />Data are insufficient to demonstrate whether short term decreases in stream <br />flow are due to initial filling of ponds and resaturation of backfilled <br />spoils. However, it is apparent that any depletion is extremely short-lived. <br />Variability in annual amounts of precipitation supplied to the watershed makes <br />it difficult to assess short term changes; however, long-term changes in flow <br />are expected to be minimal. <br />Three primary impacts on surface water quality were identified: 1) increased <br />erosion and sediment yield; 2) exceedences of applicable water quality <br />standards; and, 3) increased total dissolved solids loading. <br />Increased Erosion and Sediment Yield <br />During the active mining phase, and until sufficient vegetation can be <br />established in the watershed, sediment yields above pre-mining levels are <br />expected. Annual sediment surveys of the sediment ponds at the Seneca II Mine <br />were initiated in 1985. These surveys will allow actual amounts of sediment <br />deposited to be quantified. With continued monitoring, comparisons to <br />estimated design volumes and erosion rates can be made. Throughout mining, <br />the ponds will ensure compliance with effluent limitations prior to discharge <br />from the mine site. <br />Exceedence of Water Quality Standards <br />Minor exceedences of NPDES effluent limitations have occurred at the Seneca II <br />Mine. The Northwest Impoundment (NPDES 003) exceeded the oil and grease <br />standard three times during 1983. An oil-sorbant boom placed at the outlet <br />corrected the problem. Total suspended solids were exceeded twice at the <br />Wadge Impoundment (NPDES 002). One occasion was storm event related while the <br />other was associated with pumping water to be used for dust suppression. A <br />single exceedence of pH at the Pond 004 is considered to related to algal <br />activity, when a pH value above 9.0 was observed. <br />A comparison of water quality conditions observed in Grassy Creek to the <br />standards in effect for this stream reach indicate dissolved manganese to be <br />above the 0.30 mg/1 level at times, particularly in winter. Total iron has <br />reached concentrations as high as 11 mg/1 in April which exceed the standard <br />of 1 mg/1. Both of these metals are common mineral constituents and would be <br />expected to appear in water from mining activities. Sulfate also exceeds the <br />standard of 250 mg/1, particularly during spring and fall, in both Grassy <br />Creek and Cow Camp Creek. <br />-20- <br />