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~~. !~ • • -3- 2. The proposed mining operation would not materially damage the quantity and quality of surface and ground water which supply the alluvial valley floor. During mining operations, the proposed underground mine would slightly deplete flows in the stream/alluvial aquifer system, and the discharge of water from the underground workings would affect water quality in the Carbon Ditch and in Ward Creek. After mining operations, when the natural ground water flow pattern ins restored, water passing through the underground workings would~+ffect water quality in the Williams Creek and Ward Creek stream/alluvial aquifer system. A quantitative assessment of these effects is provided in the discussion of probable hydrologic consequences contained in this findings document. That assessment predicts the streamflow depletion to be less than 2 percent of the average streamflow in Ward Creek, and less than 7 percent of the lowest streamflow observed in Ward Creek. The predicted water quality resulting from the mining activities would not prevent the use of surface water for flood irrigation. The essential hydrologic functions of the alluvial valley floors would be preserved. The most important characteristics of the Williams Creek and Ward Creek alluvial valley floors are the integrity of the present irrigation system and the ability of the surface water system to supply sufficient irrigation water to farming activities on the alluvial valley floors. As discussed above, surface disturbances would be located outside of the alluvial valley floor, and mining would be limited to 50-55 percent extracton to avoid surface subsidence effects. As a result, the surface topography and the integrity of the existing irrigation system would be preserved. Since no signficant effect to the quantity or quality of the water which supplies the alluvial valley floor is expectgted, the supply of irrigation water would also be preserved. Alluvial Vallev Floor Monitori The proposed hydrologic monitoring plan (Table 10 of the addendum) with the Division's stipulations and the proposed subsidence monitoring plan (Section 2.05.6(6)(f)(ii)(c) of the permit application) are sufficient monitoring to protect the alluvial valley floor. Additional monitoring of agricultural productivity will not be required at this time. Should the hydrologic monitoring or the subsidence monitoring indicate that the effects of mining-be=6~-greater than predicted, additional monitoring may be required by the Division to ensure compliance with the alluvial valley floor performance standards. /ep cc: Jim Pendleton Brian Munson Roy Cox