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i • <br />Design rationale and parameters for the enlarged sedimentation pond are <br />• presented in Table G-3. Exhibits G-2 and G-3 illustrate the pond design. <br />Based upon the site specific hydrology, this sedimentation pond will contain <br />the 10-year, 24-hour storm flood volume (1.07 acre-feet). The emergency <br />spillway will convey the 25-year, 24-hour storm flow (7.5 cfs) at a velocity <br />of 3.66 feet per second. <br />Ground water in the permit area is of minimal concern. During the site <br />evaluation, ground water was encountered in the gravel only in Drill Hole <br />No. G-44 located in the northeastern portion of the permit area. There was <br />only 1.5 feet of saturated grovel encountered at Drill Hole No. G-44; the <br />gravel is 6.8 feet thick at this location. The present mining operations have <br />encountered little ground water to date. Since the Mesa Gravel operations <br />began in a topographically low section of the permit area and little ground <br />water has been encountered, it is anticipated that essentially no ground <br />water will be encountered as mining moves westward. <br />• The mining operations require no water. Drinking and sanitation water is <br />provided by a well located near the environmental office (see Exhibit D-2). <br />The hydrologic balance of the permit area and adjacent lands will be pro- <br />tected during mining by routing runoff from disturbed lands through the <br />sedimentation pond. Timely backfilling, contouring, topsoiling, and revege- <br />tation will limit soil erosion and downstream sediment loads. Following <br />mining, the entire disturbed area will be reclaimed to approximate original <br />contour in a configuration facilitating farming. The sediment pond will be <br />left in place for wildlife use. A permanent mining-related alteration of the <br />local drainage system will be an increase of 71.6 acres in the drainage area <br />of the unnamed ephemeral stream in the northeast portion of the permit <br />area. The increased drainage area of this stream will be equal to the de- <br />creased drainage ores of the ephemeral stream in the northwest part of the <br />permit area. The small areas involved comprise only a small portion of the <br />larger Sage Creek and Hutchinson Gulch drainages, so off-site impacts <br />resulting from the alteration in drainage area will likely be insignficant. <br />• <br />G-II <br />