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-4- 6. To estimate sediment yield, the concentration of suspended solids in runoff water was assumed by the applicant to be 10,000 mg/l, and multiplied by annual runoff volume to approximate the annual sediment accumulation. Neither the runoff estimates nor the suspended solids concentration were substantiated. The resultant sediment estimate is considerably lower than estimates for sediment delivery per acre of disturbance for other mines in the vicinity. The applicant will be required to substantiate the suspended solids assumption and the annual runoff estimate or revise the estimates according to more acceptable methods. Diversion and Collection Ditches A number of ditch sections have 10 year, 24 hour storm velocities which are erosive. Erosive velocity would not be a concern where ditches are in bedrock sandstone, but ditch sections which have erosive velocities and are in unconsolidated material should be lined with rock riprap. Numerous channel sections have slopes in excess of the listed slopes on page III-18 and as a result have higher than indicated velocities. The ditch section near the northeast corner of Pit 4 has a slope of over 15% and an estimated velocity of 9 fps. Potentially erosive ditch sections and channel types should be identified by the applicant and an acceptable plan should be submitted for minimizing channel erosion in segments of concern. Culverts. 1. The application does not contain information on specific methods used for estimation of peak flaws at culvert installations. The applicant should specify how each peak flow estimate was obtained and give the input parameters and equations used. Rule 2.04.7 - Surface Water Baseline 1. The application does not contain a spring and seep survey. The applicant should provide a map showing the location of all springs and seeps in the permit and adjacent area. In addition, as required under Rule 2.04.7(2)(a), the applicant should provide sufficient baseline information to identify the seasonal variations in water quality and quantity for the springs. 2. The applicant has indicated that mining operations could affect water quality in Grassy Creek (p. II. F-3). Water percolating through the mined area would discharye as springs and seeps in the Grassy Creek drainage. This discharge would raise the concentration of total dissolved solids in Grassy Creek. The applicant should provide a quantitative estimate of this effect on the Grassy Creek stream/alluvial aquifer system under normal seasonal fluctuations. In addition, the applicant should identify downstream water users that could be affected, and propose mitigations, if appropriate.