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t The grading plan (Exhibit 3) shows the proposed grading of the site. The grading plan is the same for all of the potential composting options. A 1-percent minimum grade is pro- vided at all points in the composting site. The composting Windrows are arranged normal to the site slope to allow free drainage and minimize ponding. The site is sloped to drain to two ditches designed to carry the water from the 25-year 10-minute duration storm (0.25 inches, 1.5 inches per hour) assuming all the water that falls on the composting site runs off. The ditches will carry the water to a retention pond designed to hold the net average annual rainfall during months where rainfall exceeds evap- oration (5.67 inches) assuming all the water that falls on the site runs off. This exceeds the 25-year, 24 hour rainfall event (2.28 inches). As described in Section II.A.6 of this permit application, the composting pad will be con- structed of compacted native earth. The permeability of soils below the surface of the composting pad is low. Operations will be suspended if necessary during periods of wet weather to prevent damage to the composting pad or the surface will be paved with gravel or asphalt if the economics justify the higher cost. Due to the low permeability of native soils and depth to groundwater, monitoring of soil moisture is not warranted. Soil sam- pling for ammonia, nitrates, metals and trace organics could be conducted to confirm the lack of movement of these constituents. Given the slow annual movement of groundwater at this site, soils could be removed and disposed of if movement of pollutants was found through soil monitoring. 9. Leachate Control If leachate generation is expected, provide a schematic drawing of the leachate col- lection and removal system. If necessary, indicate the location of drainage blan- kets, piping, drainage swales, collection points, etc. Describe the method of monitoring, collection, treatment, and disposal. Note: (1) Facilities that use amendments or additives in their composting operations shall include in their active composting areas a compost leachate management control system which is designed to contain and collect twice the anticipated volume of compost leachate, and the precipitation from a 25 year 24-hour rainstorm event, and (2) Shall have leachate holding facilities with a liner with a permeability of no greater than I x 10" cm/sc, which is constructed of a thickness and type of material that is resistant to damage due to weather or machinery, other stan- dard allowed in writing by the regional board. Because additional water will only be applied to the compost when necessary for main- taining adequate internal moisture, significant leachate generation is not anticipated. As stated in Section ffi. E.3, the site is in a region of net evaporation. Additionally, compost a ea10029M4.wp5/33 9/28/94 33