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Joaquin County Department of Public Works Storm Drainage Plan does not currently include the g Master Morada Area to the west of the Project site. All af .� the project is within this boundary . For these areas, the Master Storm Drainage maps show storm drain systems terminating either in lower Mosher Creek to the north or to the Calaveras River in the south. The vast majority of these Installation. acilities shown are currently unbuilt, but are planned for future 2. Environmental Impacts F Development of the project as proposed will result in higher t runoff rates for the site. Based on the rational method, the projected peak runoff is expected to increase by less than 15%. Runoff from the project site can be accommodated by conventional drainage measures. FThe proposed drainage plan - incorporates the use of individual on-site retention ponds for each of the 36 additional lots. Adequately sized retention ponds would E accommodate design storm runoff from the project site and would mitigate the decrease in area available for percolation. Based on County Design Criteria and a composite runoff coefficient, the S` required' storage volume for most lots would be approximately ., 11 , 000 cubic feet, as shown on the applicant' s preliminary subdivision map. 6 The utilization of on-site retention ponds to accommodate storm runoff could adversely affect adjacent water supply wells and on-site sewage disposal systems. The close proximity of the standing water in these retention ponds to water wells increases the potential for contamination of. the well supply with surface water. Retaining , storm water on-site has the potential for further raising the groundwater level during the winter months. If seepage pits are utilized due to soil constraints, this situation could render them less effective. 3. S, uRyeste _a 'on Measures The use of on-site retention ponds for this project may result in conflicts with the proposed individual water wells and on-site sewage disposal systems. For lot sizes less than 2 acres, adequate setbacks between individual facilities can usually be obtained, but the close proximity of the many lots increases the concern for contamination of .groundwater or surface waters. To offset these potential health-related hazards it is recommended that: F a . Individual lot sizes should be increased in size to at least 2 acres to allow maximum physical setback between facilities. Based upon past experience, the Local Health District has determined that a two-acre minimum parcel size can adequately accommodate on-site services. ' 47