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4.6 WATER RESOURCES coinciding with periods of above normal rainfall, Tracy Lakes may fill on their own accord from local runoff, and the need for diversions from the Mokelumne River would be minimal. Given these circumstances, appropriation and diversion of water from the Mokelumne River would be expected to be allowed only under limited conditions (as determined by the Division of Water Rights) and it would J not likely be sufficient to preclude the need for dry season make-up water (pumping or diversions) in critical and/or successive dry years. By increasing the storage volume of Tracy Lakes over current conditions,the project would capture and retain a greater percentage of Jahant Slough runoff than presently occurs. This would require an appropriative permit from the Division of Water Rights. Since Jahant Slough is tributary to the Mokelumne River (at high flows), the Division of Water Rights would need to determine that such additional water to be stored in the Tracy Lakes is surplus water available for appropriation. Under current conditions,the lakes overflow into the Mokelumne River generally only in wet years, when the overflow is likely to be considered surplus. The project would decrease the frequency and amount of this overflow. Impact 4.6-1 Domestic water supply is to be provided by individual wells. This is not a significant impact. Groundwater is easily attainable and relatively minor amounts will be withdrawn for domestic uses. Domestic water usage is expected to be between 25 to 30 acre feet per year. This is a minimal amount of withdrawal when compared to surrounding uses and on-site recharge. Approximately 100 acre feet of water is lost to seepage from the lakes and causes recharge of the underlying aquifer (Table 4.6-4). The remainder of the site contributes groundwater recharge (from rainfall percolation) of approximately 80 acre-feet per year (based on 4" of deep percolation);thus, the combined recharge at the site is estimated to be about 180 acre-feet per year. The existing agricultural use of water on the site is approximately 130 acre-feet per year. Combined with 30 acre-feet for project site domestic use, total future domestic and agricultural use would be about 160 acre-feet, roughly the amount of recharge expected on-site. Thus, the domestic water use is not expected to impact groundwater resources. Should the County find that the proposed use of individual water wells for the planned development is inconsistent with the specifics and intent of County development policies, then a public water system may be required. Such a system would be expected to utilize on-site groundwater as the source. The water system would have to be permitted by the San Joaquin County Public Health Services; but no other significant impacts would be anticipated. 4.6-14