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4.8 Hydrology and Water Quality Groundwater would supply additional process water as needed. At the beginning of operations, — groundwater would be used as the primary source until the siltation ponds can provide necessary supply. Exclusive use of groundwater for processing would only occur during the limited period of time that the siltation basins are filling. After the ponds are filled, groundwater use would be — reduced to a supplemental supply. Under normal operations, the quarry would need to replenish the amount of water lost to — evaporation and the amount remaining in and hauled off with the product(production loss). Added together,the evaporative and production losses would be approximately 283,000 gallons per day (gpd) with a production rate of 1.7 mty and 400,000 gpd with production at 4 mty. These quantities of replenishment water would be supplied by groundwater pumped from onsite wells. Under the 1.7 mty processing scenario, the project would require 226 acre-feet per year(ac-ft/yr) of groundwater and 320 ac-f lyr under the 4 mty of processing scenario.I1 These groundwater demands are well below the pre-project, average annual agricultural demand of 570 ac-ft/yr, and therefore,under project conditions including functioning siltation ponds,the impact to _ groundwater resources is below the environmental baseline and therefore less than significant. During the time required to establish and fill the siltation ponds, groundwater pumping would be the only available water supply. Considering proposed process water demands ranging from 3.8 mgd(1.7 mty production condition)to 8.9 mgd(mty production condition),there would be an increase in daily groundwater extraction over that of the preexisting agricultural groundwater demand. This may temporarily draw the groundwater table down but that drawdown would not lower the groundwater below the current pumping depth. The impact analysis for this EHZ partly relies on a groundwater supply study 12 completed by Montgomery Watson Americas,Inc (MWA)in 1993 for a similar gravel mining project proposed by Granite Construction Company (GCC). The previously proposed GCC mining project was to be located south and adjacent to the currently proposed project site but was ultimately not implemented. The GCC mining project was similar to the proposed project because, in addition to its location adjacent to the project site, it also proposed to mine aggregate material up to a depth of 100 feet. Further,GCC proposed that 20 percent of the water demand for processing was to be supplied by — groundwater from the two onsite agricultural wells. Because of this similarity,data from the GCC analysis can generally apply to the proposed project site and can be used to evaluate the potential for project-related impacts associated with the depletion of groundwater resources. MWA analyzed whether groundwater pumping proposed for the GCC mining project would result in drawdown of local groundwater wells or represent a long term depletion of a groundwater resource. For its study,MWA obtained regional and local hydrogeologic data I 1 The estimate of 226 ac-ft/yr(1.7 mty scenario)assumes 283,000 gpd(0.87 ac-ft)for 260 days per year(0.87 ac-ft x 260 days=226 ac-ft/yr.The estimate of 320 ac-ft/yr(4 mty scenario)assumes 400,000 gpd(1.23 ac-ft)for 260 days per year(1.23 ac-ft x 260 days=320 ac-ftlyr). — 12 The report on the groundwater analysis for the Granite Construction Company is available for review at the County Planning and Community Development Department. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-27 ESA/203015 Draft Environmental Impact Report May 2006