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4.Environmental Analysis him including information on geology,groundwater levels and trends, soil conditions,groundwater recharge and aquifer properties. The MWA study included a regional groundwater well inventory that identified a total of 17 groundwater wells within the vicinity of the proposed site including nine domestic wells, seven irrigation wells, and one industrial well. MWA used an accepted ` mathematical approach in conjunction with known aquifer properties in the Vernalis area to evaluate this potential impact. It is important to note that MWA did not conduct actual well pump tests to determine the physical response of the wells to groundwater pumping. Specifically, MWA used equations developed by C. V. Theis, known as the Theis Equations,13 to estimate the potential drawdown in vicinity wells, based on certain pumping rates. The Theis Equations r. incorporate many assumptions regarding well depth,aquifer extent, and groundwater extraction rates.14 For its analysis,MWA also applied data from the Central Valley Groundwater Simulation Model(CVGSM) to obtain a hydraulic conductivity of about 20 feet per day, aquifer thickness ranging from 600 to 800 feet,and transmissivity of 100,000 gallons per day per foot. The MWA study analyzed groundwater withdrawal rates of 60 gallons per minute(gpm), 100 gpm, 500 gpm,and 1,000 gpm from multiple wells assuming 240 days of continuous pumping with no local recharge from deep percolation or stream percolation during this period. The MWA results indicated that up to 6 feet of drawdown could occur in groundwater wells located 500 feet from the discharge well extracting 1,000 gallons per minute. The calculated drawdown values represent the long-term drawdown in both the confined and unconfined aquifer and it was anticipated that the drawdown in the two aquifer systems would be similar.However, these estimates do not include the effect of other nearby pumping wells or regional drawdown. Alternative Groundwater Pumping Scenario San Joaquin County,Water Resources Division requested that this EIR consider and evaluate the feasibility of pumping the shallow, unconfined groundwater for wash water supply at the ►. proposed milting operation rather than pumping groundwater from the deeper,confined aquifer. . Under this scenario, two new onsite wells would be constructed in the shallow, unconfined zone. Based on the depths of existing groundwater supply wells and the current understanding of local hydrogeologic conditions, wells in the unconfined zone would tap groundwater at depths between 100 feet and 200 feet. The conclusions of the following preliminary assessment are based on the benefits described by San Joaquin County in its memorandum dated May 20, 2004: • Although groundwater extraction from the unconfined zones could reduce demand on the higher quality groundwater in the confined zone,it is likely that the demand .. reduction would be insignificant, given the size and potential yield of the vast, confined groundwater aquifer system. • Using the unconfined zone would certainly reduce the amount of higher-quality ` groundwater needed for product wash water; however,the use of the lower quality shallow groundwater could present operation disruptions due to the effects of high hardness and TDS on metal equipment. r 13 The Theis Equations are widely used in the study of groundwater flow.These equations analyze radial flow to wells in confined aquifers. 14 Although this mathematical approach has been applied with relative accuracy in many groundwater evaluations,the use of this approach can not substitute data obtained through site-specific well yield and aquifer testing. r. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-28 ESA/203015 Draft Environmental Impact Report May 2006