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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALrrY MWA determined the primary potential impact to the proposed GCC gravel mining operation was localized aquifer drawdown in areas adjacent to the project site. MWA used an accepted mathematical approach in conjunction with known aquifer properties in the Vernalis area to .. study this potential impact. Specifically,MWA used equations developed by C. V.Theis, known as the Theis Equations,to estimate potential draw-down in vicinity wells based on certain pumping rates.12 The Theis Equations incorporate many assumptions regarding well depth, r aquifer extent,and assumptions on groundwater extraction rates.13 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 time period. 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,estimates do not include the effect of other nearby pumping wells or regional drawdown. The results of the analysis, presented in Table 4.8-4 show the amount of drawdown that would occur at distances of 500 to 10,000 feet from the pumping wells with varying pumping rates. TABLE 4.8-4 RESULTS FOR DRAWDOWN ANALYSIS AFTER 240 DAYS FOR 60 GPM,100 GPM, 500 GPM,AND 1,000 GPM CONTINUOUS PUMPING Pumping Rate Drawdown(feet) Distance from Pumping Wells(feet) 60 gpm 100 gpm 500 gpm 1,000 gpm 500 0.4 0.6 3.1 6.3 1.000 0.3 0.5 2.4 4.7 2,000 0.2 0.3 1.6 3.1 5,000 0.1 0.1 0.6 1.3 ` 10,000 0.0 0.0 0.1 0.3 a Assumes that project wells will draw primarily(but not necessarily exclusively)from the confined aquifer. Source: Montgomery-Watson Consulting Engineers, 1993. For purposes of this EIR,drawdown results developed in the MWA study were graphically plotted and the drawdown curves extrapolated to include the proposed project pumping rates. 12 The Theis Equations are widely used in the study of groundwater flow. These equations analyze radial flow to wells in confined aquifers. 13 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 Vemahs Quany Mining and Reclamation Project 4.8-27 FSA/703015 Adm'„+ .+ w D ft EIR March 2004 r