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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALITY Local The primary sources of groundwater recharge to the upper,unconfined aquifer in the project area are percolation of irrigation water and intermittent stream flows originating from the Coast Range to the west. In the area of the proposed project,both the Delta-Mendota Canal and the California Aqueduct are lined, and hence do not contribute to the local groundwater recharge. Due to the uncertainties in both the future human factors, (i.e., land and water use,groundwater pumping,changes in water allocations from water projects,etc.)and future climatic conditions, it is not possible to accurately predict future groundwater levels in the vicinity of the project site. In addition,the aquifers in the vicinity of the project are part of a larger,regional aquifer system; hence,future water levels may be influenced by regional groundwater conditions in the Central Valley. Groundwater recharge data are not available in the project site area,however estimates of the recharge to the unconfined aquifer in the vicinity of the project location was previously modeled using the Central Valley Ground-Surface Water Model(CVGSM)(J. M. Montgomery, 1990)8. The model considers the primary factors affecting groundwater recharge including land use(i.e., agricultural areas,urban areas and undeveloped areas),rainfall distribution, soil properties, and amounts of applied water for irrigation. Based on these factors,the model estimates the average infiltration in agricultural areas at approximately 11 inches of water per year and in undeveloped areas is estimated to be approximately 1 inch. Infiltration of natural excess rainfall in the undeveloped areas is very limited in comparison to the agricultural sources. Based on yearly records from 1997 to 2003 for irrigation, the average annual applied irrigation is 1,051 AF. This equals approximately 18 inches of water per year available for infiltration, recharge, and evapotranspiration (water used by plants). Groundwater Budget A groundwater budget is a method of quantitatively accounting for water movement in a hydrologic system. DWR's Bulletin 160-93,estimated usable storage capacity for the San Joaquin River Region to be approximately 24 million acre feet(maf)(DWR 1994 in SJRG, 2003). There have been numerous attempts to estimate what is sometimes referred to as the safe yield9 of the San Joaquin River Region. The most recent estimate for annual safe yield, or the J amount of water that the aquifer can supply, is approximately 3.3 maf of annual yield(DWR t 1994 in SJRG, 2003). DWR estimates that municipal and irrigation uses extract 3.5 maf of water per year in the San Joaquin River Region. The estimated extraction volume of 3.5 maf of water s —' The Central Valley Ground-Surface Water Model(CVGSM)is a computer"simulation"model that characterizes groundwater levels and stream flows in the entire California Central Valley. CVGSM takes into account all of the primary man-made and natural factors that affect groundwater conditions and stream flows in the Central Valley, including the Vernalis area. The model has been utilized by both the USBR and the California Department of 1 Water Resources for planning purposes. A copy of the results of the model is available in the County project file. 9 The safe yield of a groundwater basin is the amount of water that can be withdrawn from it on a yearly basis ' without producing an undesirable result. Undesired results include the depletion of groundwater reserves,the intrusion of poor quality water,depletion of stream flow,and land subsidence(Freeze and Cherry 1979). RMC Pacific Vemaas Quarry Mining and Reclamation Project 4.8-14 FSA/203015 Administrative Draft EIR March 2004