SU0004094 - Laserfiche WebLink

Home Browse Search

4.8 Hydrology and Water Quality 112 feet and 184 feet below ground level during the 40 years of recorded data(1960 to present). The 40-year average depth in the Costa well is approximately 145 feet. The groundwater depths in the Baines well have fluctuated between 87 feet and over 140 feet during the 50 years of recorded data(1946 to 1998) with an average groundwater depth of 120 feet. As with the unconfined aquifer,the elevations of groundwater in the lower confined zones appear to correlate with climatic conditions such as the drought of 1977 and increased precipitation amounts in the mid-1980s. The groundwater elevation in the confined aquifer reached its lowest recorded level during the 1950s and gradually increased through the mid-1970s. Groundwater levels decreased in response to the drought conditions of the late 1970s and the rose to stable levels in the 1980s. The hydrographs for the wells in confined zones also indicate that water levels have remained stable or have gradually declined since the early 1980s. Groundwater levels in the Barnes and Costa well rose during the 1990s. In contrast, water levels in the Costa well have decreased since 1999 and are currently at an elevation of approximately 20 feet above msl (173 feet below ground level). _ Groundwater Storage Infiltration and percolation of rainfall, stream flow,and water from canals,ditches,and irrigation water provide recharge to the groundwater basin. The deep percolation of irrigation water is a major source of groundwater recharge in the San Joaquin Valley. The primary source of recharge _ to the lower,confined aquifer system is subsurface flows from the east side of the San Joaquin Valley. Rivers, such as the Tuolumne River and Stanislaus River,contribute to the recharge of this zone originate in the Sierra Nevada and flow westward to the San Joaquin River. 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 were previously modeled using the Central Valley Ground-Surface Water Modell (CVGSM) (J. M. Montgomery, 1990). 7 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 Vemalis area.The model has been utilized by both the USBR and the California Department of Water Resources for planning purposes.A copy of the results of the model is available in the County project file. RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-13 ESA/203015 — Dmft Environmental Impact Report May 2006