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6.2 Cumulative Impacts realized in the gradual decline of water levels in the lower Tulare Formation as proposed 6' municipal and industrial projects are developed in the future. A gradual decline in groundwater elevations caused by a cumulative increase in groundwater use could eventually reduce the capability of some users to extract groundwater.Excessive cumulative groundwater extraction ` can also lead to aquifer overdraft and subsidence, as was experienced in some areas of the San Joaquin Valley during the height of agricultural groundwater use in the mid-1900s. The construction of the Delta-Mendota Canal and California Aqueduct helped reduce groundwater use and arrest further subsidence in the affected regions. As discussed in the Hydrology and Water Quality chapter, Section 4.8, subsidence has not affected the region surrounding the project site or the Tracy area. Dependence on groundwater is typically reduced by increased availability and use of higher r, quality imported surface water supplies,reduced agricultural groundwater pumping, water conservation,and efficient agricultural irrigation practices. Raised awareness of potential groundwater depletion has encouraged groundwater studies designed to determine the sustainable ` aquifer yields and avoid adverse impacts to underlying aquifers. Throughout the San Joaquin Valley region,including the area surrounding the project site, municipalities and water districts are implementing efforts to reduce dependency on groundwater and provide surface water 6. sources, as is currently the case in the city of Tracy. Municipalities, such as Tracy and other entities have developed or are in the process of developing Groundwater Management Plans as required under the Groundwater Management Act [Sections 10750-10756 of the California Water ` Code(AB 3030)]. These plans assist in establishing sustainable groundwater yields to reduce adverse groundwater resource impacts. These efforts are allowing groundwater levels to increase in certain areas that had experienced water level declines through the mid- 1900s. Proposed or existing projects in the region that use groundwater could cumulatively contribute to r increased groundwater pumping and an eventual decline of groundwater levels in the lower Tulare Formation. However,the potential for this to occur and result in cumulatively significant effects is low considering the extent and volume of the lower Tulare Formation aquifer and its ` increased ability to accommodate increased extraction due to regional efforts to reduce groundwater dependency and use. Furthermore,the mitigation measures developed for this project(see Hydrology and Water Quality,Section 4.8)would reduce the potential that the project would contribute cumulatively because the prescribed mitigation requires that the project determine an acceptable drawdown to avoid adverse impacts on the underlying aquifer. With mitigation, the project would not create a cumulatively considerable contribution to regional adverse impacts on the lower Tulare Formation aquifer. Cumulative impacts would he less than significant. Groundwater Quality The project would limit mining to at least 5 feet above the seasonally fluctuating groundwater 6w level and therefore reduce the potential that mining operations would contribute to cumulative water quality degradation. With the implementation of the mitigation measures discussed in Section 4.8,Hydrology and Water Quality,project impacts related to groundwater quality will not he cumulatively considerable and the cumulative impact will be less than significant. RMC Pacific Vernalis Quarry Mining and Reclamation Project 6-13 ESA/203015 Draft Environmental Impact Report May 2006 r