Ground-water Hydrology of the Hollister and San Juan Valleys 1913-68
Chabot Kilburn | August 31st, 1972
The Hollister and San Juan Valleys are within the Gilroy-Hollister ground-water basin. That part of the ground-water basin underlying the valleys consists of three subbasins each of which contains two or more ground-water subunits. The subbasin and subunit boundaries are formed by known or postulated faults, folded sedimentary rocks, and igneous rocks.
The principal water-bearing units are lenticular beds of sand and gravel interbedded with clay, silt, sand, and gravel, or their locally consolidated equivalents, which range from Pliocene to Holocene in age.
Ground water occurs mainly under artesian or semiartesian conditions but also under unconfined (water-table) conditions in areas adjacent to most surface streams and, locally, under perched or semiperched conditions.
In 1968 the depth to water in wells ranged from approximately 20 feet above land surface to more than 200 feet below land surface. Water-level differences in wells across the boundaries of adjacent subunits ranged from about 10 to more than 100 feet.
Withdrawals of groundwater for irrigation began in 1878. Since that time water levels in wells have declined more than 180 feet in the Hoilister Valley and more than 100 feet in the San Juan Valley. Serious declines in water levels probably did not begin, however, in most of the area until after 1945. Since 1945 large cones of depression have formed in each of the major subbasins. The centers of the cones are approximately one-half mile northeast of Hoilister in the Hollister subbasin, 1% miles northwest of Hollister in the Gilroy-Bolsa subbasin, and one-half mile east of San Juan Bautista in the San Juan subbasin. Ground-water movement beneath both valleys is now toward and into the cones.
Ground water in the eastern part of the Hollister Valley locally contains objectionable concentrations of boron and chloride. However, available data indicate that significant changes in the distribution patterns of these constituents have not occurred since 1939.
Keywords
Central Valley Project (CVP), Groundwater Exchange, groundwater recharge, subsidence, water quality