Ground-water hydrology of the Hollister and San Juan Valleys, San Benito County, California, 1913-68

Abstract

The Hollister and San Juan Valleys are within the Gilroy-Hollister groundwater basin. That part of the groundwater basin underlying the valleys consists of three subbasins each of which contains two or more groundwater 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. 2 GROUNDWATER HYDROLOGY, HOLLISTER AND SAN JUAN VALLEYS, CALIF. Withdrawals of ground water 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. Groundwater 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. INTRODUCTION According to present plans, 273,000 acre-feet of water will be delivered annually through the U.S. Bureau of Reclamations San Felipe Division of the Central Valley Project to be apportioned to Hollister and San Juan Valleys in San Benito County, to the Pajaro Valley area of Santa Cruz and Monterey Counties, and to Santa Clara County. The Bureau's feasibility report of the San Felipe Division of the Central Valley Project, California, recommended that 46,000 acre-feet of this water be allocated to San Benito County for supplemental irrigation of approximately 33,000 acres of land (County of San Benito Imported Water Advisory Commission, 1968, p. 15). Most of this water will be used for irrigation and municipal supply; the imported water will constitute nearly one-third of the total estimated water requirement of the area under conditions of anticipated development. Some of the imported water may be percolated to recharge the groundwater basin.

Comparison of phase velocities from array measurements of Rayleigh waves associated with microtremor and results calculated from borehole shear-wave velocity profiles

“…The sediment consists of clay, sand, gravelly sand, and sandy gravel layers. Kilburn (1972) gives additional geologic information from well logs of four deep oil and gas test holes. (The closest of these four holes, the V. I. Gandrup, O'Connell No.l, is located at -1.4 km northeast of the 60 m borehole mentioned above; see Figure 9).…”

Comparison of Phase Velocities from Array Measurements of Rayleigh Waves Associated with Microtremor and Results Calculated from Borehole Shear-Wave Velocity Profiles

“…Figure 9 shows the V S and geologic logs from the borehole. Kilburn (1972) gives additional geologic information from well logs of four deep oil and gas test holes. Kilburn (1972) gives additional geologic information from well logs of four deep oil and gas test holes.…”

“…Curve #2 is calculated using an earth model constructed from the borehole logs for the top 60 meters and estimated for the lower depths to 1160 m guided by the geologic information from Kilburn (1972) separate time segments (light error bar represents ‫1ע‬ standard deviation of the phase velocity and heavy error bar represents ‫1ע‬ standard deviation of the mean). The dispersion curves are calculated from earth models constructed from borehole logging data and site geologic information (see text).…”

“…Generally, the measured phase velocities fall within the dispersion curves calculated using the estimated lower and upper bounds of the velocity profile. V S estimates for lower depths to 1160 m (a lower estimate to the left and an upper estimate to the right) are guided by the geologic information from Kilburn (1972) and the velocity information from Yamamizu and Goto (1978) and Yamamizu et al (1981). V S models for calculating fundamental-mode Rayleigh-wave dispersion curves in Figure 10.…”

Status and understanding of groundwater quality in the South Coast Interior groundwater basins, 2008: California GAMA Priority Basin Project

“…In the southern region of the GIL study area, engineered recharge occurs from the San Benito River as a result of river regulation at the Hernandez Reservoir, which is located approximately 20 mi southeast of the study area boundary (California Department of Water Resources, 2004a, b, c, d). Groundwater discharge occurs primarily as pumping for municipal and irrigation supply (Kilburn, 1973;California Department of Water Resources, 2004a, b, c, d).…”