An Overview of the Groundwater Flow and Chemistry of the Central Part of the Western San Joaquin Valley, California.
By Randolph B. Flay
Setting and Geology (Belitz, Heimes et al. 1990)
The San Joaquin Basin is an
asymmetrical basin enclosed by the Sierra Nevada Mountains on the east, the
Coast Ranges on the west, the Tehachapi Mountains on the south, and the San
Francisco Bay-Delta region on the north.
The Central Part of the Western San Joaquin Valley, as defined by the
source report, is bounded by the San Joaquin River on the east the Coast Ranges
on the west. The Pleistocene Corcoran
Clay layer of the Tulare Formation divides the groundwater flow system into an
upper semiconfined zone and a lower confined zone. Above the Corcoran Clay layer, three hydrogeologic units can be
identified: Coast Range alluvium (marine), Sierran sand (micaceous), and
flood-basin deposits.
The
Coast Range alluvium is generally oxidized and range in thickness from 850 feet
along the Coast Ranges to 0 feet closer to the valley’s axis. These deposits range greatly in texture and
permeability based on position along the alluvial fan. The Sierran sand is 400 to 500 feet thick in
the valley center thins to west, as the alluvial deposits of Coast Ranges
increase in thickness. The Sierran sand
is reduced in the valley trough.
Erosion through time has created a largely interfingered system in the
valley of Sierran sand and Coast Range alluvium. The varied textual and geochemical properties of each have
created a fairly complex situation.
The Corcoran Clay was formed as lake deposits of clayey silt, and creates a low permeability boundary of 20 to 120 feet in thickness, found 900 feet deep along the Coast Ranges and 400 feet in the valley trough.
Ground-water Flow System
The
San Joaquin Valley is not the place that it once was. Human induced stresses have greatly altered the chemistry and
flow regime of the region. Several
major human activities should be noted for their influence: “percolation of
irrigation water past crop roots, historical pumping from below the Corcoran
Clay Member of the Tulare Formation, delivery of surface water, and
installation of regional subsurface tile-drain system. This summary will focus on the
pre-development groundwater flow system of the semiconfined zone.
Recharge
was largely derived from infiltration of stream water from intermittent
streams, and possibly from ephemeral streams of the Coast Ranges. None of these streams, flowing west to east,
actually reach the San Joaquin River in the valley trough (the eastern boundary
of the study area). Earlier studies
estimated that recharge from the four major streams of the region accounts for
about 40,000 acre-feet of recharge per year.
The soil salinity data and the presence of low permeability mudflows in
interfan areas have been used to infer that recharge was limited mostly to
streams. Recent visitors to the valley
will find it hard to imagine the extensive marshes and artesian wells that once
predominated.
The ground-water gradients in the semiconfined zone were from the southwest to northeast, reflecting the topography of the region. Low rates of recharge contributed to a gradient of only 1 to 3 feet per miles. Other reports indicate similar features of the confined zone, although heads were about 10 to 20 feet lower in the confined zone along the Coast Ranges and 0 to 10 feet higher along the valley trough (Williamson, Prudic et al. 1989). Slug tests from another study reveal that hydraulic conductivity ranges from 10-4 to 10-5 ft/s for the Coast Ranges alluvium and from 10-3 to 10-4 ft/s for the Sierran sand (Belitz, Phillips et al. 1993).
The Upshot
The
Central Part of the Western San Joaquin Valley is a complex hydrogeological
setting that has been influenced greatly by anthropogenic activities, mostly
for agriculture. This sub-region and
the San Joaquin Basin as a whole are valuable resources for California and have
an estimated usable groundwater supply of 80 million acre-feet (Department of Water Resources 1975). These
groundwater resources are vital for agricultural, domestic (many cities from
Bakersfield to Sacramento drink it), and for in-stream uses in the San Joaquin
Basin. Despite its importance, the State
of California has failed to remedy conditions of overdraft, more than one
million acre-feet per year (Department
of Water Resources 1998). Other
serious issues affecting groundwater in the region are salinity, trace elements
(e.g. selenium), and nutrient and pesticide contamination from agricultural
activities (Domagalski 1992;
Dubrovsky, Kratzer et al. 1995; San Joaquin Valley Drainage Implementation
Program and University of California. Salinity/Drainage Task Force 2000). To ensure
the longevity of these resources, the role of the State in groundwater
management and the effect of agricultural activities on groundwater quality and
flow should be examined.
References
Belitz, K., F. J. Heimes, et al. (1990). Character and evolution of the ground-water flow system in the central part of the western San Joaquin Valley, California. Regional aquifer-system analysis. Washington D.C., U.S. Geological Survey: vi, 28.
Belitz, K., S. P. Phillips, et al. (1993). Numerical simulation of ground-water flow in the central part of the western San Joaquin Valley, California. U.S. Geological Survey water-supply paper ; 2396. Washington, D.C., U.S. Geological Survey: vi, 69.
Department of Water Resources (1975). Bulletin 118: California's Ground water. Sacramento, State of California: 135.
Department of Water Resources (1998). Bulletin 160-98: California Water Plan Update. Sacramento, State of California.
Domagalski, J. L. (1992). Pesticides in Surface and Ground Water of the San Joaquin-Tulare Basins, California: Analysis of Available Data, 1966 through 1992, U.S. Geological Survey.
Dubrovsky, N. M., C. R. Kratzer, et al. (1995). Water Quality in the San Joaquin-Tulare Basins, California, 1992-95, U.S. Geological Survey.
San Joaquin Valley Drainage Implementation Program and University of California. Salinity/Drainage Task Force (2000). Final report : evaluation of the 1990 drainage management plan for the westside San Joaquin Valley, California. Sacramento, California Dept. of Water Resources.
Williamson, A. K., D. E. Prudic, et al. (1989). Ground-water Flow in the Central Valley, California. Reston, VA, U.S. Geological Survey: 1-127.