By M.W. Newhouse, R.T. Hanson, C.M. Wentworth, Rhett R. Everett, C.F. Williams, J.C. Tinsley, T.E. Noce, and B.A. Carkin
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5250
Sacramento, California 2004
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To better identify the three-dimensional geohydrologic
framework of the Santa Clara Valley, lithologic, geologic, geophysical, geomechanical,
hydraulic, and water-chemistry data were collected from eight ground-water multiple-well
monitoring sites constructed in Santa Clara County, California, as part of a
series of cooperative studies between the U.S. Geological Survey and the Santa
Clara Valley Water District. The data are being used to update and improve the
three-dimensional geohydrologic framework of the basin and to address issues
related to water supply, water chemistry, sequence stratigraphy, geology, and
geological hazards. This report represents a compilation of data collected from
1999 to 2003, including location and design of the monitoring sites, cone penetrometer
borings, geologic logs, lithologic logs, geophysical logs, core analysis, water-chemistry
analysis, ground-water-level measurements, and hydraulic and geomechanical properties
from wells and core samples.
Exploratory cone penetrometer borings taken in the
upper 17 to 130 feet at six of the monitoring sites identified the base of Holocene
as no deeper than 75 feet in the central confined area and no deeper than 35
feet in the southern unconfined areas of the valley. Generalized lithologic
characterization from the monitoring sites indicates about four to six different
aquifer units separated by relatively fine-grained units occur within the alluvial
deposits shallower than 860 feet deep. Analysis of geophysical logs indicates that
coarse-grained units varied in thickness between 10 and 25 feet in the southeastern
unconfined area of the valley and between 50 and 200 feet in the south-central
and southwestern areas of the valley. Deviations from temperature-gradient logs
indicate that the majority of horizontal ground-water flow occurs above a depth
of 775 feet in the south central and above 510 feet in the southeastern areas
of the valley. Bulk physical properties from more than 1,150 feet of core samples
indicate an average primary-wave velocity of about 5,515 feet per second, a
bimodal distribution of density between 2.19 and 2.32 grams per cubic centimeter
with an average of 2.16 grams per cubic centimeter, and a magnetic susceptibility
that generally ranged between
9 and 40 with an average of 22.
Water-chemistry data indicate that the ground water in the alluvial aquifers
generally is low in total dissolved solids and chloride and of good quality.
Isotopic data indicate that water from artificial recharge is present throughout
the shallower parts of the aquifer system but may not be present toward the
center of the valley. The percentage of water from artificial recharge present
in ground water ranges from 0 to 61 percent for water-supply wells. The age
of most shallow ground water is less than 2,000 years before present, and the
age of deeper ground water is as much as 39,900 years before present, as determined
from carbon age dates.
Initial water-level data from the multiple-well monitoring sites indicate seasonal
water-level fluctuations as great as 60 feet and water-level differences between aquifers as great as 10 feet. The
water-level hydrographs indicate different water-level changes and relations
between aquifers in different parts of the basin. However, most of these hydrographs
indicate the potential for downward water-level gradients, with lower hydraulic
heads in the deeper monitoring wells.
Hydraulic properties of selected new monitoring wells
indicate that horizontal hydraulic conductivities range from 0.1 to 583 feet
per day. Hydraulic testing of selected core samples yielded vertical hydraulic
conductivity values ranging from 8 x 10-4 to 0.3 feet per day, and effective
porosity values ranging from 0.21 to 0.4. Geomechanical properties estimated
from one-dimensional consolidation tests of selected core samples resulted in
geometric mean inelastic and elastic specific storage values of 1.5 x 10-4 and
1.2 x 10-5 per foot, respectively.
Abstract
Introduction
Purpose and Scope
Approach
Description of Study Area
Acknowledgments
Design of Monitoring Sites
Geologic Data
Lithologic Descriptions
Geophysical Logs
Coyote Creek Outdoor Classroom (CCOC) Site
Willow (WLLO) Site
McGlincey (MGCY) Site
Santana Park (STPK) Site
Guadalupe (GUAD) Site
Eleanor Park (ELNR) Site
Saratoga (STGA) Site
Evergreen (EVGR) Site
Core Measurements
Physical Data
Observational Data
Core Subsample Data
Water-Chemistry Data
Selected Chemical Attributes
Major, Minor, and Trace Elements
Major Ions
Selected Minor Elements
Selected Trace Elements
Dissolved Nitrogen and Carbon
Isotopes
Hydrologic Data
Ground-Water Levels
Hydraulic Properties of Aquifers
Accessing Data
Summary and Conclusions
References Cited
Tables
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