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By Christopher D. Farrar, Loren F. Metzger, Tracy Nishikawa, Kathryn M. Koczot, and Eric G. Reichard
With a section on Basement Rock Configuration Interpreted from Gravity Data
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5092
Sacramento, California 2006
In cooperation with the Sonoma County Water Agency
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The Sonoma Valley, located about 30 miles north of San Francisco, is one of several basins in Sonoma County that use a combination of ground water and water delivered from the Russian River for supply. Over the past 30 years, Sonoma Valley has experienced rapid population growth and land-use changes. In particular, there has been a significant increase in irrigated agriculture, predominantly vineyards. To provide a better understanding of the ground-water/surface-water system in Sonoma Valley, the U.S. Geological Survey compiled and evaluated existing data, collected and analyzed new data, and developed a ground-water flow model to better understand and manage the ground-water system. The new data collected include subsurface lithology, gravity measurements, groundwater levels, streamflow gains and losses, temperature, water chemistry, and stable isotopes.
Sonoma Valley is drained by Sonoma Creek, which discharges into San Pablo Bay. The long-term average annual volume of precipitation in the watershed is estimated to be 269,000 acre-feet. Recharge to the ground-water system is primarily from direct precipitation and Sonoma Creek. Discharge from the ground-water system is predominantly outflow to Sonoma Creek, pumpage, and outflow to marshlands and to San Pablo Bay. Geologic units of most importance for groundwater supply are the Quaternary alluvial deposits, the Glen Ellen Formation, the Huichica Formation, and the Sonoma Volcanics. In this report, the ground-water system is divided into three depth-based geohydrologic units: upper (less than 200 feet below land surface), middle (between 200 and 500 feet), and lower (greater than 500 feet).
Synoptic streamflow measurements were made along Sonoma Creek and indicate those reaches with statistically significant gains or losses. Changes in ground-water levels in wells were analyzed by comparing historical contour maps with the contour map for 2003. In addition, individual hydrographs were evaluated to assess temporal changes by region. In recent years, pumping depressions have developed southeast of Sonoma and southwest of El Verano.
Water-chemistry data for samples collected from 75 wells during 2002–04 indicate that the ground-water quality in the study area generally is acceptable for potable use. The water from some wells, however, contains one or more constituents in excess of the recommended standards for drinking water. The chemical composition of water from creeks, springs, and wells sampled for major ions plot within three groups on a trilinear diagram: mixed-bicarbonate, sodium-mixed anion, and sodium-bicarbonate. An area of saline ground water in the southern part of the Sonoma Valley appears to have shifted since the late 1940s and early 1950s, expanding in one area, but receding in another. Sparse temperature data from wells southwest of the known occurrence of thermal water suggest that thermal water may be present beneath a larger part of the valley than previously thought. Thermal water contains higher concentrations of dissolved minerals than nonthermal waters because mineral solubilities generally increase with temperature. Geohydrologic Characterization, Water-Chemistry, and Ground-Water Flow Simulation Model of the Sonoma Valley Area, Sonoma County, California
Oxygen-18 (δ18Ο) and deuterium (δD) values for water from most wells plot along the global meteoric water line, indicating that recharge primarily is derived from the direct infiltration of precipitation or the infiltration of seepage from creeks. Samples from shallow- and intermediate-depth wells located near Sonoma Creek and (or) in the vicinity of Shellville plot to the right of the global meteoric water line, indicating that these waters are partly evaporated. The δ18Ο and δD composition of water from sampled wells indicates that water from wells deeper than 200 feet is isotopically lighter (more negative) than water from wells less than 200 feet deep, possibly indicating that older ground water was recharged under cooler and (or) wetter climatic conditions. Alternatively, isotopically lighter water could represent recharge originating from higher elevations of the Sonoma Creek watershed.
A simulation model of ground-water flow in the Sonoma Valley was developed using MODFLOW-2000. The eightlayer model was parameterized to represent the three geohydrologic units. Model development required estimating model fluxes (pumpage and recharge) and hydraulic parameters (hydraulic conductivity and storage) for the area. The hydraulic barrier created by the Eastside Fault was incorporated into the model. In general, the calibrated model simulated waterlevel declines that matched measured values. The cumulative volume of water pumped from the ground-water basin between 1975 and 2000 was about 1.97 × 105 acre-ft; of this total pumpage, the model simulated that about 9 percent (1.73 × 104 acre-ft) was removed from storage. This fairly small decrease in storage explains the localized nature of the water-level declines. A sensitivity analysis indicated that the model would most benefit from additional data collection in the northern part of the basin.
Abstract
Introduction
Location of the Study Area
Purpose and Scope
Land and Water Use
Climate
Previous Investigations and Databases
Acknowledgments
Physiography and Geologic Setting
Geology
Basement Rocks
Basin Fill
Tertiary Sedimentary Rocks
Sonoma Volcanics
Huichica Formation
Glen Ellen Formation
Quaternary Alluvial Units
Bay Mud Deposits
Geologic Structure
Hydrology
Surface-Water Hydrology
Ground-Water Hydrology
Water-Bearing Properties
Effects of Geologic Structures on Ground-Water Movement
Recharge
Discharge
Streamflow Gains and Losses
Methods of Data Collection and Analysis
Streamflow Measurements and Estimated Gains and Losses
Watershed Hydrologic Budget
Ground-Water Levels and Movement
Comparison of Water-Level Contour Maps: 1950, 1980, and 2003
Long-Term Changes in Ground-Water Levels in Different Parts of the Sonoma Valley
Surface-Water and Ground-Water Chemistry
Methods of Water Sampling and Analysis
General Chemical Composition of Surface, Spring, and Ground Water
Constituents of Potential Concern
High-Salinity Waters
Ground-Water Temperature
Chemical Composition of Thermal Waters
Oxygen-18 and Deuterium
Background
Stable Isotope Results
Ground-Water Flow Model
Model Discretization
Spatial Discretization
Temporal Discretization
Model Boundaries
Subsurface Properties
Hydraulic Conductivity
Storage Coefficient and Specific Yield
Faults
Stream-Aquifer Interactions
Model Inflow
Model Outflow
Model Calibration
Simulated Hydraulic Heads
Areal Distribution: Steady State and 2000
Simulated Hydrographs
Simulated Water Budget
Model Fit
Streamflow Gains and Losses
Sensitivity Analysis
Uses and Limitations of the Ground-Water Flow Model
Summary
References Cited
Appendixes
Appendix A. Basement Rock Configuration Interpreted from Gravity Data
Appendix B. Water Levels at Selected Wells in the Sonoma Valley
Appendix C. Construction data, and spring 2003 water levels for selected wells used for geologic cross sections, water-level monitoring, chemistry sampling, and temperature logging in the Sonoma Valley, Sonoma County, California
Appendix D. Field measurements and laboratory analyses of samples from streamflow-measurement stations, springs, and ground-water wells, Sonoma Valley, Sonoma County, California, 2002–04
Appendix E. Summary of specific conductance and temperature measurements in samples from springs, ground-water wells, and miscellaneous sources, Sonoma Valley, Sonoma County, California, 1969–2004
Appendix F. Summary of delta deuterium and delta oxygen-18 values in samples from streamflow-measurement stations, springs, ground-water wells, and miscellaneous sources, Sonoma Valley, Sonoma County,California, 2002–04
Appendix G. Methodology for Estimating Pumpage for the Ground-Water Simulation Model
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