Geohydrology and Water-Chemistry of the Alexander Valley, Sonoma County, California

U.S. GEOLOGICAL SURVEY

Scientific Investigations Report 2006-5115

Sacramento, California 2006

In cooperation with the Sonoma County Water Agency

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Abstract

This study of the geohydrology and water chemistry of the Alexander Valley, California, was done to provide an improved scientific basis for addressing emerging water-management issues, including potential increases in water demand and changes in flows in the Russian River. The study tasks included (1) evaluation of existing geohydrological, geophysical, and geochemical data; (2) collection and analysis of new geohydrologic data, including subsurface lithologic data, ground-water levels, and streamflow records; and (3) collection and analysis of new water-chemistry data.

The estimated total water use for the Alexander Valley for 1999 was approximately 15,800 acre-feet. About 13,500 acre-feet of this amount was for agricultural use, primarily vineyards, and about 2,300 acre-feet was for municipal/industrial use. Ground water is the main source of water supply for this area.

The main sources of ground water in the Alexander Valley are the Quaternary alluvial deposits, the Glen Ellen Formation, and the Sonoma Volcanics. The alluvial units, where sufficiently thick and saturated, comprise the best aquifer in the study area.

Average recharge to the Alexander Valley is estimated from a simple, basinwide water budget. On the basis of an estimated annual average of 298,000 acre-feet of precipitation, 160,000 acre-feet of runoff, and 113,000 to 133,000 acre-feet of evapotranspiration, about 5,000 to 25,000 acre-feet per year is available for ground-water recharge. Because this estimate is based on differences between large numbers, there is significant uncertainty in this recharge estimate.

Long-term changes in ground-water levels are evident in parts of the study area, but because of the sparse network and lack of data on well construction and lithology, it is uncertain if any significant changes have occurred in the northern part of the study area since 1980. In the southern half of the study area, ground-water levels generally were lower at the end of the 2002 irrigation season than at the end of the 1980 season, which suggests that a greater amount of ground water is being pumped in the southern half of the study area in recent years compared with that pumped in the early 1980s.

Water-chemistry data for samples collected from 11 wells during 2002–04 indicate that water quality in the study area generally is acceptable for potable use. Two wells, however, each contained one constituent (241 µg/L of manganese and 1,350 µg/L of boron) in excess of the recommended standards for drinking water (50 µg/L and 1,000 µg/L, respectively).

The chemical composition of water from most of the wells sampled for major ions plot as a mixed cation-bicarbonate, magnesium-bicarbonate, or calcium-bicarbonate type water. The ionic composition of the historical and recent samples from wells in the Alexander Valley is similar to that of the historical surface-water samples collected from the Russian River near Healdsburg. This suggests a similar source of water, particularly for wells that are less than 200 feet total depth and perforated in Quaternary alluvial deposits. Water from deeper, non-alluvial wells may contain slightly higher concentrations of sodium as a result of cation exchange.

Water samples collected from several wells over an approximately 30–year period suggest a progressive change in water chemistry over time. Samples from the southern part of the valley show a trend towards higher ionic concentrations and increasing concentrations of particular constituents such as sulfate.

CONTENTS

Abstract

Introduction

Purpose and Scope

Location of the Study Area

Land and Water Use

Climate

Previous Investigations and Data Bases

Acknowledgments

Physiography and Geologic Setting

Geology

Basement Rocks

Basin Fill

Sonoma Volcanics

Glen Ellen Formation

Quaternary Alluvial Units

Quaternary Landslides

Geologic Structures

Hydrology

Surface-Water Hydrology

Geohydrology

Water Budget

Ground-Water Levels and Movement

Long-Term Changes in Ground-Water Levels

Ground-Water and Surface-Water Quality

Methods of Water Sampling and Analysis

Ground-Water and Surface-Water Chemistry

Water-Quality Monitoring

General Chemical Composition of Ground and Surface Water

Summary and Conclusions

References Cited

Appendix

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U.S. Department of the Interior, U.S. Geological Survey Persistent URL: https://pubs.water.usgs.gov/sir20065115 Page Contact Information: Publications Team Last modified: Thursday, 01-Dec-2016 19:06:19 EST

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