California Water Science Center
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An essential aspect of county-wide management is the relationship between the Russian River basin—the main source of water supply—and the other "satellite" basins that are dependent on Russian River water. Although many areas within the county have specific needs for more detailed and up to date information on water resources than currently available and would benefit from more rigorous analysis and quantification of the hydrologic system, Sonoma Valley was chosen as the first "satellite" basin to study and model because it is a relatively small and well-defined hydrologic basin that SCWA has already developed a GIS for. The basin has some areas of declining ground-water levels, potential water quality problems from sea water intrusion and upwelling of geothermal waters, and ground-water/surface-water interaction, all of which are issues that could be better managed with additional information, analysis, and a numerical model of the system.
Approach: Specific tasks were: (1) Develop a geographic information system to compile relevant data for the ground-water basins of interest; (2) Collect data, including water-quality sampling from streams and wells, seepage runs, and ground-water levels, (3) Data interpretation and geohydrologic characterization, and (4) Develop a simulation model of the ground-water flow system in Sonoma Valley.
Results: 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 USGS 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, ground-water 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.
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. 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.
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 water level 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.
Farrar, C.D., Metzger, L.F., Nishikawa, Tracy, Koczot, K.M., and Reichard, E.G. 2006, Geohydrologic characterization, water-chemistry, and ground-water flow simulation model of the Sonoma Valley area, Sonoma County, California, with a section on basement rock configuration interpreted from gravity data by Victoria E. Langenheim: U.S. Geological Survey Scientific Investigations Report 2006-5092, 167 p.