Use of ground water in Sonoma County has been increasing steadily for the last 25 years. Today, with more than 10,000 water wells identified in the county, ground water continues to be an important natural resource. Ground water in Sonoma County is used for domestic, municipal, industrial, and agricultural purposes. Municipal water needs of the cities of Santa Rosa, Cotati, Sonoma, and Petaluma are met primarily from surface water supplies provided by the Sonoma County Water Agency. The cities of Cloverdale, Healdsburg, Sebastopol, and Rohnert Park rely principally on ground water.
The continuing population growth in Sonoma County has been accompanied by an increasing stress on the ground water resource. In order to obtain better information on which to base decisions, the Sonoma County Planning Department requested the Department of Water Resources to undertake a cooperative investigation of the ground water resource of the county, including the effect on this resource caused by urbanization in rural areas served by individual domestic well and septic tank systems. Information requested by the county included (1) the identification of all water purveyors; (2) the identification of all sanitary sewer systems; (3) the geologic parameters involved in the recharge, transmission, and withdrawal of ground water; (4) the chemical and bacteriological quality problems in the county; and (5) the identification and evaluation of the various ground water basins in the county.
The study was conducted on a cooperative basis and accomplished two goals. First, the study developed ground water data that the county will need in order to implement water development guidelines and the Department will need to evaluate the extent of the ground water resource for use in statewide planning. Second, the fact that the study was undertaken cooperatively ensured that planning would be based on local conditions and that local agencies would be involved in the effort and acquainted with the results and the bases upon which they were reached.
Sonoma County is situated north of San Francisco Bay, as shown on Figure 1. Santa Rosa, the county seat, had a 1970 population of so, oo6 . The city is the center of commerce of the North Bay area, and is served by U. S. Highway 101, two railroads, and two scheduled airlines. Other population centers include Petaluma (population 24,870) in the southern part of the county, Sonoma (population 4,112) in Sonoma Valley, Sebastopol (population 3,993), Healdsburg (population 5,438), and Cloverdale (population 3,251).
Rapid urbanization currently is taking place in much of the southern part of the county, which is within the commuter range of San Francisco. This urbanization has caused the development of such cities as Rohnert Park (population 6,133) and Cotati (population 1,368). Sonoma County historically has developed along agrarian lines; viticulture, orchards, poultry, dairying, and livestock are important to the local economy. California State College, Sonoma, located at Cotati, serves the area as a center for advanced learning.
The overall objective of the current study is to provide the county with guidelines regarding ground water resources and their use on which to base rational planning for the future. The scope of the study, which was identified in the contract between the Department and the County, listed the following items:
1. Determine the location and extent of the various ground water units.
2. For each ground water unit, determine
a. Well yields,
b. Recharge rates,
c. Storage capacity,
d. Water quality,
e. Depth to ground water.
3. Define areal and subsurface geology as it affects ground water.
4 Identify existing water quality problems.
5. Identify areas of septic tank acceptability and possible interference between domestic water wells and septic tanks.
Previous investigations related to the ground water resource in Sonoma County have been limited chiefly to the collection of data bearing on specific water resource problems. The earliest published report dealing with the water resources of the area was by Waring (1915). (Refer to Appendix A for bibliographic listings.) This report summarized the characteristics of a number of springs in the county as part of a statewide bulletin on springs.
More recently, Gamer (1942) made a study of the feasibility of using wells for drainage of poorly drained areas in the northern part of Santa Rosa Valley. Shortly thereafter, the U. S. Bureau
of Reclamation (1945) made a brief study of the water resources and land use of the Russian River watershed. This was followed by a report by the U. S. Corps of Engineers (1948) of the potential effects and benefits of water storage reservoirs proposed for the Russian River system.
The first comprehensive study of the geology of Sonoma County was conducted by Weaver (1949). Subsequently, three water-supply papers were published by the U. S. Geological Survey dealing with ground water geology of various parts of the county. The first was by Cardwell (1958) and covered the Santa Rosa Plain-Petaluma Valley area. The second, by Kunkel and Upson (1960) dealt with the Sonoma Valley area. The last, also by Cardwell (1965), included the entire Russian River watershed, as well as areas to the north of Sonoma County.
A bibliography on geology and subjects related to ground water resources, water wells, and septic tanks appears as Appendix A to this bulletin.
The current investigation of the ground water resources of Sonoma County has led to the following summary and conclusions:
1. Geology. Sonoma County is underlain by an assortment of geologic materials ranging in age from Jurassic to Recent. Most, if not all, of these materials yield ground water in some degree to wells. The quality of the water ranges from unpot able to excellent, with much of the unpotable water occurring near San Pablo Bay or the Pacific Ocean. The best yields of water are from wells drilled adjacent to the flowing streams; adequate supplies of water also can be derived from wells drilled in the hills near Sebastopol.
2.Water Supply Systems. Sonoma County has 191 water systems which could be identified. These range from large municipal systems with more than 19,000 service connections to very small mutual water companies with only two service connections. These systems rely on both ground water and surface water for their supply; the total annual demand by these systems exceeds 24,000 acre-feet (30 cubic hectometers = 30 million cubic meters)
3. Sanitary Waste Disposal Systems. Sonoma County has 19 sanitary waste disposal systems, ranging from large municipal systems with 17,500 service connections to small industrial systems with only one connection. The total capacity of these systems is nearly 24 million gallons per day (91,000 cubic meters per day). The potential for continued installation of septic tanks and leach lines in the county is extremely limited because only 0.61 percent of the total area of the county has soil characteristics suitable to the unqualified placement of such septic systems.
An additional 6.64 percent of the county has soil conditions which may be acceptable for septic tanks, but on-site tests should be required to determine if this is the case. The remaining 92.75 percent of the county is underlain by soils that are totally unacceptable for the satisfactory placement of septic tanks and leach lines due to inadequate percolation rates, steepness of slope, depth to rock, or depth to water. Recommended acceptable areas for septic tank siting are presented on Plate 2.
4. Ground Water Resources. Seven ground water basins in the county have been identified; these comprise 16 percent of the total area of the county. Adjacent upland ground water areas, underlying an additional 26 percent of the county, also have been identified. Most of the 10,199 identified water wells are located in these seven ground water basins and in the upland areas. The average depth to water was tabulated for each section for which water level data were available. In many cases, adjacent wells of differing depths had markedly different water levels.
Well records dating back to 1949 show that there have been few significant changes in water levels over the years. Natural recharge areas were identified in the valley areas of the Russian River and Dry Creek as well as the hill area southwest of Sebastopol. Previous studies of this latter area indicate that its recharge capability is about 21,000 acre-feet per day (26 cubic hectometers per day).
The density of water wells, as well as the percentage of wells with sanitary seals, also was determined. In several mile-square sections of land, there are more than 100 water wells; one section southwest of Sebastopol contains 180 water wells. In many sections with numerous wells, less than half of them have sanitary seals.
In Sonoma County, there are at least 400 springs ings, many of which yield potable ground water. There are also thermal springs which yield highly mineralized, unpotable ground water.
The total amount of ground water in storage in Sonoma County was determined through the use of a computer-assisted program. The program.. indicated that there are about 19 million acre- (23,000 cubic hectometers)) of ground water in storage. However, usable storage capacity could not be determined due to a lack of adequate data on recharge, transmissivity, pumpage,, and safe yield.
5. Water Quality Hazards. Most ground water developed in Sonoma County is usable for domestic purposes. Only in a few areas are chemical constituents present which render the water unpotable.. Boron is present in the water from a number of wells; this constituent, although not a hazard
to drinking water, may be injurious to toxic to a variety of plants and trees. Sodium, which also is an agricultural hazard, is present in a number of wells throughout the county. Water used for domestic purposes decreases in quality with an increase in salinity, iron and manganese, hardness, and total dissolved solids. Each of these four hazards was found in certain localities in the county.
Ground water is an important resource in Sonoma County. Many rural areas are entirely dependent on ground water. The use of ground water for storage in combination with surface water supplies may provide more efficient utilization of water resources. There is a need to continue the current investigative effort to develop additional ground water data, determine the usable ground water storage capacity, ascertain the dynamic response of the various ground water basins and contiguous ground water areas, and develop water resources management plans for Sonoma County. The program should include:
1. Establishment of the following priority for the further investigation of the various ground water basins because usable ground water storage capacity could not be determined during the present study:
Group I: Santa Rosa Valley (including Dry Creek and Rincon Valley), Petaluma Valley, Sonoma Valley, and the Kenwood Valley-Glen Ellen area.
Group II: Alexander Valley, Knights River Valley.
Group III: Ground water areas outside of boundaries water basins.
Investigation of the Group I basins should include the study of the entire geohydrologic system and an evaluation of the capability of the aquifer system to support municipal, and other high demand, water systems. Investigation of the Groups II and III basins and areas should include an identification of the aquifer systems and an evaluation of efficient use of ground water.
Establishment of a network of monitoring wells of sufficient density to provide data on the identification and configuration of the potentiometric surfaces of the various aquifer systems and the determination of the direction of groundwater flow in them.
-of the various aquifer ion of the direction of ground
3. Development of mathematical models of the Group I more intensely developed basins in order to evaluate alternatives to meet the water demands of these basins to the year 2000. Data needed for each model include land use, water use and pumpage, population projections, precipitation, aquifer characteristics, and transmissivity values.
4. Development, testing, and evaluation of conjunctive use plans for the Group I basins and recommendation of the type of ground water basin plan most appropriate for each basin.
5. Development of hydrologic data as in No. 3, above, for use in estimating usable ground water storage and safe yield in less- developed basins where the development of a ground water model is not justified at this time.
6. Identification of natural and artificial recharge sites and determination of their infiltration characteristics. Sources of water for recharge (both natural and treated) should be identified and evaluated. The effects of artificial and natural recharge of water on ground water withdrawals should be evaluated.
Additional study of water quality problems, such as iron and manganese, should be made to determine their cause, effect, and remedy.
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