Sonoma County, being a combination of urban and rural development, contains both municipal sewage treatment systems and homes served by individual septic tanks. The municipal systems range from those such as the Windsor County Water District, which has 2,320 service connections and a treatment plant of 0.75 million gallons per day ( 2,835 cubic meters per day) capacity, to the City of Santa Rosa, which has 17,500 service connections and three treatment plants with a total capacity of 9. 67 MGD ( 36, 600 m^3/d).). Table 6 presents data on all of the public and private sanitary sewer systems in the county; Figure 9 shows the locations of these systems.
It long has been suspected that much of rural Sonoma County (that part of the county not served by sewer systems) is unsuitable for the construction and operation of septic tank and leach field systems. Soils data developed by Miller (1972) support this view. One of the principal aims of the current investigation is to use these soils data to identify rural areas where septic tank problems exist, and where such systems should be prohibited.
As part of the data developed regarding septic tank practice in Sonoma County, reports documenting 14 septic tank failure areas were obtained from the Soinoma County Public Health Service. Table 7 presents data on the documented septic tank failures; the locations of the failure areas are shown on Figure 10.
To augment the failure data, surface water samples were collected at 33 locations throughout the county and were analyzed for total coliform by the County Public Health Service. Sampling areas were selected on the basis of the probability of the surface water having become polluted by failing septic tanks, and usually two samples were taken, one upstream of the potential pollution to establish a "background" coliform count, and one downstream to identify any pollution. For example, a sample was taken on the unnamed stream just north of Bloomfield (see Figure 10); this sample showed a Most Probable Number (MPN) of coliform of 7,000 per 100 milliliters (ml). A sample taken on the same stream just south of Bloomfield showed an MPN of 24,000, indicating a probability of septic tank failures in the Bloomfield area. Although a certainty of septic tank failures could not be determined based solely on the presence of total coliform,, it is suspected that samples which showed an MPN in excess of 1,000 indicated some degree of failure. The locations of the 33 sampling stations and the MPN of total coli- at each sampling station are shown on Figure 10.
click here for west side Sewer Service Areas FIGURE9A.JPG
click here for east side Sewer Service Areas FIGURE9B.JPG"
: : Number of : : : :
: :: Septic Systems: Satisfactory: Questionableb/: MalfunctionC/:
Area : Location : Failure: Ir~spected : No. %: No. %: No. %: Remarks
F1iddle Rincon Road Assessment Dist. Rincon Valley 4-1971 61 26 42.5 6 10,0 29 47.5
Edgewood Farms Rincon Valley 2-1966 110 79 72 16 1S 1S 14
Bodega Bay Bodega Bay 11-1972 233 170 71.5 - - 68 28.5Effluent flows into 80dega asy.
Salmon Creek Salmon Creek 11-1972 81 63 77.8 - - 18 22.2 Effluent present in roadside ditches.
Hollard Heights Gennett Valley 1-1972 101 S1 SO.S 11 10.9 39 33,6
Lake Street Cloverdale 4-1970 25 12 48 3 12 10 40 Probably 100% failure rate during winter.
Windsor Windsor 1961 388 202 52.1 - - 186 47.9
South Edison St. Graton 2-1965 31 6 16.1 - - 26 83.9 71% of wells sampled showed presence of
col if orm.
Graton Graton 2-1972 148 45 30.4 103 69.6 50% of wells sampled showed presence of
collform. 11 surface water samples
showed presence of coliform.
Glen Ellen Glen Ellen 4-1967 SO 34 68 - - 16 32 Effluent flows into Sonoma Creek.
Glen Ellen Glen Ellen 10-1972 260 161 61.7 43 16.7 56 21.6
Fircrest Avenue Sebastopol 1969 21 19 90.S - - 2 9.S
Belmont Terrace Sebastopol 2-1971 44 23 53 3 6 18 40
Rinconada Avenue Rincon Valley 1966 57 33 62 2 1 22 37
Montecito Avenue Rincon Valley 1971 229 188 82 1S 7 26 11
Penngrove Penngrove 1971 9'3 10 11 - - 87 89
b/ Questionable: Evidence of previous sewage observed. Malfunction probably occurs after periods of heavy rain and/or heavy use.
c/ Malfunction: Sewage Observed on surface of ground.
To determine whether septic tanks can be efficiently constructed, operated, and maintained in a given area, a knowledge of the soil and topographic characteristics of that area is essential. Physical factors, such as soil type, soil depth, ground slope, and presence of ground water, have bearing on the siting of septic tanks and leach fields. Directional factors, such as distance to property lines, cuts, fills, water wells, and bodies of water also are important.
The physical factors are, perhaps, the most critical in the determination as to whether a septic tank system can be placed on a particular parcel of property. The soils in Sonoma County range from alluvial valley and basin soils to residual mountain soils; each has its own unique physical characteristics. One criterion for the division of soils into septic tank acceptability groups is that of permeability, which is the measure of the ability of the soil to absorb and transmit fluids. Obviously, if a soil is of such low permeability that it cannot absorb fluids,
click here for a west side map of Septic Tank Failures
click here for a east side map of Septic Tank Failures
it is totally unsuitable for the placement of a septic tank system. Conversely, soils with a rate of percolation of up to 60 minutes per inch (mpi) or 25 minutes per centimeter (m/cm) have adequate permeability to accept and transmit effluent.
Because of the critical nature of the percolation rates of soils, it is important that percolation tests be performed correctly. Tests should be carried out during the most adverse time of the year (during a time when soils are wet and standing water is at its highest level).
A septic tank percolation test measures the rate of drop of the level of water in a test hole excavated into the area of a proposed leach field. Thus, the rate of drop of the water level is related to the percolative capacity of the soil in the proposed leach field area. Standards of percolation test hole construction and testing have been established by the Sonoma County Public Health Service. These standards are presented in Appendix E to this bulletin.
The depth of soil cover is important for a leach field system because there must be sufficient soil mantle underlying the leach field area to filter and purify effluent. A great number of the bacteria contained in sewage effluent are effectively removed by downward percolation through several feet of soil. The removal process involves mechanical and biological straining (a result of soil clogging) and/or destruction by environmental changes (soil defense mechanisms). Soil clogging is the increase in physical resistance to flow and generally occurs within the first six inches (15 cm) of soil near the leach line. Soil defense mechanisms occur at a greater distance and involve the destruction of harmful bacteria by: (1) the latter's inability to adjust to changes in temperature; (2) oxygenation and nitrifi-; and (3) destruction by naturally-occurring soil bacteria. Thus, water of a bacterial quality suitable for drinking purposes can be obtained after passing coliform-laiden effluent through a thickness of soil which is dependent on soil structure and conditions. Furthermore, coliform can be effectively removed from effluent even if the soil is a fairly coarse-grained dune sand. Experiments illustrating this latter fact were conducted by the University of California, Sanitary Research Engineering Laboratory (1955) who reported that coliform was reduced from 70,000 to 700 after percolation through 12 feet (4 m) of dune sand.
In view of the above depth requirements, soils which are less than five feet (1.5 m) deep, this is, two feet (0.6 m) m) deep below the invert of a 3-foot (0.9 m) deep leach line, are unsuitable for the placement of a septic tank and leach field. In contrast, as the soil gets progressively deeper, it becomes more acceptable; the most ideal depth of soil for septic tanks is that in excess of 22 feet (6.5 m).
The ground slope is extremely important with regard to septic tank siting. If slopes are greater than 30 percent, there is a possibility that untreated effluent may move laterally from the leach line and emerge on to the ground surface. The effluent then could move downslope onto adjacent property, pond at the base of a cut slope, or run into a body of water. Furthermore, slopes steeper than 30 percent could be subject to sheet erosion, which might wash out a leach line. Hence, if the horizontal distance from the bottom of the leach line to the ground surface is 10 feet (3 m) or less, the land is too steep for a septic system to operate efficiently.
The depth to ground water is a vital factor in siting septic tanks because without adequate filtration in the zone of aeration (above the water table), septic effluent could enter the ground water body and spread down gradient. Romero (1970) reported that fecal Bacillus cold and other pollutants travel as a thin sheet on the surface of the zone of saturation (the water table). This sheet can spread down gradient, and Bacillus cold has been reported as far as 80 feet (24 m) from the source of pollution. Thus, in order to provide a sufficient depth for filtration, the minimum seasonal depth to water must be not less than 5 feet (1.5 m) below the leach line invert, or 8 feet (2.4 m) below the ground surface.
Using soils data developed by Miller (1972), all of the soils in Sonoma County were classified according to their respective permeability, slope, depth to rock, and minimum depth to seasonal high water. The classification resulted in three basic septic tank acceptability groups being identified; one of the groups was further subdivided into three subgroups. Each septic tank acceptability group and subgroup is briefly described below. Table 8 presents the relationship of each soil series with its septic tank acceptability group; Table 9 shows the total area for each group and subgroup. Plate 2 shows the general areal extent of each septic tank acceptability group. For specific septic tank siting, the soils maps at a scale of 1:20,000 in the report by Miller (1972) are recommended for use in connection with data on Table 8 in this bulletin.
click here for a diagram explaining how septic tanks can pollute groundwater
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