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Information on Winter Water Table Testing in Santa Cruz County

High winter water tables are probably one of the most significant and problematic constraints to proper septic system functioning in Santa Cruz County, particularly in the San Lorenzo Valley. Inadequate separation between the bottom of a leachfield and groundwater can cause septic failure or contamination of groundwater and surface water.

In most areas of Santa Cruz County, groundwater levels rise significantly during the rainy season, often coming within 1-6 feet of the ground surface for several weeks to several months, depending on the season. If a leachfield is flooded by groundwater, it greatly limits the ability of the sewage to soak into the ground and a failure may occur with untreated sewage on the ground surface. If sewage reaches groundwater before it has been adequately treated in the soil, bacteria, virus, and pathogens can contaminate wells or nearby streams. Studies in the San Lorenzo Valley have shown very high levels of bacteria within 25-50 feet of leachfields that are in direct contact with groundwater. In some studies, pathogenic viruses have been shown to travel several hundred feet under high groundwater conditions. However, where soils are not saturated, it has been found that bacteria and virus are reduced to safe levels within only 2-5 feet of a leachfield.

Many older septic systems in this county were installed before standards were in effect and are too deep and are wholly or partially saturated by groundwater for significant periods during the winter. For new systems and repairs, it is important to keep them shallow enough to provide an adequate amount of unsaturated soil between the bottom of the leachfield and high groundwater to reduce the overall occurrence of failures and to improve the water quality in the San Lorenzo River. How much separation is enough, particularly when trying to repair a system to serve an existing house, is somewhat a judgement call, and was one of the main points of discussion between the County and the State Regional Water Quality Control Board during preparation of the San Lorenzo Wastewater Management Plan and County septic repair standards. The amount of separation required was ultimately determined by a subcommittee consisting of members of both the Board of Supervisors and the Regional Board. This determination was ratified by both Boards. The required separation for new systems is 5 to 50 feet (depending on soil and proximity to a well) and for repairs it is 3 to 5 feet, depending on proximity to a stream (Code Sections 7.38.095.B.1 and 7.38.150.B.7). At distances of more than 250 feet from a stream, a separation of as little as 1 foot can be allowed on a repair as a nonconforming system, with no major building additions allowed.

Maintaining an adequate separation from groundwater has long been a basic component of standards for new septic systems throughout the state and the nation. A specific period for winter water table testing has been required for new septic systems in Santa Cruz County since at least 1974. For most areas of the County, the depth to the winter table is one of the most critical septic system design parameters, and often one of the more difficult to determine. If groundwater is expected to occur within 10-15 feet of the ground surface, a septic permit cannot be approved until a determination is made of the expected winter groundwater level. On parcels with shallow groundwater, the determination of groundwater level may mean the difference between a conventional septic system or an alternative system, a haulaway system, or complete denial of any new development.

Water tables can fluctuate by as much as 10-20 feet during the year and can also fluctuate fairly significantly over short horizontal distances, depending on geology and topography. (An attached page shows these fluctuations from a monitoring well in Boulder Creek.) Although there are methods to try to estimate expected winter water table levels based on observations of soil characteristics during the dry season, these are not particularly dependable. Over the years we have installed groundwater monitoring wells in parts of the San Lorenzo Valley and have developed a database of all prior winter water table observations. Staff uses this information, when available, to attempt to estimate the expected winter water table level for septic applications that are received during the dry season (which is usually 10-11 months of the year). However, the range of error of these extrapolations is expected to be about 2-3 feet. In conditions of shallow groundwater, the estimated level may result in a requirement for an alternative system, a haulaway system, or permit denial. An applicant for a repair or upgrade may disagree with an estimate made by County staff and wish to wait until winter time to make actual observations of winter groundwater levels. For new development, actual observations of winter groundwater are required for approval on any parcel where the winter water table is likely to rise to within 10-15 feet of the surface.

Because groundwater levels can fluctuate significantly during the winter in response to rainfall, it is important to carefully define the period when water table testing can be done to obtain appropriate measurements for design purposes. The period when winter water table testing can be done is specifically defined in Section 7.38.120.B of the County Code: at least 60% of the average annual rainfall must have fallen and at least 6 inches of rainfall must have fallen within the previous 30 days. Environmental Health maintains records of seasonal rainfall accumulations from stations throughout the county and maintains a computer spreadsheet program of accumulated rainfall which allows a prediction of the date at which the 30 day antecedent rainfall will drop below 6 inches. Thus when criteria are met for beginning the winter water table testing season, the date can also be specified when the testing period will close unless substantially more rainfall occurs. During the testing period, the spreadsheet is updated as rainfall occurs to determine if the testing period can be extended.

Environmental Health provides notification that winter water table testing season is open (including a specification of the minimum time that it will be open) in the following ways:

  1. Press release to all the local papers (copies attached).
  2. Copy of press release to all 15 septic consultants and 50 septic contractors.
  3. Notice to all applicants or property owners who have previously been notified that they will need to have winter water table testing before their permit can be approved. A file of these names is maintained by the Land Use Program Manager based on information supplied by the District Specialists.
  4. Direct responses to telephone inquiries about winter water table testing, which become more and more frequent as rainfall accumulates during the season.

If enough additional rainfall occurs to extend the testing period for at least one week, a press release is issued and notice is given to the consultants, who are the ones doing most of the testing. Contractors and property owners who are interested in water table testing are already working with staff and are notified directly through consultations with staff of any extensions of the allowable testing period.

Currently most people tend to wait until the latter part of the testing period to request County staff observation of groundwater levels, in the expectation that the levels will be lower as conditions dry out at the end of the testing period. The testing procedure for alternative systems such as mound systems. is somewhat different in that groundwater observations throughout the winter testing period are required. For such systems the minimum allowable separation from groundwater is only one foot. Because groundwater levels can easily vary by several feet during the water table testing period, it is necessary to measure levels throughout the period to ensure that the very narrow design parameters are maintained. Testing is done by installing several observation wells (piezometers), and logging regular readings. The information is then evaluated to determine the appropriate groundwater level for design purposes. If groundwater separation requirements can be met, system design is completed and the permit is approved.

Example of groundwater table fluctuations and variations in nitrate concentrations from 1988 through 1998 at a monitoring well in downtown Boulder Creek: