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Overview of Wastewater Management

In January 1978, the RWQCB adopted Resolution 78-1, allowing for the continued use of onsite wastewater treatment and disposal systems in Stinson Beach under the management authority of the District. Initially, the District was granted authority for inspections, monitoring and abatement activities; this was later expanded in 1987 to include the authority to establish regulations and issue permits for all existing and new onsite wastewater systems. The current District Wastewater Ordinance was adopted in 1996.

The present study was commissioned to evaluate the water quality impacts of onsite wastewater disposal systems in the rapidly permeable sandspit areas of Stinson Beach, which encompasses the Seadrift, Patios, and Calles areas. Prior to the District assuming management authority for onsite wastewater systems, most of the study area had been developed using conventional gravity septic tank-leachfield systems. This practice continued with new development in the Seadrift area until 1987, when the concern was raised that the groundwater depth in the sandspit area was insufficient to meet the minimum guidelines of the RWQCB. In response, the District adopted the practice of requiring the use of an intermittent sand filter treatment unit ahead of the leachfield to provide a higher and more reliable level of effluent renovation prior to discharge to the sandy soils. The District issued a set of interim design criteria for sand filters and agreed to conduct ongoing monitoring of these systems and their effects on the groundwater quality within the Seadrift area.

Periodic problems were experienced with some of the sand filters, mostly related to equipment failures and sand clogging. This led to modification of the design criteria in 1995, incorporating the use of coarser sand media and shorter, more frequent dosing of the filter bed. These criteria were adopted in the District code in 1996. At this time, it was also brought to the District's attention that the RWQCB (and the new District code) prohibits the use of any onsite wastewater system in areas having a percolation rate faster than one minute per inch. This rapid percolation condition was found to exist throughout the sandspit area, requiring that the District process formal variance requests for any new wastewater systems in these areas. At about the same time, the District intensified their sand filter monitoring efforts to answer questions from the RWQCB about the viability and impacts of these systems in the Seadrift area that previously had not been answered satisfactorily.

Alternative Treatment-Disposal Methods

The existing District Wastewater Ordinance requires the use of an intermittent sand filter ahead of the leachfield for the study area, and includes specific criteria for design and construction of these systems. The District staff has intensively monitored several of the sand filter systems over the past few years documenting the treatment performance, which has generally met all expectations. The results of this monitoring are summarized in several reports, the last of which (Final Report) was completed on June 30, 1997.

Sand Filter Design Criteria. Sand filters are in common usage throughout many areas of California and the U.S., but the design criteria are by no means standardized. The criteria contained in the Stinson Beach Title IV Code are more specific than in most areas. Greater latitude is generally afforded to the system designer in other jurisdictions (e.g., County of Marin). As a matter of comparison, the differences between the Stinson Beach Code and Marin County Regulations are as follows:

  • Media Size. Stinson Beach requires a special coarse sand media that is not available locally. Marin County specifies a medium-coarse sand equivalent to the sand specification that was initially followed for sand filters at Stinson Beach.
  • Dosing. Stinson Beach requires timed dosing; Marin County allows timed or flow-actuated dosing.
  • Distribution System Design. Stinson Beach Code contains very specific design requirements for the pressure distribution system. Marin County provides general parameters for the pressure distribution system design, but leaves many of the specifics to the system designer.
  • Leachfield Loading Rate. Stinson Beach allows sand filter effluent to be discharged to leachfield systems in the study area at a rate of 2.4 gpd/sq ft, roughly double the rate previously permitted for septic tank effluent. Marin County Regulations also allow a doubling of the wastewater application rate under certain conditions. However, in rapidly permeable soils with high groundwater conditions, Marin County actually requires a reduction in the loading rate to spread the wastewater flow and slow migration of wastewater to the groundwater.

The District's sand filter monitoring study has shown good to excellent performance for systems installed under the current sand filter design criteria. With only a few exceptions, the sand filters installed under the former design criteria (finer textured sand and flow-actuated dosing) appear also to be performing satisfactorily. The main purpose of the sand filters is to provide greater pathogen reduction before disposal to the sandy soils. The sand filters are clearly achieving this goal. Additionally, the sand filters are providing measurable reduction in nitrate- nitrogen concentrations in the wastewater effluent as well as high levels of solids and organic (BOD) removal. Our review of the monitoring and performance data revealed no compelling reason to change the current design criteria or to require redesign of the older sand filters to meet the current standards; there is nothing inherently wrong with either set of criteria.

Possible Wastewater System Improvements. Maintaining consistent, standardized design criteria has advantages for initial design review, construction quality control and on-going system maintenance. However, given that on-site wastewater technology is an ever-changing field, the District should remain flexible and open-minded with respect to innovations and adjustments in local design practices. Some of the current possibilities that are not covered in the District's existing code are reviewed below.

Leachfield Wastewater Application Rate. As noted previously, the District code currently allows sand filter effluent to be dosed at 2.4 gpd/sq ft into leachfields in the sand spit, and requires a dual 200 percent leachfield system (per RWQCB requirements). Two leachfield systems are provided to assure that one is always available in reserve in the event that the other field fails. Given the high degree of treatment provided by the sand filters, the possibility for a leachfield to fail is extremely remote. A greater spread of effluent and enhanced treatment in the dune soils could be achieved by utilizing the entire field as a single system, with an effective application rate of 1.2 gpd/sq ft. This would not change the overall amount of leaching area required, but it would increase the amount of treatment received by the effluent during vertical migration through the sands. Given the small amount of leachfield required, the system could be designed (through valving and pump selection) to operate with either half or all of the leachfield "open". This design concept would have the most value in the higher groundwater areas.

Recirculating Sand Filters. The District code requires intermittent sand filters as the standard system in the study area. However, the District has permitted the use of recirculating sand filters in other repair situations. Recirculating sand filters are becoming more and more common for individual residential applications than they have in the past. They have several advantages: (1) they require less land area due to the use of a very coarse sand media and high loading rate (4 to 5 gpd/sq ft); (2) they provide a greater degree of nitrogen removal than intermittent sand filters (generally producing less than 20 mg/l total N); and (3) they are more resilient to fluctuations in wastewater flow rates. The District should consider incorporating criteria and offering recirculating sand filters as a design option throughout the sandspit area.

Aerobic Treatment Systems. There are a variety of proprietary aerobic treatment systems available. Some units produce standard secondary effluent quality, while others produce advanced secondary effluent similar to that from sand filters. Some of these systems are reported to provide higher nitrogen removal than recirculating sand filters. However, aerobic treatment systems are not generally as effective as sand filters in regard to pathogen removal. The main advantage of aerobic treatment units at Stinson Beach would be the ability to produce high quality aerated effluent in very tightly constrained lots. Along with sand filters, such systems have been documented to be very effective in renovating existing failed leachfield systems. The District should entertain the possibility of utilizing aerobic treatment units, beginning first with repair situations. Those systems producing advanced secondary effluent quality should be given primary consideration.

Disinfection. Simple UV (ultraviolet) disinfection units are now commercially available for incorporation in residential wastewater systems (Cruver, 1997). Such units could be incorporated following sand filter (or other secondary+) treatment. There is no demonstrated need to incorporate disinfection units as a routine requirement. Nonetheless, their use may be justified in connection with repairs or waiver situations to overcome high groundwater constraints. Disinfection would also be required in connection with the use of subsurface drip irrigation as a disposal option (see below).

Subsurface Drip Irrigation. Subsurface drip irrigation has recently been approved for a residential application in Stinson Beach and should be given further consideration in other cases. The best opportunity for its use may be in the study area where the soils are very uniform. The wide-spread, low-rate distribution of the treated effluent through drip tubing would eliminate the concentrated, near-saturated flow of effluent through leaching beds. Other advantages of drip irrigation are as follows:

  • It is resistant to root intrusion, which is a common problem in the sandy dune soils.
  • The wastewater flow can supply some of the landscaping irrigation needs.
  • As compared with normal leachfields, additional nitrogen removal is obtained through plant uptake.

One possible action for the District to take at this time would be to permit sub-surface drip irrigation to be installed as a substitute for the second half of the normal dual leachfield system in the sand spit area; the second leachfield is largely redundant and this would offer an opportunity for cost savings as well as valuable experience with the application of this technology in a situation where it seems ideally suited.

Onsite System Monitoring

Time and money limits the amount of monitoring of on-site systems that can be performed by the District staff. Therefore, it is important that the monitoring that is performed be performed in an efficient manner and that the District make use of all available resources and technology. There are two major options to expand the amount of data that are available to the District.

Data Recording Control Panels. The standard pump control panel has an event counter and an elapsed time meter, which gives useful but limited data. Microprocessor-based control panels are available (at a competitive price) that provide full data-logging capabilities, as well as remote access via modem. Such systems are being developed specifically for application in the onsite wastewater industry and could greatly expand the ability to monitor and diagnose system problems. Conversion to this type of control panel (with or without modem access) should be considered for new installations and commercial system repairs.

Maintenance/Pump-out Reporting. Septic tank maintenance work and pump-outs are performed periodically on systems located within the District but the performance of and observations during the work is not necessarily reported to the District. Oftentimes the maintenance people will observe conditions that should be recorded and may be important for future preventative action. Some counties have expanded the reporting requirements for septic tank pumpers, requiring that they complete a separate form covering key observations during their work. The District may want to consider establishing a similar practice by providing the specific inspection forms to be completed and instructions to the contractors who work in the area.

In terms of the actual monitoring activity done by the District, the recent emphasis on sand filter performance monitoring can be scaled back or eliminated; the data collected has verified the adequacy of these systems. Future water quality monitoring should be reserved for spot checks on problem systems and greater attention to groundwater monitoring. New sand filter installations should continue to provide for the installation of onsite monitoring wells, but it may be more beneficial for the District to retain the option of directing the system owner to install the monitoring well in a strategic off-lot location to expand the network of groundwater monitoring wells throughout the study area.

STEP System Alternative

It is clear from the monitoring data collected in this study and from past water quality sampling that wastewater disposal in the Old Town and Highlands portion of Stinson Beach continue to have an adverse impact on Easkoot Creek and Bolinas Lagoon. Management of onsite sewage disposal systems in these areas is made difficult by clayey soils, high groundwater, steep slopes, limited land area and other constraints. Many of these properties, especially in the Old Town area, could benefit greatly from a community collection/treatment/disposal system that provides for offsite disposal in a location having more suitable conditions. Some properties in the Patios and Calles, including the District office itself, are operated on holding tanks or other marginal systems and could also benefit from access to a community wastewater system.

Part of this study was devoted to a preliminary examination of the potential feasibility of a community system for these portions of Stinson Beach. The concept explored was based upon the use of a STEP (Septic Tank Effluent Pump) collection system. In this type of system the septic tanks at each property would be maintained for primary wastewater treatment, and only the effluent would be collected (by gravity or by a small pump unit) in a network of small diameter pipes (typically 2 to 4-inch diameter) and lift stations, as needed, leading to a central point for treatment and then disposal in one or more locations. This type of system contrasts with a conventional sewer system in which larger sewer pipes (6 to 8-inch diameter) are used to collect all liquid and solids, eliminating the need for on-lot septic tanks. The STEP system concept makes sense for Stinson Beach due to the existence of septic tanks at all (or nearly all) properties that might be served by such a system. It has other important advantages including the following:

  • The use of small diameter pipes minimizes construction costs and disruption during installation.
  • It can be conformed more easily to hilly terrain since it is designed and installed for conveyance of liquids only, much the same as water distribution piping.
  • Lift station and treatment facilities are less costly and less complex due to the elimination of solids handling.
  • Infiltration and inflow is negligible due to the watertight joints and the elimination of manholes in favor of clean-outs along the pipeline route.

Although the collection system is generally one of the most significant costs of a community wastewater system, its feasibility is rarely in question. The key issue is the method of treatment and final disposal. This is a particularly difficult problem for Stinson Beach because of the existing level of development, the geography and terrain, and the limited availability of suitable land for disposal of large volumes of wastewater. A review of maps along with field reconnaissance was completed to identify potential disposal sites. The following were initially identified:

  1. GGNRA open space lands in the vicinity of the Highlands Water Tank.
  2. GGNRA open space lands south and east of Panoramic Highway
  3. Open space land within Seadrift
  4. Beach dunes

Of these potential alternatives, only the first is considered reasonably viable and worthy of further investigation. The other options represent possible sites having sufficient land area for a community wastewater disposal system; however, there are likely to be overwhelming environmental constraints that would preclude their use for this purpose. The Highlands area, on the other hand, appears to have the most desirable physical capabilities and the least environmental constraints.

The Highlands site is indicated on Figure 25. The area of interest encompasses approximately eight acres of a gently sloping (10 to 15 percent) colluvial fan immediately west of the Highlands Water Tank. The published USDA Soil Survey identifies the soils to be gravelly loams of the Cronkhite-Barnabe complex. These soils are reported to be moderately well drained, which is substantiated by field observations. No soil profile investigation was included within this preliminary evaluation, but the area was walked and surface soils were examined and appeared consistent with the Soil Survey description. A recent geotechnical investigation of the Highlands Tank site by Kleinfelder (March 1997) revealed deep colluvium (old landslide debris), in excess of 30 feet deep, with groundwater encountered at a depth of 23.5 feet. The groundwater level is probably closer to the surface in the identified potential disposal area, which is about 50 feet lower in elevation than the tank site. The site is vegetated with annual grasses and scattered brush. There are no defined water courses in the area, which suggests very well drained soil conditions. Drainage from the area eventually emerges in a wetland-willow thicket before concentrating in a small drainage channel that drops down to Highway 1 and Bolinas Lagoon, just north of the District office.

From available information, the Highlands site appears to be very suitable for wastewater disposal. However, further confirmation would be needed in the form of soil profile inspection, groundwater observations and percolation testing. The field studies would also be needed to establish the disposal capacity that could be provided at the site and the degree of wastewater treatment needed preceding disposal (i.e., primary or secondary treatment). As a rough preliminary guide, deep well drained soils as are expected at this site can generally support wastewater loading rates of 5,000 to 10,000 gpd per acre of septic tank effluent. This potentially could be increased with the incorporation of sand filter or equivalent secondary treatment. Thus, if soil and groundwater conditions are consistent with preliminary expectations, the 8-acre area identified may have potential disposal capacity for 40,000 to 80,000 gpd, or more. Based on a conservative unit wastewater flow of 200 gpd/dwelling, this would equate to a capacity for 200 to 400 dwelling units (or the equivalent). This should be more than sufficient to accommodate the critical Old Town area of Stinson Beach as well as other selected problem lots from the Highlands, Calles and Patios.

The Highlands site is considerably higher in elevation than the Old Town area, which will require significant pumping. The disposal site elevation is approximately 350 feet above sea level; therefore, a minimum of two lift stations would be required. Although a variety of routes are possible, the most likely routing of a force main from the Old Town to the site would appear to be via Calle del Mar, Buena Vista, Lincoln, and Avenida Farralone. One lift station would be located in the Old Town area and could collect flow by gravity or pumping (e.g., a STEP collection line from the Calles area). Additionally, an intermediate booster lift station would be located along the force main route and could also be used to collect flow from selected properties in the Highlands.

An alternative approach might be to locate a central lift station in the vicinity of the District office, with the force main following an overland route on the west side of the Highlands. Finding a suitable overland route is likely to be difficult, but there may be cost savings or other advantages that warrant consideration of this piping alternative.

Technically, the above-described conceptual plan for a community wastewater system for portions of Stinson Beach appears very feasible. The major obstacle is gaining the concurrence of the Park Service to permit the identified Highlands site to be used as a wastewater disposal area. The potential environmental impacts of wastewater disposal in the identified area are likely to be minimal in comparison with existing impacts within the community and Bolinas Lagoon from the marginal septic systems functioning in portions of the town. Additionally, it is conceivable that the development of this community wastewater site could also provide capacity to serve the Stinson Beach Park rest rooms, which currently discharge septic tank effluent to absorption beds in the sand dunes. If the District is committed to pursuing a long-term permanent wastewater solution for the Old Town and other problem areas, the next step should be to initiate discussions with the Park Service about the concept outlined here. Enlisting the support of the Regional Water Quality Control Board, other regulatory agencies, environmental groups and local legislative representatives should also be considered due to the general resistance and controversy that typically accompanies any proposal to utilize Park land for public improvements such as this.

Water Resource Monitoring

This Hydrologic Survey was focused on Seadrift and the Calles and Patios. In addition, the survey was planned to extend through the summer and into late autumn 1997. Findings of the survey indicate that wastewater impacts to Easkoot Creek are significant, and that additional hydrogeologic work, examination of wastewater systems, and water quality monitoring should be conducted in the Old Town, Highlands, and Panoramic areas that contribute to the creek. In addition, the water levels and quality monitoring should be extended through the wet season. Accordingly, a second phase of the Hydrologic Survey is warranted to establish the hydrogeologic context for wastewater disposal in the Old Town and Highlands area, to evaluate impacts on water quality, and to develop an overall program for water monitoring and protection and wastewater management that will include all of Stinson Beach. Planned groundwater exploration for supplemental District water supply wells could be conducted concurrently, thereby providing a more comprehensive water resources management plan.

The second phase should include installation of additional monitoring wells aligned in one or more transects through eastern Stinson Beach, with the possibility of siting some wells to tie into the existing Calle del Resaca and Seadrift transects. No more than ten wells are envisioned at this time. The installation and logging of the new monitoring wells, consistent with the procedures established in this study, would allow development of additional geologic cross sections and accompanying geologic analysis. One or more relatively deep wells are recommended to document the depth to bedrock. The wells would be sited, designed, and installed to provide reliable, year-round, long-term data collection. Each well would be identified on maps and fully documented in terms of well construction, geology, and wellhead elevation, allowing meaningful analysis of water level and water quality data. A pumping test also would be useful in determining aquifer characteristics.

Phase 2, possibly extending from January or February 1998 through the summer, would include not only hydrogeologic studies, but also continued monitoring and analysis. The water level and water quality monitoring of the nine existing monitoring wells should be continued in order to document winter and spring conditions. In addition, the monitoring should be extended to the new wells. During the second phase, water levels should be taken monthly in the nine existing wells, ten new wells, and existing staff gages. Subsequently, water level measurements probably would be reduced to four times a year. Water level measurements also would be taken from existing piezometers (e.g., G4) to continue documentation of seasonal trends in water levels.

One or more synoptic surveys of Easkoot Creek, involving a series of streamflow measurements at gage sites along the creek, should be conducted to assess the relationship between streamflow and groundwater. These data will be useful in assessing the existing stream stations (S1 through S5).

Water quality sampling also should be continued in the existing monitoring wells and surface water stations, and extended possibly to additional surface water stations along Easkoot Creek. Water levels should be taken concurrently with water quality samples to determine groundwater level and flow conditions at time of sampling. Water quality procedures used in this study, and involving pumping of the wells to extract samples, should be applied. Three sampling events probably would be sufficient. The water quality analyses for the new wells should include the same suite of general mineral constituents, physical parameters, nitrogen species, and bacteriological examination as in the first phase. The analyses for existing monitoring wells probably would be reduced to key parameters (e.g., specific conductance, nitrogen, MBAS, coliform). In the future, complete analysis probably would be warranted only periodically, while sampling and analysis of key parameters would occur on a quarterly basis.

The water levels and quality monitoring programs should include prompt compilation of data into spreadsheets and analysis through means such as plotting of hydrographs or plotting of quality data on cross sections or maps. This would ensure that any errors or problems in data are detected and remedied quickly.

The second phase of the hydrologic survey would be summarized in a final report, presenting all data, analyses, findings and recommendations. Thereafter, the water quality monitoring program could be summarized in an annual report that would present the data and analysis to all interested parties. A brief mid-annual report would keep the District and RWQCB informed promptly of any changed conditions.

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