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Beneficial Uses of Groundwater

The Stinson Beach County Water District was established not only to provide water and wastewater management, but also to prevent water contamination and improve water quality. The District cooperates and is guided by the California Regional Water Quality Control Board, San Francisco Bay Region, whose primary planning and policy document is the Water Quality Control Plan for the region (RWQCB, June 1995). This plan includes definition of beneficial uses of water resources, including surface waters, groundwater, and wetlands.

Beneficial uses are defined for the Bolinas Bay basin, of which Stinson Beach is a part, and specifically for Bolinas Lagoon, which include the following:

  • Ocean, commercial, and sport fishing
  • Marine habitat
  • Fish migration
  • Preservation of rare and endangered species
  • Water contact recreation
  • Noncontact water recreation
  • Shellfish harvesting
  • Fish spawning
  • Wildlife habitat

The beneficial uses of the Bolinas Lagoon wetland areas also are identified, including marine habitat, contact and noncontact water recreation, wildlife habitat. Although the Stinson Beach area is not identified as a groundwater basin, the use of local groundwater for municipal and domestic supply is a beneficial use.

Based on the water quality study sponsored by the District in 1976, water quality criteria were established for the surface water and groundwater quality stations. As noted in the District's Fifteenth Annual Report for the OWMP, the criteria have been revised subsequently.

In brief, the criteria for the surface water stations along Easkoot Creek and in Seadrift Lagoon refer to contact recreation. For the nontidal stations of the creek (S1, S2, S3, and S4 shown in Figure 3) fecal coliform concentrations shall not exceed 400 Most Probable Number (MPN)/100 milliliters (ml). For the tidal stations S5, S6, and S7, the total coliform shall not exceed 10,000 MPN/100 ml and the fecal coliform concentrations shall not exceed 400 MPN/100 ml (Stinson Beach County Water District, Fifteenth Annual Report). It should be noted that the RWQCB water quality objectives for fecal coliform bacteria with regard to shellfish harvesting call for median concentrations less than or equal to 14 MPN/100 ml and 90th percentile counts of 43 MPN/100 ml or less. The objectives for total coliform similarly are 70 and 230 MPN/100 ml. Federal drinking water standards include maximum permissible limits that disallow more than ten percent of standard 10-ml samples in one month showing coliform (Todd, 1980).

For Stinson Beach surface water and groundwater stations, the criteria also state that concentrations of selected constituents shall not cause nuisances or adversely affect beneficial uses.

The selected constituents and criteria are:

Nitrate + Nitrite (mg/L as Nitrogen) 10 mg/L
Ammonia (mg/L as Nitrogen) Not to exceed levels observed during the 1976 study
(averaging 0.30 mg/L)
MBAS 0.05 mg/L threshold concentration
0.5 mg/L limiting concentration.

The ammonia limits were established with reference to specific monitoring wells (e.g., G1, G2), some of which no longer exist, so the applicability of these limits to current conditions is debatable. The 0.30 milligrams per liter (mg/L) criteria is very close to the detection limit of 0.20 mg/L used in this study.

Groundwater Quality

Major factors affecting groundwater quality include the relative quantity and quality of recharge sources, the chemical characteristics of the aquifer materials, and the amount of time and distance that groundwater flows through the aquifer. In Stinson Beach, the last two factors are relatively constant, while the first factor, the character of groundwater recharge is variable through time and from place to place. Major sources of recharge are rainfall infiltration, stream recharge, landscape irrigation percolation, septic return flow, and seawater intrusion.

Water quality sampling and analysis for the Hydrologic Survey included analysis of general mineral quality in order to understand background or existing groundwater quality in Stinson Beach. The laboratory analyses, presented in Appendix C, included major cations (calcium, magnesium, potassium, sodium), major anions (bicarbonate, sulfate, chloride), hardness, alkalinity, pH, total dissolved solids (TDS), electric conductivity (EC), and turbidity. Appendix C also shows selected analyses for comparison, including standard mean ocean water and analyses from the District's Aldergrove 2 and the Beach Park wells.

Review of these analyses shows that ambient groundwater quality in Stinson Beach includes a calcium/magnesium/sodium bicarbonate type that is characteristic of Aldergrove 2 Well, Beach Park, and MW-6, which is screened in bedrock. These analyses show TDS concentrations of 180 to 250 mg/L, which meet California drinking water standards for TDS.

Local groundwater also includes a type that is dominated by seawater. This type is best exemplified by MW-2, MW-3, and MW-4 near Seadrift Lagoon. These wells are characterized by sodium chloride groundwater with TDS concentrations ranging from 1,000 to over 30,000 mg/L. This water is not suitable for drinking water purposes.

General groundwater quality in the remaining monitoring wells ranges between these two types, reflecting varying degrees of mixing between seawater and recharge from rain, streamflow, irrigation, and septic systems.

Figure 13 shows the measured TDS concentrations in the monitoring wells across Seadrift for the three sampling events on September 3, September 30-October 1, and October 21, 1997. The lagoons were not sampled on September 3 because the surface water stations had not yet been established. Because groundwater flow directions are an important factor in determining groundwater quality at Seadrift, the TDS data are displayed with reference to the three groundwater flow patterns described in the previous section on Groundwater Levels and Flow. However, it should be noted that the groundwater flow patterns were not documented at time of sampling. For example, for the uppermost cross section on Figure 13, groundwater quality was sampled on September 3, while groundwater levels were measured on September 9. Accordingly, the specific relationships between groundwater flow and groundwater quality cannot be considered definitive at this time. Nonetheless, general relationships can be discerned and warrant discussion.

The uppermost cross section showing September 3 results indicates a broad range of TDS from 290 to 30,000. Highest TDS levels occur next to Seadrift Lagoon, reflecting seawater mixing, particularly in MW-3. Lower TDS groundwater reflects recharge from rainfall, irrigation, and septic returns.

The TDS concentrations in the center cross section (September 30-October 1 sampling) are higher than in the previous sampling. This reflects in part a change in the sampling procedure; the first round of samples was collected through bailing the wells, while the second and third rounds were accomplished through pumping the wells. Pumping wells removes a greater volume of water from the aquifer and allows sampling of groundwater that represents a larger area. The higher TDS levels in the both the second and third rounds shows greater influence of seawater, indicating that fresh water exists locally as only a thin lens floating above denser seawater. Seawater is 2.5 percent denser than fresh water and naturally tends to maintain a vertical stratification of water types with a small mixing zone. Infiltration from rainfall and septic systems recharge the upper fresh zone, while brackish water persists with depth.

The convergent groundwater flow pattern shown in the center cross section also would contribute a larger portion of seawater, as indicated by the high TDS contents in MW-1 and MW- 2 on the Dipsea side. The TDS concentrations for the October 21 sampling could be representative of the divergent groundwater flow pattern shown in the lowermost cross section or of the through-flow pattern in the uppermost section.

Figure 14 shows TDS concentrations measured along Calle del Resaca for the three sampling events with reference to the two main groundwater flow patterns described in the previous section on Groundwater Levels and Flow. The uppermost cross section shows the Highlands-to-ocean pattern that prevailed before September 15. The TDS concentrations range from a low of 230 mg/L in MW-6 to a high of 710 mg/L in MW-7. The low TDS content in MW-6 reflects groundwater inflow from the bedrock Highlands, while the high TDS in MW-7 likely reflects inflow of brackish water from the tidally-influenced Easkoot Creek. The TDS decreases downgradient toward the ocean, reflecting recharge of relatively fresh water from rainfall, irrigation, and septic returns.

The TDS concentrations in the lower portion of Figure 14 are considerably higher, with the exception of groundwater in MW-6, the bedrock well. As before, the higher TDS in the second and third rounds reflect the pumping procedure for sampling, which pulled in a greater portion of underlying seawater than did the bailing procedure used on September 3. This effect is particularly pronounced in MW-9, located closest to the ocean.

In summation, available data on general mineral groundwater quality are relatively scanty, but nonetheless are revealing about major factors influencing groundwater quality. Groundwater quality in Stinson Beach is influenced by the source and character of groundwater recharge, including recharge from rainfall, streamflow, landscape irrigation, and septic return flow, as well as seawater intrusion. The particular sources affecting groundwater quality at any particular time and place is determined by groundwater flow patterns.

Impacts of Wastewater on Water Quality

Impacts of wastewater on groundwater and associated surface water quality were assessed through analysis of the current water quality sampling data generated during this study, review of existing water quality data from previous investigations and monitoring programs. The next section summarizes preparation of nitrate balances.

Current Water Quality Sampling. The water quality sampling conducted during the Hydrologic Survey focused on the nine new monitoring wells (MW-1 to MW-9) and associated surface water gages. The locations of the monitoring well transects were selected to provide a cross section through the Seadrift and Calles areas and thereby to illuminate groundwater flow conditions. Another factor in selecting the monitoring well sites was access to property; wells were sited on public property and rights-of-way when possible, or on vacant private property. The wells were not sited with regard to existing septic systems. Subsequently, it was determined that the Seadrift transect is in an area of standard gravity septic systems, as shown in Figure 15, and not sand filter systems. Similarly, the Calle del Resaca wells, situated along or within the calle itself, are bracketed by standard gravity leachfields on both sides of the calle (Figure 16). Thus, the monitoring results from these areas would tend to reflect the worst-case impact of septic systems in the dune sands.

The water quality sampling and analysis for this study included documentation of several indicators of wastewater influence including nitrogen concentrations (nitrate, ammonia, and total Kjeldahl nitrogen), total and fecal coliform counts, and MBAS, reflecting the presence of foaming agents or detergents.

Nitrogen. With regard to nitrogen, ammonia was selected for particular analysis because of its presence in many sampled wells. Nitrate was very rarely detected in the monitoring wells with the exception of the bedrock well, MW-6, which showed consistent nitrate concentrations of about 1.6 mg/L. Nitrate also was detected in Easkoot Creek at concentrations of 0.2 to 0.7 mg/L. Total Kjeldahl nitrogen was detected in many wells in concentrations paralleling ammonia concentrations, but at a higher level.

Figure 17 shows ammonia concentrations from the three sampling rounds with reference to the groundwater flow patterns described previously for Seadrift. As with the discussion on TDS, it needs to be reiterated that groundwater flow patterns were not documented at time of sampling, so the factors affecting groundwater quality can be discussed only in general terms.

As shown in all three sections on Figure 17, ammonia concentrations are very low or below detection (not detected, nd) on the Seadrift side. On the Dipsea side of the sandspit, concentrations are generally higher, ranging from less than one to seven mg/L. The most notable feature of the ammonia distribution on the Dipsea side is the decrease in concentrations along the down-gradient groundwater flow direction. This pattern is most clearly shown on the uppermost cross section, but may also exist for the convergent and divergent flow patterns. This suggests that ammonia is reduced by subsurface processes to concentrations less than one mg/L prior to entering adjacent lagoons.

Figure 18 similarly shows ammonia concentrations from the three sampling rounds with reference to the two groundwater flow patterns identified for the Calles. The uppermost graph, showing the through-flow pattern of groundwater flow toward the ocean, indicates a general decrease in ammonia concentrations downgradient from MW-7. The contribution of ammonia to groundwater is apparently greatest in the vicinity of MW-7, possibly reflecting the local shallow depths to groundwater and/or the operating condition of nearby septic systems.

The lower cross section shows the same pattern of decreasing concentrations toward the ocean, despite the changed groundwater flow condition. The samples for the latter two rounds were pumped rather than bailed, and the higher ammonia concentrations shown on the lower cross section indicates that pumping resulted in interception of wastewater-influenced from one or more septic sources.

With regard to water quality criteria established for the groundwater stations in the 1976 Stinson Beach study, the concentrations of ammonia documented in this study generally exceed the 0.30 mg/L criteria. The surface water samples analyzed for this study showed no ammonia.

Coliform. Figure 19 shows total and fecal coliform concentrations with reference to the groundwater flow patterns in the Seadrift area. As shown in all three sections on Figure 19, total coliform counts are at or below the level of detection on the Seadrift Road side, and no fecal coliform were detected. On the Dipsea Road side, fecal coliform were detected only once. Total coliform were detected consistently in MW-2, and in all three Dipsea wells during the first sampling round. The results of the first sampling round may reflect insufficient well purging and may not be representative of aquifer conditions, given that coliform were not detected subsequently in MW-1 and MW-3 and greatly reduced in MW-2. This is supported by turbidity measurements, which typically were highest in the first sampling and have declined consistently since then. Alternatively, the decline over time in coliform detections could reflect changing groundwater flow patterns or the operation of nearby septic systems.

The lagoons were sampled only in the last two sampling events, showing coliform at or below the level of detection in Seadrift Lagoon, and total coliform in Bolinas Lagoon at levels of 79 MPN/100 ml. Fecal coliform also were detected in Bolinas Lagoon during the second sampling.

Figure 20 shows total and fecal coliform counts in the monitoring wells along Calle del Resaca. The number of detections were greater in the first sampling than subsequent sampling, when no wells indicated coliform detections except MW-7, which shows rapidly declining counts of both total and fecal coliform over the sampling period. As noted in the Seadrift area, this could reflect progressive purging or cleansing of the well.

Easkoot Creek was sampled during the second and third sampling rounds; both times, total and fecal coliform counts of 1600+ and 920 MPN/100 ml were recorded.

MBAS. MBAS also was analyzed in all the monitoring wells for three sampling events and in the lagoons and Easkoot Creek for the last two sampling events. The sampling indicate that all wells except MW-1 on Bolinas Lagoon and MW-6, the bedrock well, show concentrations of MBAS in the range of 0.05 to 0.35 mg/L. The surface water samples also showed MBAS in the range of 0.05 to 0.12 mg/L. Similar to the other parameters, MBAS concentrations tend to decline along the groundwater flow pathways. Overall, the concentrations fall between the limiting and threshold criteria of 0.05 and 0.5 mg/L set in 1976 for Stinson Beach.

Review of Existing Water Quality Data. The District has initiated several water quality investigations, including the 1975-1976 Surface Water and Groundwater Study (Eutek, April 1977). This study first documented that wastewater disposal was influencing groundwater, and established seven surface water stations and twelve groundwater stations (G1-G3, G5-G13) to monitor local water quality.

Subsequently, the groundwater and surface water stations established in the 1976 study became the basis for the District's routine water sampling program, which has continued to the present. The most recent ten years (1986-1996) of data from this routine sampling program have been compiled by the District in its Summary of Groundwater Studies (1996) and updated with information from the 1997 program. The sampled wells included G1 through G10; however, the sampling period for each well varies as wells were added to or deleted from the program. The number of sampling events also varied as individual wells at times could not be sampled because they were dry or contained too little water for sampling. Analyses were performed for ammonia, nitrate and nitrite, MBAS, and total and fecal coliform; detection limits for the constituents varied over the period.

Review of the water quality data indicates considerable variability, some of which can be explained by changing conditions in nearby septic systems. In addition, the variable water quality results probably reflect the changing groundwater flow conditions documented in this study. However, at the time, such conditions were not documented. In addition, it is also likely in some cases that the analytical results do not accurately represent groundwater quality in the aquifer. Most notably, G1 is located above all residential developments and should reflect background quality without influence of wastewater. The lack of any detections of ammonia, nitrate, or MBAS over the well's 10-year sampling period support this assumption. Nonetheless, fecal coliform was detected in 51 percent of samples, suggesting contamination of the well itself and/or sampling problems.

In brief, the routine water quality data were not relied upon for this study. This is due to the lack of well construction and geologic information for the wells, absence of information on hydrologic conditions, and uncertainty over well conditions and sampling procedures.

In 1987, the District conducted an investigation of wastewater impacts on groundwater quality at Seadrift (see Stinson Beach CWD, Summary of Groundwater Studies, 1996). This investigation consisted of two parts, monthly sampling of ten selected wells and Seadrift Lagoon over the period January through October 1987, and an intensive one day survey on March 16, 1987 that included two rounds of water level measurements in the wells and Bolinas Lagoon and water quality sampling of the wells.

The 1987 study included selection of a number of existing wells for sampling, including ten wells for the monthly sampling, and nineteen wells for the March 16 survey. The wells were selected for a variety of land use and wastewater disposal intensities and grouped in five distinct areas A through E, which are shown on Figure 2. As indicated, Area A consisted of a grid of five wells (G5, G31-G34), while the wells in the other areas consisted of alignments of three to four wells. It was recognized that these alignments were similar in concept to the monitoring well transects developed for this study, and would allow extension of this study's analysis to additional cross sections along Seadrift. Accordingly, the available topographic, well location, water level, and water quality data from the 1987 study were used to develop three additional cross sections through Seadrift. The location of these cross sections (C-C', D-D', and E-E') are shown on Figure 5.

Figure 21 shows cross section C-C' through Area B, showing groundwater levels and water quality on the morning of March 16, 1987. The well depths and construction are unknown, as indicated by the query at the bottom of each well. Area B is located 700 feet east of and is similar to the Seadrift Road portion of cross section A-A' (see Figures 13, 17, and 19). As shown in Figure 21, a low groundwater mound is indicated, with divergent flow to the ocean and apparently to Seadrift Lagoon, which apparently was not measured for water levels at the time. A typical water level of about 2.5 feet MSL was assumed, consistent with current levels and descriptions of outfall facilities (Questa, 1994). TDS concentrations are lower than those measured in 1997, probably reflecting seasonal effects and sampling procedures in 1987. It is likely that the sampling skimmed relatively freshwater from the water table that had been recharged during the preceding wet months. The water quality from Well G39 shows the influence of nearby septic systems, but it is noteworthy that the groundwater quality improves in a downgradient direction.

Figure 22 combines the transects through Areas C and E to create cross section D-D', located about 1,200 feet west of cross section A-A'. The groundwater levels and flow apparently are similar to those measured along cross section A-A' on September 3, 1997, with a broad groundwater mound on the Seadrift Road side. On the Dipsea Road side, groundwater may have been slightly mounded with divergent flow to both lagoons or there may have been through-flow from Seadrift Lagoon to Bolinas Lagoon. Bolinas Lagoon experienced a rising tide on the morning of March 16, 1987, but little response in wells was recorded. As in cross section C-C', TDS levels are relatively low across the Seadrift Road side, but clearly show the influence of seawater on the Dipsea side, particularly adjacent to Bolinas Lagoon. Little influence of wastewater disposal is indicated, with the exception of nitrate.

Figure 23 shows cross section E-E' through Area D, located about 2,700 feet west of cross section A-A'. This section also shows apparent groundwater through-flow from Seadrift Lagoon to Bolinas Lagoon on the Dipsea side. TDS concentrations are elevated, particularly reflecting the seawater influence of Seadrift Lagoon. Again, little influence of wastewater disposal is indicated, with the exception of nitrate.

In summation, the reinterpreted results of the March 16, 1987 study are consistent with the findings of the 1997 Hydrologic Survey in terms of groundwater flow patterns and effects of seawater intrusion. In addition, both the 1987 and 1997 studies detected the localized influence of wastewater on groundwater quality, but suggest that groundwater quality often improves along the groundwater flow pathway.

Nitrate Loading Analysis

Although no clear geographical or temporal trends are apparent, high concentrations of nitrogen have been detected by the District from time-to-time at some of the groundwater monitoring locations in the Seadrift, Patios, and Calles areas. For example, review of the results of the routine groundwater sampling program between 1986 and 1997 indicated that ammonia concentrations exceeding 0.3 mg/L occurred periodically in G3 located near Calle Occidente and Easkoot Creek, and in G8 on Dipsea Road, while high nitrate concentrations occurred regularly in samples from G5 on Seadrift Road, particularly in summer and autumn. Additionally, the focused sampling of the sand filter treatment units over the past few years has confirmed what was generally expected, i.e., that high levels of nitrogen are contained in the septic tank effluent and, to a lesser degree, the sand filter effluent that is being discharged to leachfield systems in the sandspit areas. There has also been a concern that nitrate may be reaching the groundwater as a result of fertilizer applications to the very sandy, excessively-drained dune soils. The effect of these nitrogen sources on local water quality and beneficial uses may not be adequately addressed by current practices and this is of concern to the District.

To address these concerns a nitrate loading analysis was completed to supplement and expand the understanding of the water quality data obtained through the field sampling program. The specific purpose of the nitrate loading analysis was to estimate the contributions of nitrogen from different sources (e.g., wastewater, fertilizer, natural sources), under different scenarios of wastewater treatment (with and without sand filters), and for different levels of development (e.g, current and future conditions). From this the projected long-term effects on groundwater nitrate concentrations were estimated.

Approach. The methodology employed a water-chemical mass balance following the general approach outlined by Hantzsche and Finnemore (1992). The specific approach and assumptions for this study was as follows.

Define Hydrologic Sub-areas. The first step was to divide the study area into sub-areas, based on groundwater conditions. Using the information developed from this study and described in Section 4 of this report, five sub-areas were defined as follows and mapped on Figure 24.

Calles. The Calles sub-area encompasses 30 acres in which there are 99 developed residential lots. Groundwater flow in this area is generally toward the ocean for most of the year, and includes a lateral groundwater flow from the hillside bedrock. The sub-area extends from Easkoot Creek to the ocean. This sub-area was assumed to be fully developed.
Patios. The Patios sub-area is defined as the area on the inland side of the dune line encompassing 20 acres and 83 developed residential lots. The groundwater flow is predominantly toward Bolinas Lagoon from a groundwater ridge that coincides roughly with the dune line. This sub-area was assumed to be fully developed.
Seadrift Road. The Seadrift Road sub-area encompasses 60 acres in which most of the groundwater flow is northerly into Seadrift Lagoon. It is recognized that a portion of groundwater flow in this area is toward the ocean (see Figure 11). However, most of the septic systems are located east of the groundwater divide in the area of flow toward Seadrift Lagoon. There are presently 146 developed residential lots, and an estimated 180 total lots that may be developed at build-out.
Dipsea Road. The Dipsea Road sub-area includes the area between Seadrift Lagoon and Bolinas Lagoon. It has an area of approximately 47 acres, with 84 developed residential lots and an estimated 100 total lots at buildout. The groundwater flow is generally from Seadrift Lagoon toward Bolinas Lagoon, including lateral flow contributed by Seadrift Lagoon.
Seadrift Westend. The west end of Seadrift beyond the Lagoon, encompasses 10 acres between the dune line and Bolinas Lagoon, where the groundwater flow is predominantly toward Bolinas Lagoon. There are presently 40 developed residential lots and an estimated 50 total lots at build-out.

The above-noted build-out estimates are approximations, based on a drive-through lot count along with information supplied by the District (i.e., service connections) and Seadrift Association.

Construct Water Balance for each Sub-area. The water balance for each sub-area was constructed using the following assumptions:

Wastewater Flow (W). Examination of District water use records for a winter and a summer billing period were used as a basis for the following estimate of daily and annual wastewater flow rates:
  • Calles and Patios: 145 gallons per day (gpd) [0.16 acre-feet per year (ac-ft/yr)]
  • Seadrift Sub-areas: 175 gpd [0.2 ac-ft/yr]
Based on these unit flow estimates and previously noted lot counts, the projected daily wastewater flow for each of the sub-areas at build-out would be:
  • Calles - 99 units @ 145 gpd
  • Patios - 83 units @ 145 gpd
  • Seadrift Road - 180 units @ 175 gpd
  • Dipsea Road - 100 units @ 175 gpd
  • Seadrift West End - 50 units @ 175 gpd
Rainfall. Average annual rainfall for Stinson Beach is 28 inches, based on the last 20 years of record from data taken at Stinson Beach Park (compiled by Bonnie Jones). However, over the past three years rainfall has averaged about 43 inches per year, which is likely to be reflected in the current monitoring results and represents "wet" conditions.
Runoff Rate. Runoff of rainfall is small to negligible due to the high permeability of the dune sands. Accordingly, an annual runoff rate of 2 percent was assumed for this study.
Evapotranspiration. An actual evapotranspiration rate of 6 inches per year was assumed due to the excessively drained dune sands and generally sparse vegetation. This is about half the normal rate that would apply to well developed upland soils in the same climatic region.
Net Rainfall Recharge (R). From the preceding assumptions, the net annual rainfall- recharge to each of the sub-areas was determined to be 21.5 inches (per unit area) for average rainfall conditions and 36.5 inches for the recent "wet" year conditions.
Lateral Groundwater Inflow (G). For the Calles and Dipsea Road sub-areas, a lateral flow component was incorporated in the water balance. For the Calles, the annual lateral flow from the hillside bedrock groundwater was estimated to be approximately 100 ac- ft/year; for the Dipsea Road sub-area the annual lateral dilution flow from Seadrift Lagoon was estimated to be roughly 80 ac-ft/year.

Wastewater Nitrogen Content (Nw). Based on locally collected monitoring data from the District along with published literature values, the concentration of total nitrogen (i.e., nitrate, ammonia, and organic nitrogen) was estimated to be 60 mg/l in septic tank effluent and 30 mg/l in sand filter effluent. The District's monitoring data included sampling of nitrate and ammonia concentrations in septic tank effluent and sand filter effluent for several systems in the Seadrift area that were in regular use. Note that the actual occupancy and water use/wastewater generation was not determined. The assumption was then made that all nitrogen will be converted to nitrate (i.e., nitrification via oxidation processes) as the effluent moves through the leachfield, sandy soils and groundwater. Based on District records, it was further estimated that approximately 36 percent of the septic systems in the Seadrift area presently have sand filter systems; while there are virtually none in the Calles or Patios.

Estimate Soil Denitrification Rate (D). Denitrification is the conversion of nitrate to nitrogen gas which is carried out by certain anaerobic bacteria in an environment having a supply of organic carbon (i.e., organic matter). Marshlands are ideal for denitrification; but the sand spit areas of Stinson Beach are not likely to have sufficient organic matter to promote high levels of denitrification. Accordingly, a nominal denitrification rate of 10 percent of the wastewater nitrogen content was assumed. Higher rates undoubtedly occur at the margins of the sandspit where the groundwater enters the Bolinas Lagoon or the interior Seadrift Lagoon.

Estimate Background Nitrate Concentration (NB). The background nitrogen associated with percolating rainfall or lateral groundwater flow was estimated, based on monitoring data, to be at the detection limit, or 0.1 mg/l. This concentration was also applied to the lateral dilution flow of water from Seadrift Lagoon through the Dipsea Sub-area. However, for the Calles, the lateral groundwater flow was assigned a background nitrate concentration of 1.7 mg/l based on the monitoring results from MW-6.

Fertilizer Nitrogen Contribution. For the initial set of calculations, the fertilizer nitrate contribution was assumed to be negligible and deferred to a separate detailed analysis, which follows later.

Calculations. Using the above factors and assumptions, the estimated average groundwater nitrate concentration (Nc) was calculated for each of the five sub-areas according to the following mass balance equation:
NC =   (W)(NW)(1-D) + (R+G)(NB)
  W + R + G

Multiple calculations were made for each sub-area, covering various wastewater treatment assumptions and development scenarios. Calculation tables for each sub-area are provided in Appendix D. A summary of the results for all five sub-areas is provided in Table 4.

The following are indicated by the nitrate loading calculations:

Wastewater disposal in the Calles and the Dipsea Road sub-areas is likely to have the smallest impact on groundwater nitrate concentrations, due largely to the lateral inflow of groundwater that helps to dilute nitrate concentrations in these areas. The predicted concentrations for existing conditions and build-out is less than 5.0 mg/l in both areas. For comparison, the data collected in the field program of this project showed an average total nitrogen concentration of 3.8 mg/l for the Dipsea Road area (MW-1,2 &3), and 2.5 mg/l for the transect of wells through the Calles (MW-7,8 & 9). These data are consistent and less than the model predictions, indicating the nitrate loading analysis to be on the conservative (safe) side. The residences near these monitoring wells appeared to be regularly occupied during the sampling period of this study.
The calculations show a considerably higher potential for elevated nitrate concentrations in the Patios, along Seadrift Road and at the west end of Seadrift. The groundwater nitrate concentrations in these areas are predicted to approach, and possibly exceed, the drinking water standard of 10 mg/l. Monitoring data from the current field program was limited in these areas; the few sampling points (MW-4 and 5), did not reveal total nitrogen concentrations of more than a 2 to 3 mg/l.
The concentrations can be expected to rise slightly in the future in Seadrift with full build- out, even if sand filters are continued to be employed for all new systems. But the predicted impact would decline below drinking water limits if existing standard septic systems are converted over time to incorporate sand filters or an equivalent degree of nitrogen removal.
The Patios are essentially fully developed and no further increase in nitrate loading effects would be anticipated. In cases where existing small houses are replaced with much larger structures in the Patios and the Calles it is reasonable to assume that the District would require the incorporation of sand filters. Consequently, there would be no net increase in nitrogen loading even if there is an increase in wastewater flow. A significant reduction could be achieved, again reducing the predicted nitrate concentration below the drinking water limit, if sand filters or equivalent nitrogen removal are incorporated into the existing systems.
The ultimate pathway of groundwater nitrate in the Patios and Seadrift West End is to the marshland fringe of Bolinas Lagoon which, through natural denitrification processes, provides an ideal sink for the nitrate contained in the groundwater from the sandspit. Similarly, the groundwater from the Seadrift Road area discharges to Seadrift Lagoon, which also can be expected to provide for significant dilution and assimilation of the nitrate through denitrification along the lagoon bottom and via plant uptake. Consequently, elevated nitrate concentrations are and will continue to be confined to the shallow groundwater within the developed sandspit area.

Groundwater Impact from Fertilizer Use

To examine the potential impact of fertilizer use on groundwater nitrate in the study area, a qualitative and quantitative assessments were made.

Qualitative Assessment. The qualitative assessment involved a general canvassing of the study area and phone interviews with the Seadrift General Manager (Dick Kaminiecki) and several knowledgeable nursery people who work in the Stinson Beach area. The local nurseries contacted included Las Baulinas Nursery and Horseshoe Hill. The general findings of this survey included the following:

The vast majority of the properties in Seadrift have landscaping that consists of drought resistant ground cover (e.g., ice plant, beach grasses) and low maintenance ornamentals. Lawns, which have generally the greatest fertilizer use, are rare; but there are a few.
Some property owners have imported topsoil to improve the conditions and options for landscaping.
Many of the properties utilize services of local nurseries, such as Las Baulinas and Horseshoe Hill, who rely predominantly on organic, slow-release fertilizers. This type of fertilizer (as opposed to commercial fertilizers) release the nutrients (nitrogen, phosphorus, potassium) as a slow rate, making nitrate more available to the plant and less likely to be leached past the root zone into the groundwater. The nurseries also report that their use of fertilizers is very selective and generally in small quantities. The general opinion of the local nursery people, based on their practice and knowledge of the area, is that nitrate leaching from fertilizers is likely to be very limited.
Many of the property owners do their own gardening, and may employ traditional commercial fertilizers and at heavier rates. Also, some properties have little or no landscaping at all.

Quantitative Assessment. The nursery people interviewed were unable to quantify the typical fertilizer usage at properties in Stinson Beach. Therefore, to obtain some sense of the relative impact that fertilizer use might have, a worst case scenario was constructed. This scenario assumes that all properties in Seadrift have, or could have, a large lawn area of 5,000 square feet (sq ft), and that the lawn is maintained in a fashion similar to a golf course green or tee, for which there is reasonably good information on fertilizer use rates and leaching potential. This information was used to predict the nitrate loading through a similar mass balance calculation as presented previously for the wastewater nitrate loading analysis. The assumptions and calculations were as follows:

All 326 of the potentially buildable lots in Seadrift were assumed to have 5,000 sq ft of maintained turf grass;
Annual nitrogen fertilizer application rate was assumed to be 5 lbs N/1,000 sq ft, or 25 lbs nitrogen per lot per year;
The leaching fraction was assumed to be 10 percent of the applied nitrogen, based on test plot studies of golf course greens (Brauen, 1995);
Additional water use for turf maintenance was assumed, and 25 percent of the irrigation water (approximately 0.1 ac-ft per year) was assumed to percolate and add to the recharge of the local groundwater;

The fertilizer nitrogen and irrigation return flow was calculated to amount to an annual flow of 0.1 ac-ft/lot, with a nitrate-nitrogen concentration of about 9.1 mg/l. This was then combined (in a mass balance calculation) with the previously estimated rainfall recharge volume and the assumed background concentration of 0.1 mg/l, to give an adjusted worst case background nitrate-nitrogen concentration of 0.95 mg/l. This calculation demonstrates the relatively insignificant impact on groundwater nitrate concentrations from fertilizer use, even in a very conservative, worst case scenario that greatly overstates the existing or potential fertilizer use in the study area. Based on this analysis, further investigation or focus on fertilizer practices in Stinson Beach does not appear warranted, at least from the standpoint of groundwater quality protection.


In brief, the results of this Hydrologic Survey and earlier investigations and monitoring confirm that there are wastewater impacts on local water resources, most notably indicated by the presence of nitrogen and MBAS in groundwater and surface water.

With regard to beneficial uses, the impacts on groundwater of onsite wastewater disposal systems in the Seadrift, Calles, and Patios areas do not represent a threat to human health through drinking water. The nitrate loading analysis predicts, using conservative assumptions, that concentrations in the study area under existing conditions approach but do not exceed drinking water standards. Predicted concentrations under buildout conditions could exceed the drinking water standard, but could be mitigated by use of sand filter systems. Onsite wastewater disposal systems apparently are contributing coliform bacteria locally to groundwater and surface water. However, analyses of groundwater from the monitoring wells for this survey indicated few and declining detections of fecal coliform, suggesting insufficient well purging. Furthermore, the groundwater in the Seadrift, Calles, and Patios areas is not a likely source of drinking water because of seawater intrusion. In addition, there is little risk of contamination of the municipal supply. The municipal Aldergrove 2 well is located upgradient and is afforded an additional degree of protection by the clay layer underlying the well site. This study indicates no threat to human health through drinking water pathways under current conditions or in the reasonably foreseeable future, and provides no technical basis to support the building moratorium.

Nonetheless, numerous beneficial uses exist in Bolinas Lagoon. These include water contact recreation and shellfish harvesting, for which concentrations of coliform bacteria are a concern. Sampling of Bolinas Lagoon for this Hydrologic Survey included one detection of fecal coliform; however, this survey shows that fecal coliform is far less likely to be contributed to the lagoon by groundwater than by Easkoot Creek, which was documented to be most significantly affected by bacteria.

Accordingly, additional study should be focused on Easkoot Creek and areas tributary to the creek in the Old Town and Highlands areas. It should be noted that the only known potential domestic wells are in Highlands, and the impact of wastewater on these wells is not known. In addition, inclusion of the Stinson Beach Park also is recommended, given its proximity to Easkoot Creek and considerable wastewater discharge during the summer season.

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