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Installation of Monitoring Wells

Weeks Drilling and Pump Company, Sebastopol, California, drilled nine 4-inch diameter monitoring wells (MW-1 through MW-9) in Stinson Beach between August 19 and August 25, 1997. The purpose of these wells was to obtain geologic information, monitor water quality, and document groundwater levels and flow directions in two representative locations. Five monitoring wells (MW-1 through MW-5) were constructed in a north-south transect from Dipsea Road to Seadrift Road and four wells (MW-6 through MW-9) were drilled in a northeast-southwest transect along Calle del Resaca. Locations of the monitoring wells are shown on Figure 2 and Figure 3; note that these two maps portray the single, elongated study area and that a match line linking the two maps is provided. Construction details for the monitoring wells are summarized in Table 1. A tenth well near Seadrift Road was not completed due to lack of owner permission to access the site.

Drilling was expected to encounter a low-permeability clay layer underlying the beach and sand dune deposits and overlying a sand and gravel aquifer. To preserve the integrity of the clay layer and to protect the underlying aquifer, drilling was terminated after penetrating approximately three feet of gray organic clay. However, two 50-foot borings (MW-5 and MW-9) were planned to determine the local stratigraphy. MW-5 was drilled to 51 feet. MW-9 was drilled only to 36 feet after the driller encountered difficult drilling conditions. The clay layer was not encountered in either MW-5 or MW-9.

The monitoring wells were drilled with a Mobile Drill hollow stem auger rig using a 13- inch diameter bit and 10-inch inner diameter (ID) augers. Figure 4 shows a typical geologic and well construction log. Appendix A includes the geologic and well construction logs for MW-1 to MW-9 and water well completion reports. Soil samples were collected with a split spoon sampler at five foot intervals and inspected in the field for lithologic characteristics. The well casings were constructed of 4-inch ID, schedule 40 PVC pipe with flush threaded couplings. The screens consist of 5 or 10 feet of 0.020-inch (aperture width) slot PVC placed near the top of the saturated zone. MW-6 was drilled to weathered bedrock and the screen was set in the bottom of the hole. With the exception of MW-6, all other wells were fitted with a 3 to 5-foot tail pipe to act as a silt trap. To ensure that sufficient water will be available to collect water samples throughout the year, each well was drilled to a depth greater than ten feet below mean sea level.


All Depths Are in Feet

Well No. DWR Report No. Date Drilled TD of Boring CD of Well Stick Up Screened Interval SWL TOC Elev.
MW-1 509448 8/20/97 20 20 1.4 7 - 17 6.0 9.92
MW-2 509447 8/19/97 25 21 1.1 6 - 16 4.4 8.02
MW-3 509446 8/20/97 20 20 1.6 7 - 17 5.0 9.5
MW-4 509442 8/25/97 25 25 - 0.7 10 - 20 4.7 6.97
MW-5 509443 8/22/97 50 25 - 0.4 10 - 20 4.0 6.49
MW-6 509450 8/25/97 20 20 - 0.4 15 - 20 1.8 4.8
MW-7 509433 8/21/97 13 13 - 0.4 5 - 10 3.4 4.46
MW-8 509432 8/21/97 25 25 - 0.3 10 - 20 4.7 6.73
MW-9 509449 8/21/97 35 25 - 0.8 10 - 20 8.9 10.16


TD - total depth with respect to ground surface
CD - completed depth with respect to ground surface
SWL - static water level measured from TOC on date well was constructed
TOC - top of casing
Stick up is the reference point above (+) or below (-) ground surface
All wells were drilled by 10-inch diameter hollow stem auger
All casings are 4-inch ID, schedule 40 PVC
All screen aperture sizes are 0.020 inch
All wells are gravel packed with No. 2 Monterey sand
All elevations surveyed in August 1997
All elevations surveyed with respect to mean sea level

Each well has a sanitary surface seal between the 4-inch PVC casing and the 13-inch borehole consisting of at least 4 feet of Portland cement and 0.5 to 1 foot of bentonite pellets. Each well has a filter pack of No. 2 Monterey sand extending from approximately 1 to 2 feet above the top of the screen to the bottom of the hole. The wells were logged and field activities were recorded by Todd Engineers. MW-1 through MW-3 were fitted with stovepipe-type locking well covers since these wells are not in areas of vehicular traffic. The balance of the wells were constructed with locking caps and flush-mounted, traffic-rated well covers.

Well development was performed on September 2, 1997 by Blaine Tech, Inc. The wells were developed by pumping at a constant discharge of 6 gallons per minute (gpm) with a 3-inch submersible electric pump to remove fine-grained materials adjacent to the well screen, filter pack, and surrounding formation materials. Blaine Tech pumped approximately 20 casing volumes (200 to 300 gallons) from each well. However, MW-2 and MW-7 had very low yields, producing less than 50 gallons of water during development. Turbidity, pH, and electrical conductivity were measured at regular intervals during development. Pumped water was essentially clear and silt free by the end of development for each well. Formal pumping tests were not conducted following development.

Identification of Existing Wells

Numerous PVC wells, or stand pipes, are known to exist at Stinson Beach as a result of building permit requirements to document depth to groundwater. However, there is a general lack of construction information for most existing PVC wells, making it difficult to know, in many cases, whether the water in the well is groundwater or merely standing rain water. Existing stand pipe wells were field checked by Todd Engineers together with District staff to determine whether they would make suitable points for monitoring water levels or water quality. The water level and total depth of the well was measured. Together these measurements were used to compute the depth of water encountered in the well and to assess the well's potential usefulness for monitoring. Nine PVC stand pipe wells were determined to be sufficiently deep to provide water level information. Water quality samples were not collected from any existing wells, because of the lack of construction details documenting the zone being sampled.

In addition, Todd Engineers acquired all available water well drillers reports from the California Department of Water Resources for production wells in the vicinity of Stinson Beach. Known production wells in Stinson Beach are discussed in the section Groundwater Use and Wells.

Water Level Monitoring

Reference point elevations were surveyed by Questa for the nine selected existing stand pipe wells, the nine new monitoring wells, and three staff gages. A staff gage consists of a graduated scale mounted on a vertical board or plate that is placed in a surface water body and used to measure surface water levels. In Stinson Beach, groundwater and adjacent surface water bodies were expected to be hydraulically continuous; for example, with tides affecting groundwater levels. Accordingly, staff gages were located in the major water bodies including Bolinas Lagoon adjacent to MW-1, Seadrift Lagoon near MW-3, and Easkoot Creek at the Calle del Arroyo bridge (see Figure 2 and Figure 3).

Water level monitoring was conducted on September 3, September 9, September 17, October 14, and October 28, 1997. All monitoring events included the nine new monitoring wells (MW-1 through MW-9). Staff gage readings were begun on September 9, 1997. Four to nine existing stand pipe wells were included in the last three monitoring events. Because it was recognized that ocean tides can affect groundwater levels, especially in wells located close to the lagoons and beaches, water levels were measured rapidly in each of the monitoring rounds. The two lagoon staff gages were also measured at the beginning and at the end of each of the latter monitoring rounds to record the total amount of tidal fluctuation during the monitoring event. Wells MW-1 and G-45, both located immediately adjacent to Bolinas Lagoon, were found to show significant tidal water level changes exceeding two feet. Due to the lack of an ocean tide gage, the Bolinas Lagoon gage was assumed to approximate ocean tide levels.

Water Quality Sampling

Three water quality sampling events were planned to occur during the late summer and autumn of 1997. Sampling was planned for the new monitoring wells plus additional selected surface water and groundwater monitoring sites. The new monitoring wells were sampled immediately following installation and development on September 3. Two additional sampling events occurred on September 30/October 1 and October 21/22. These later sampling events included the new monitoring wells plus three surface water sampling sites including Easkoot Creek near Calle del Resaca, Stinson Lagoon near MW-3 and Bolinas Lagoon near MW-1. No additional groundwater monitoring sites were selected, largely because of the lack of well construction information for existing wells.

The monitoring wells (MW-1 through MW-9) were first sampled September 3, 1997 by Questa Engineering personnel. Each well was first measured for depth to groundwater by use of an electric coaxial water level sounder. Additionally, any monitoring wells within a few hundred feet were also measured for depth to groundwater before any removal of groundwater began. The amount of water present in the well casing was calculated for each well to determine how much groundwater to remove before sampling. Through the use of pre-cleaned disposable plastic bailers, each well was purged of approximately five well casing volumes of groundwater before samples were collected. Questa staff used sterile disposable latex gloves for the handling of bailers and sampling equipment to minimize potential cross-contamination of samples. For subsequent monitoring events on September 30/October 1 and October 21/22, a centrifugal pump was used in place of bailers to purge groundwater from the wells. Pumping of the wells allowed sufficient drawdown of groundwater to obtain a sample representing a larger volume of the aquifer, In addition, pumping reduces handling of sampling equipment and thereby lessens the potential for cross-contamination. Clean plastic tubing was placed down the well casing to pump 10 volumes of groundwater per well. The pump was decontaminated between each well by pumping chlorinated water (1:50 ratio of bleach and water) through the pump and tubing.

Groundwater samples were collected from the bailer or pump discharge line into 8 ounce sterile plastic containers for total and fecal coliform analysis; 1-liter plastic bottles for nitrates, total Kjeldahl nitrogen, and ammonia nitrogen analyses; and into 2-liter plastic bottles for total dissolved solids, MBAS as LAS, alkalinity, major cations and anions, turbidity, electrical conductivity and pH. Samples were labeled and stored on blue ice at 4 degrees Celsius until delivered later the same day to a laboratory courier for transport to Brelje and Race Laboratory, Santa Rosa, a State Certified Laboratory.

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