LAKE MERCED HYDROLOGY AND WATER QUALITY

Vivian Matuk and Nick Salcedo
Graduate Students
Geography
San Francisco State University

Abstract. This paper discusses the water within Lake Merced from a qualitative and quantitative perspective. The information within this paper was principally derived from the extensive studies prepared by or on behalf of the City of San Francisco to assess the current water quality and lake level issues at Lake Merced. How much water is in the lake is of great interest; however, lake quantities are not that easy to determine. This is largely because the lake’s relation to the Westside Basin aquifer is hard to define. Water quality in the lake generally suffers from turbidity, alkalinity, phosphorus and high organic content.

INTRODUCTION

Lake Merced is one of an estimated 131,600 lakes in the United States and they are invaluable ecological resources that also serve many human needs. Lakes and reservoirs provide 68 percent of the water used by the nation’s largest utilities (more than 50,000 customers) (National Research Council 1992). Lakes and reservoirs enhance our lives by providing opportunities for swimming, boating, and fishing. In the United States, fishing alone has an estimated annual economic impact of $28 billion (Mays 1996).

This paper focuses on the water in Lake Merced, specifically in terms of its hydrology and quality. In the first section, the paper discusses the pre-altered, or natural, conditions that made up Lake Merced’s hydrology. Next, it touches on the history of the use and development of the lake and its water, and how the uses of the lake water have changed over the last several decades responding to the changing needs of the surrounding urban communities. Then it touches on the current issues associated with of the lake’s water – how much water is there? how much water was there? Where does the water come from? What is the water used for? Unfortunately, there does not seem to be an easy or consistent answer to these questions. Finally, the first section of the paper concludes by discussing some of the current thinking surrounding the use and management of the water in the lake and some of the alternatives being considered to resolve the current dilemmas.

The second section of the paper discusses the water quality of Lake Merced. The water quality section briefly touches on the importance of water quality, its associated benefits, and the water quality management program. The paper then discusses many potential contaminants from turbidity to lead. A summary of each identified parameter is included with a corresponding graphic.

LAKE MERCED HYDROLOGY

"If five thirsty kids are given five straws and a quart of water, they will suck so hard that the pitcher will go dry before anyone asks for a refill."( Lynn Ludlow, 1994)

This analogy of five kids sucking as hard as they can is a favorite of hydrologists when they try to explain why too much groundwater pumping can empty an aquifer. In the following pages we will try to summarize some of the past and present issues surrounding the development, use and management of Lake Merced’s water resources.

Pre-altered, or Natural, State of Lake Merced’s Water Resources

In the beginning, the lake was much smaller, possibly non-existent, it may have been just a coastal stream. At the end of the last ice age, about 15,000 years ago, the ocean flooded Merced Valley creating a small inlet or bay. At that time the lake used to extend further east and was open to the ocean. Only small streams flowed into the lake. In dry times there may have been no flow to the ocean due to decreased input and evaporation. Eventually the sand laden ocean currents moved across the creek depositing a sand bar across the mouth of the valley, or inlet. In the summer the sand bar cuts off the creek from the ocean and in the winter the creek  breaches the sand bar and flows to the ocean. This is the same process that occurs at the mouth of most small streams and lagoons on the coast as they are seasonally sealed off from the ocean by a flow of sand (Gilliam 1967).  In the case of Lake Merced this sand bar eventually became permanent. Freshwater gradually replaced the ocean water after the two were separated. In 1852, the last historic connection to the ocean, an earthquake breached the sand spit resulting in a lowering of the lake level.

Lake Merced is often referred to as a lagoon. The word "lagoon" is defined in the Funk and Wagnalls Standard College Dictionary as: "a body of shallow water, as a bay, inlet, pond or lake usually connecting with a river, larger lake or the sea." Lake Merced’s connection to ocean is very important, it’s fundamental to the formation of the lake, and it cannot be overlooked. The forces of erosion, corrosion and earthquakes all will play an important role in the future evolution of the lake and its connection to the ocean.

Prior to the urbanization of the south-western portion of the San Francisco peninsula, the Lake Merced watershed was predominantly covered with a dense coastal scrub community of scattered shrubs, subshrubs and herbs often developing considerable cover (Holland, 1986). The thick vegetative cover, combined with the high permeability of the sand and gravel soils in the watershed, probably kept the surface water runoff in the original watershed at a lower level.

Lake Merced's most important hydrological connection is to the to Westside Basin Aquifer, the major groundwater basin that stretches from Golden Gate Park to the City of South San Francisco. There is some evidence that the surface of the lake reflects the water table of the shallow aquifer and it may be a surface feature of the aquifer (CMP 1998). The original watershed has been estimated to cover 6,320 acres (Pezzetti and Bellows 1998). The United States Geologic Survey estimated the original watershed to range from anywhere between 2,176 acres to 5,248 acres (Yates et al 1990). The lake also is estimated to have had a natural fluctuation of anywhere from 1.2 feet to 5.2 feet (Pezzetti and Bellows 1998).

Post European Use and Development

The first European use of the lake’s water began in the late 1700s. Fernando Rivera, one of the first Spanish explorers, noted in his journal that he found the water of the lake "sweet and fresh" (Shoup and Baker 1981). The lake was used for recreation as early as 1850, primarily for trout fishing. It was not until the late 1800s that the lake began to undergo significant changes to its natural condition. In 1868, the Spring Valley Water Company purchased the water rights to Lake Merced and began to buy up the surrounding lands. From 1870 to 1930 the Spring Valley Water Company sold the water of the lake to residents of San Francisco for municipal purposes (Shoup and Baker 1981, Campo 2000).

In 1880's the lake was permanently dammed off from the ocean. Later it was divided into four lakes: North Lake, East Lake, South Lake and Impound Lake, Impound Lake being the last lake created in 1935 (Gilliam 1967, Campo 2000). Over the course of the 1880's and into the 1890's,  the Spring Valley Water Company made major hydro-modifications to the lake including two dams, pipelines, flumes, ditches, pumps, tanks, wharfs, bridges, powerhouse and pumphouse buildings and a rail line. Only the South Lake is directly connected to the Lake Merced Pump Station. The blocking of the lake’s connection to ocean helped the company provide a more dependable supply of water. The construction of canals, ditches and flumes around the perimeter of the lake helped protect the quality of the water by diverting surface water around the lake and through a tunnel in the dunes for discharge into the ocean (Shoup and Baker 1981).

Water production at Lake Merced steadily grew until the turn of the century. In 1877, the Spring Valley Water Company was exporting approximately 295 million gallons a year (mgy). In 1887, it was exporting 1,588 mgy and by 1902 it was up to 1,774 mgy (Shoup and Baker 1981). However, it was about this time that the Spring Valley Water Company was undergoing criticism for its monopoly on the water service in the City. This was likely exasperated by the 1906 earthquake and fire that put unrealistic demands on the damaged water system. The water company began a series of battles with the public and City officials and it began to sell off its holdings to golf courses, the zoo and the gun club after it began to realize it was no longer wanted as a water service provider. The approval of the Hetch-Hetchy project in 1908 and the completion of the Hetch-Hetchy aqueduct in 1935 culminated in the collapse of the Spring Valley Water Company as a water provider.

Eventually the lake’s water was no longer needed as the City’s municipal water supply, and in the late 1930s the City acquired Lake Merced and gave jurisdiction of the lake to the San Francisco Public Utilities Commission (SFPUC). Today the SFPUC monitors the water quality of the lake because the water is still identified as a potential municipal water supply. In 1950, the City’s Recreation and Park Department was given management responsibility of the Lake Merced lands (SF PUC Reso. No. 10,435) to develop and carry out recreational opportunities at the lake including fishing, boating, hiking, golfing and other related activities (Ecology and Environment 1993).

The Lake Today

"Here at Lake Merced … there is no stream flowing to the sea." ( Harold Gilliam, 1967)

Nor are there any streams flowing into the lake. Nearly all of the natural stream inputs to the lake have been diverted, and those that are left have been highly modified and many are a only a minute fraction of what they once were. Pinning down a precise description of Lake Merced and its water today proved to be very difficult.

The current watershed is estimated to be approximately 650 acres in size (Pezzetti and Bellows 1998, Ecology and Environment 1993) (Figure 1).

Figure 1. Lake Merced Watershed Comparison 1935 to 1995  (Source: Pezzetti and Bellows 1998).

The development of the lake’s watershed with impervious surfaces and the diversion of most of the runoff to the City’s combined stormwater system have had an effect upon the surface runoff into the lake. It also likely to have had an effect on the groundwater recharge rates of the Westside Basin aquifer that serves the region from Golden Gate Park to South San Francisco (Figure 2).

Figure 2. West Basin Aquifer and Groundwater Extraction Points ( Source: S. F. Public Utilities Commission and Recreation and Park Department 1998).

More than a hundred wells tap into the Westside Basin aquifer, which as noted, is the same aquifer that feeds Lake Merced. These wells range from 50 feet deep to 300 feet deep and draw from either the shallow aquifer, the deep aquifer, or in some cases both (Figure 3). Many of the wells are monitoring wells; not all are production wells or in operation at this time. Some of these wells are located around the lake, with others extending as far North as Golden Gate Park along 44th Avenue. The more nearby wells tap the groundwater to serve Daly City and the adjacent golf courses. A San Francisco Technical Memorandum lists 140 of the wells, yet only half are listed with a pump rate, and the memorandum notes that it is difficult to estimate the annual extraction of groundwater because many of the wells are not equipped with flow meters (Brown et al 1997). An estimate 9 million gallons a day are being extracted from the aquifer (SF PUC 1998).

Figure 3. Schematic Cross-Section across the Westside Basin in the Lake Merced Area (Source: S. F. Public Utilities Commission and Recreation and Park Department 1998).

At least three computer models have been constructed to model the Westside Basin aquifer. Geo/Resource Consultants, Inc. (1993) constructed its ground water model using data from three different years – 1983 to represent a wet year, 1985 to represent a normal year and 1990 to represent a dry year. The results of the model indicate that the lake level would go up by 4 to 5 feet if extraction for golf courses is stopped. Without pumping for Daly City, the lake level would go up approximately 10 feet. In periods of droughts (40% of normal rainfall) the lake could go down as much as 8 feet. In the event of an emergency, the lake would lose approximately 7 feet if 50 mgd were pumped for 84 days. A more recent model constructed for the area indicated that the lake level would increase 1 to 1.5 feet with no golf course pumping, 1 to 1.5 feet with no pumping by the City of Daly City, and increase up to 5 feet if pumping was stopped altogether (Brown et al. 1997).

Estimates of the total surface area of water range from 244. 94 acres of open water (EIP Associates 2000), to 266 acres (Camp Dresser and McGee 1998) to 273 acres (Yates et al. 1990). This range is likely due to differences in lake level and surrounding topography. The North and East Lake, with a surface area of 88 acres, ranges in depth from 3 to 20 feet, and the average depth varies from 9.8 feet average to 11.4 feet. The South Lake, with a surface area of 163 acres, ranges in depth from 3 to 21 feet, and the average depth ranges from 13 feet to 14.8 feet. The Impound Lake, with a surface area of 15 acres, ranges in depth from 2 to 10 feet, and the average depth ranges from 5.5 feet to 6.1 feet (Ecology and Environment 1993 and CDMc). Only one reference was found that indicated any modifications of the bottom of the lake, that being when dredging was conducted in the South Lake to remove lead shot from in front of the gun club (Ecology and the Environment 1993).

Estimates of the capacity of the lake also vary greatly from a low of 768 million gallons to high of 1.93 billion gallons (Ecology and Environment 1993). According to Camp Dresser and McKee (1998), the volume of the North and East Lake is approximately 280 million gallons, the South Lake approximately 700 million gallons and the Impound Lake is approximately 26 million gallons, for a total of approximately 1 billion gallons of water in Lake Merced. Yates et al (1990) estimates the lake’s capacity at 1.2 billion gallons. Again, the range is likely due to differences in lake level and surrounding topography.

The best information on input of water into the lake is from the USGS report (Yates et al 1990) and the SF PUC Watershed Sanitary Survey (CDM 1999). Based on a 1998 model constructed on the hydrology of the lake, the USGS estimated that yearly 261 mgal of water are added to the lake from rainfall (based on a 67% of normal rainfall year), 3.9 mgal from surface runoff and 512 mg from groundwater for a total input of 371 mgal/y. Camp, Dresser and McKee (1998) estimate that about 42 mgal/y are added to the lake from groundwater, approximately 126 mgal/y from rain and 5 mgal/y from runoff for a total input of 173 mgal/y. This is quite a bit lower than the 1,774 mgal/y that the Spring Valley Water Company was extracting in the early 1900s. However, a certain degree of variation is to be expected because the lake was being used as an aquifer. The SF PUC and Recreation and Park Department report (1998) estimates that as much as 3,285 mgal/y is being extracted today from the aquifer.

In any event, the different models and simulations all illustrate the hydrologic complexity of Lake Merced. Several reports (CH2Mhill 1997, Yates 1990) note that Lake Merced provides some recharge to the shallow aquifer, that there is the possibility that the shallow aquifer may recharge the lake, and that the shallow and deep aquifers may also be connected.

Measuring lake levels is an important and complex issue too. Technical Memorandum 17, Vol. 1 (1997) includes several paragraphs on this subject because of the difficulty relating lake levels to a variety of datum. A depth of approximately 26 feet is recommended for Lake Merced per the 1950 SFPUC and Recreation and Park District agreement, as measured at the lake’s water gauge board. However, the lowest level recorded has been 15.5 feet (Geo/Resource Consultants 1993) (See Figure 4). The water board datum is approximately 8.75 feet higher than Mean Sea Level, a datum commonly used to measure marine water levels. A third datum, the National Geodetic Vertical Datum, is commonly used to measure surface elevations of land. It is unclear whether the bottom of the lake and the sub-surface ground water measurements are based on this datum as well.

Figure 4. Lake Surface Level Elevations

It is important to note that water quantity figures are derived from different computer models running programmable simulations. Many of the reports recommend that additional simulations be run and that further refinement of the models be made. There now appears to be enough data available to model the hydrology of Lake Merced, providing an opportunity to construct computer models and simulations, adding insight to the figures listed above.  Recently, workshops dedicated to developing a better understanding and use of modeling for the hydrology of the lake and the Westside Basin aquifer, have taken place. Anyone is interested in researching this issue further, please refer to the reports referenced in the back as they contain a detailed description on how the computer models and simulations were developed.

Beneficial Uses of the Water in Lake Merced

The SF Public Utilities Commission manages the water in the lake to achieve the "Beneficial Uses" of the water as identified by the Public Utilities Commission and the Regional Water Quality Control Board. The uses are as follows:

1.    emergency water supply

2.    habitat for endangered and threatened species

3.    recreation, education and aesthetics

4.    habitat for non-endangered species

As noted earlier, the water in the lake is affected by groundwater pumping from the coastal aquifer, to supply Daly City, South San Francisco, California Water Service Company and the Olympic Club, Lake Merced Golf and Country Club, San Francisco Golf and Country Club, and Harding Golf Course (Ludlow 1994). Indecently, none of these uses fall into one of the identified beneficial uses, except recreation. Needless to say, there is plenty of demand on the water in the coastal aquifer for purposes not directly associated with the lake. Since the coastal aquifer is partly responsible for water input to the lake, the groundwater extraction is a hot issue.

Current Issues

"A lake without an adequate volume ultimately becomes a mudflat, and all the efforts in the world to fix the other problems in the area will have been largely futile if what is eventually left is Mudflat Merced, rather than Lake Merced." (Committee to Save Lake Merced)

The most recent alarm over reduced lake levels was first sounded in the early 1990’s. The "Committee to Save Lake Merced" was created by a concessionaire located adjacent to the lake in 1993. The "Friends of Lake Merced," dedicated to preserving the natural and recreational value of Lake Merced, was formed a year later in 1994. These groups have partnered with the Dolphin Rowing Club, the Pacific Rowing Club and the St. Ignatias Prep Rowing Club.

A groundwater management plan (AB 3030) and a reclaimed water use ordinance (SF Sup. Ord. No. 390-91 and 391-94) have been passed to ensure that the City and County of San Francisco, the golf courses and the City of Daly City all work together in the study of the water resources at Lake Merced.

Current reports (Pezzetti et al. 1998, CDM 1999) cite the main reasons for low lake levels as (1) drought, (2) groundwater pumping (3) evaporation and (4) urbanization (by decreasing the amount of groundwater recharge and by diverting stormwater into the City’s combined sewer and stormwater system). Proposed solutions include (1) adding Hetch-Hetchy water, (2) injecting recycled water and (3) diverting stormwater from the Vista Grande Channel.

The Friends of Lake Merced (FOLM) have been studying lake levels intensely since 1993. They note the past four years have been some of the rainiest seasons on record. As one would expect, when it rains the level of Lake Merced should increase. In very rainy months the increase in lake level exceeds the direct input from rainfall, indicating a significant runoff or groundwater input. When it is not raining, however, the level of the lake typically falls at the rate of 2" - 3" per month. The net effect of four very wet years (1994 - 1998), according to PUC reports, has then been a negligible increase in lake level (FOLM 2000).

FOLM also notes that during this four-year period there is no apparent effect of adding Hetch Hetchy water to Lake Merced either. One report (Pezzetti et al 1998) states, "These periodic additions have provided short-term benefits, but no sustained increase in the lake level." In fact, during months following infusion of Hetch-Hetchy water into Lake Merced the rate at which the water level falls is observed to more than double the normal rate, to as much as 6" or 7" per month. It appears, then, that the lake is able to absorb large quantities of water, presumably returning surplus to the aquifer. The Friends of Lake Merced have concluded that treating Lake Merced as a reservoir, relying on the infusion of Hetch Hetchy water during dry periods, will prove to be ineffective in maintaining lake levels (FOLM 2000).

The lake’s connection to the West Basin aquifer is also important in terms of water quality – both in the lake and of the groundwater. A recent report notes that in 1994, groundwater sampling noted that the Westside Basin groundwater generally meets drinking water quality standards (King 1994). According to the FOLM, the only viable long-term solution is to restore the aquifer to its normal condition (emphasis added, as a "normal" is a subjective term and can mean pre-1852 earthquake condition, pre-reservoir condition, etc.).   Only in that way can one count on Lake Merced remaining as a great natural and recreational resource, thereby meeting the objectives outlined in the "beneficial uses."

LAKE MERCED WATER QUALITY

Importance of Lake Merced Water Quality

Lake Merced has served a number of functions for the city of San Francisco, from a municipal water supply in the late 1800’s to its present use as a potential emergency water supply (Camp Dresser & McKee 1999). California requires a watershed sanitary survey be performed for every water supply watershed, although there is no specific requirement for a watershed survey of a potential emergency supply. However, the sanitary survey provides important information to determine potential sources of contamination of drinking water within a watershed and information on the overall water quality and condition of the watershed. Studying water quality of the lake is helping to protect and maintain the aquatic ecosystem and the other resources this lake provides to the society (The California Regional Water Quality Control Board 1995).

Potential Contaminant Sources in the Lake Merced Watershed

Camp Dresser and McKee (1999) identified a number of potential contaminant sources at Lake Merced. They pointed out that there is not sufficient water quality data or contaminant source management detail to quantify sources to the lake. Appendix 1 includes some observations and a number of recommendations for further study of the potential contaminant sources.

Lake Merced Water Quality Monitoring Program

The Lake Merced Basin Plan follows the water quality objectives for surface waters in the San Francisco Bay Basin that protect the beneficial uses of high quality waters of the state.

The water quality of Lake Merced is governed by its physical configuration: 1) as a shallow surface impoundment, 2) by the water quality of inflowing groundwater, 3) by impacts imposed by recreational uses and resident fish and wildlife, and 4) to a small extent, by stormwater discharge (GEO/Resource Consultants 1993).

The Water Quality Bureau (WQB) has conducted weekly water quality monitoring of Lake Merced since 1958. Samples are collected at the surface of both the North and the South Lakes by single surface grab samples. Limnology profile sampling is conducted quarterly by collection of samples at various depths in North, South, and East Lakes. These samples have been analyzed for the following parameters: Temperature, pH, conductivity, alkalinity, hardness, chlorides, total Coliform bacteria and color (San Francisco Public Utilities Commission & San Francisco Recreation & Park Department 1998).

Beginning in February 1997, quarterly limnological monitoring began on North, East, and South Lakes. Four locations at the Lake are profiled with samples collected at five-foot intervals of depth. Profiling locations were selected at the deepest point in each lake, with two locations needed in the South Lake due to its large size.

The following testing parameters are typically measured in the limnological monitoring at Lake Merced:

- Temperature - pH - Conductivity - Turbidity

- Alkalinity - Hardness - Chlorides - Dissolved Solids

- Dissolved oxygen - Fluoride - Bromide - Orthophosphate

- Sulfate - Manganese - Chlorophyll a - Algal biomass

- Nitrate Nitrogen - Oxidation Reduction Potential - Plankton

- Iron - Total Phosphorous

- Ammonia Nitrogen - Total Organic Chemicals (TOC)

- Methyl-Tertiary Butyl Ether (MTBE) - Secchi Disk Measurement

Nitrogen and phosphorous concentrations in a lake are very important limnological parameters. These nutrients often serve to trigger algae blooms that can frequently impact the aesthetic qualities of a lake. Dissolved oxygen concentrations and pH levels are equally important parameters to be measured since their levels can correlate with the start of a sudden phytoplankton population increase that could trigger an eutrophication process. Monitoring of dissolved oxygen and pH levels is of particular importance as the lakes move into the summer stagnation period (period that occurs as soon as temperature rises and thermal stratification sets in). During this period the oxygen content of the water in the hypolimnion (the lower part) decreases (Mays 1996). The Basin Plan states that testing for chlorophyll a, which is a critical indicator of algal biomass in a lake or reservoir, is important because it decreases the beneficial recreational uses when present in excess (Camp Dresser & McKee 1999).

Based on literature review and previous information developed for this particular project, parameters such as dissolved oxygen, pH, alkalinity, turbidity, conductivity, temperature, nitrogen, phosphorous, plankton count, lead and Coliform bacteria have been chosen for this analysis. From the literature, these parameters clearly show the dynamic of a lake, its balance and trends (Margaleff 1996, Chapman 1997).

To provide a summary of the water quality of Lake Merced, the water quality information from 1960, 1970, 1980, 1997, 1998 and 1999 has been discussed and displayed graphically (See Figure 5 through Figure 15). The information was provided by the San Francisco Water Department archive (Via. Patrick Law (comm. per. 2000) and Dave Dingman (comm. per. 2000). Four months (March, June, September and December) of 1960, 1970 and 1980 water quality information were selected to evaluate the lakes flucuations during the different seasons. [Relevant data collected for this report are shown in Appendix 2.]

Lake Merced Water Quality Parameters

Turbidity

Turbidity is a measure of water clarity: the more material suspended in water the less light can pass through the water column. Turbidity units are NTU (Nephelometric units). Suspended material include soil particles (clay, silt, and sand), algae, plankton, microbes, and other substances. These materials are typically in the size range of 0.004 mm (clay) to 1.0 mm (sand). Turbidity can also affect the color of the water. Higher turbidity increases water temperatures because suspended particles absorb more heat. This, in turn, reduces the concentration of dissolved oxygen (DO) because warm water holds less DO than cold. Higher turbidity also reduces the amount of light penetrating the water, which reduces photosynthesis and the production of DO. Suspended materials can clog fish gills, reducing resistance to decrease, lower growth rates, and affecting egg and larva development. As the particles settle, they can blanket the lake bottom, and smother fish eggs and benthic microinvertebrates (USEPA 1991, APHA 1992). Turbidity data from Lake Merced was collected from the surface of the lake and is shown in Figure 5.

Figure 5. Turbidity at Lake Merced (S. F. Water Dept 1960, 1970, 1980, 1997, 1998, and 1999).

The graphic clearly shows how turbidity has increased over the sampling period. Turbidity increases in all the sampling points during autumn and wintertime. This might be due to the wind and rain incidence into the lake during these times. Camp Dresser & McKee (1999) mention that sediments can be washed off trails without erosion management control. Also, some gullying and soil erosion is visible along the steep embankment between the golf course and South Lake, suggesting that surface runoff from the golf course and its associated sediment, sometimes occurs (GEO/Resource Consultants 1999). Other sources, such as contaminants from uncovered waste, stormwater from Vista Grande Channel, or combined sewer overflows can increase turbidity. Dingman (comm. per. 2000) mentions that turbidity is one of the factors that makes the lake not a good potential emergency water supply, even though there was a storm filter treatment unit installed in the parking lot area north of the Boathouse Restaurant on Harding Park Road in the spring of 1998.

Dissolved Oxygen and Temperature

Due to the important correlation between DO and temperature, it is important to analyze both parameters together. Oxygen in aquatic systems is measured in its dissolved form as dissolved oxygen (DO, units: mg/L). DO is important in natural water because oxygen is required by many microorganisms and fish. Typical dissolved oxygen concentrations reported for natural waters throughout the world are 3 to 9 mg/L, which is the concentration of dissolved oxygen in fresh water at 20 oC (68 oF). The observed range of dissolved oxygen concentrations reported worldwide is 0 mg/L (anoxic conditions) and 19 mg/L (supersaturated conditions). Super saturated conditions are caused by algal bloom. Under anoxic conditions, or periods of zero dissolved oxygen in the water, reduced the number of chemical species being formed and frequently leads to the release of undesirable odors until oxic conditions or aerobic conditions develop (Mays 1996).

The rates of biological and chemical processes also depend on temperature. Aquatic organisms from microbes to fish are dependent on certain temperature ranges for the optimal health (APHA 1992). Optimal temperatures for fish depend on the species: some survive best in colder water, whereas other prefer warmer water. Benthic macroinvertebrates are also sensitive to temperature and will move in the lake to find their optimal temperature. If temperatures are outside this optimal range for a prolonged period of time, organisms are stressed and can die (USEPA 1991, Chapman 1997).

Temperature affects the oxygen content of the water (oxygen levels become lower as temperature increases); the rate of photosynthesis by aquatic plants; the metabolic rates of aquatic organisms; and the sensitivity of organisms to toxic wastes, parasites and diseases (USEPA (1991), Margaleff (1996) and Chapman (1997).

The following results are for 1997, 1998 and 1999, seeing as dissolved oxygen was not evaluated before this time. The graphics show DO readings at surface and at 15 foot depth, providing profiled information for this parameter of the lake.

Figure 6. Dissolved Oxygen at Surface in Lake Merced (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

Figure 7. Temperature at the Surface in Lake Merced (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

Figure 8. Dissolved Oxygen at 15 feet in Lake Merced  (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

Figure 9. Temperature at the15 feet in Lake Merced (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

Normal DO levels in freshwater are between 8 and 10 mg/L. Analyzing figure 6, it is possible to observe that the lake, during the three year sampling period, has had a DO range between 6 and 14 mg/L. Therefore, it is possible to conclude that at the surface the lake has generally normal DO levels. The highest DO value (14 mg/L) was reported in May 1997. By late-spring and summer the lake water level decreases creating ideal conditions for algae growth. This situation might explain the high DO levels during that period of time. High amounts of algae produce more dissolved oxygen in the aquatic systems as it was mentioned previously (Margaleff 1996). It is important to point out that, generally, comparing figures 6 and 7 the inverse relationship between temperature and DO is not applicable at the surface of Lake Merced.

Analyzing figures 8 and 9, shows an inverse correlation between temperature and DO. For example, higher temperatures at 15 feet in the lake are mostly reported by late summer. During this period, there are some occasional anoxic conditions. This means that some fish are not able to survive at these depths because they are not able to seek colder water at lower levels.

While the surface of the water warms first in the spring, producing a seasonal stratification in water temperature, the lake is not deep enough to maintain that stratification, and by mid-to late summer, the water is consistent temperature throughout. This particular fact shows that Lake Merced does not have a vertical stratification. Literature reports that deep lakes exhibit vertical stratification because the density of water varies with temperature, with a maximum of 4 oC. This stratification is often distinct, resulting in an upper, warm epilimnion, and a lower cool hypolimnion (Wetzel 1983). Because conditions are frequently windy and water depth is fairly shallow, the water in Lake Merced is probably well-mixed most of the time. Therefore, Lake Merced does not have a vertical stratification. As a consequence, the exchange and diffusion of water or solutes is not restricted (Mays 1996, GEO/Resource Consultants 1999).

pH

pH is a term used to indicate the alkalinity or acidity of a substance as ranked on a scale from 1.0 to 14.0. Acidity increases as the pH gets lower. pH affects many chemical and biological process in the water. For example, different organisms flourish within different ranges of pH. The largest variety of aquatic animals prefer a range of 6.5-8.0. pH outside this range reduces the diversity in the lake because it stresses the physical system of most organisms and can reduce reproduction. Low pH can also allow toxic elements and compounds to become mobile and "available" for uptake by aquatic plants and animals. This can produce conditions that are toxic to aquatic life, particularly to sensitive species (USEP 1991). pH from 1970 is shown in Figure 10.

Figure 10. pH in Lake Merced (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

The pH values during the sampling period analyzed have been between 7.9 and 9.0; therefore, it is possible to conclude that the lake water alkaline or basic (when pH is above 7.0). The highest values have been reported during summer time (8.7 - 9.0). Literature reports that in shallow lakes, algae and microphytes reduce the amount of carbon dioxide as a result of the photosynthesis reducing the production of carbonic acid, thereby increasing the pH , to confirm this fact, one should analyze the Lake Merced plankton count.

Plankton

Figure 11. Plankton Count in Lake Merced (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

Even though data about plankton count is only available for the 1997-1999 period, there is a is possibility that high values of turbidity, DO and pH previously reported during late-spring and summer could be caused by plankton, specially blue green algae or oscillatoria (Dingman comm. per. 2000, FOLM 2000). GEO/Resource consultants (1993) report that the primary physical factors governing algae formation include: lake depth, ratio of water volume to surface area, detention time, temperature, light penetration, and attachment sites.

Deeper lakes are generally less productive for algae than shallower ones because they have a greater volume of nutrient inflow and have less surface area in which algae can live. Droughts would increase algae productivity because of a decrease in lake volume. Lake Merced fits the description of a higher-algae shallow water body. The droughts and consequent low lake levels appear to have increased algae content. Thus, the main quality impacts of extensive algae blooms in Lake Merced are odor, turbidity, DO and pH confirming previous analyses. Moreover, algae blooms can cause fish-kill during decomposition due to the higher organic chemicals and related oxygen depletion. This situation complicates the disinfection process of the lake water due to the potential formation of trihalomethanes (THMs). THMs can clog filters, which is of particular concern in the event that the lake water is needed for the San Francisco water supply system.

Nitrogen

Nitrogen is sometimes the limiting nutrient to algal growth, comprising 1 to 10 percent of the dry mass of algae. The process of nitrification which coverts NH4+ to NO3- , is important because it consumes oxygen and can cause substantial oxygen depletion in aquatic systems. Nitrogen is found in several forms in terrestrial and aquatic ecosystems: including ammonia (NH3), nitrates (NO3), and nitrites (NO2). Free ammonia (NH3) is toxic to fish and other aquatic organisms at concentration that are sometimes encountered in hypereuthrophic systems (Effler et al. 1990). Together with phosphorus, nitrogen in excess amounts can accelerate eutrophication, causing dramatic increase in aquatic plant growth and changes in types of plants and animals in the lake. This, in turn, affects dissolved oxygen, temperature, and other indicators. Excess of nitrogen can cause hypoxia (low levels of dissolved oxygen) and can become toxic to warm-blooded animals at a higher concentrations (10mg/L or higher) under certain conditions. The natural level of ammonia or nitrate in surface water is typically low (less than 1 mg/L). Sources of nitrogen include wastewater treatment plants, runoff from fertilized areas, failing on-site septic systems, runoff from animal manure storage areas, and industrial discharges that contain corrosion inhibitors (USEPA 1991). Figure 12 shows ammonia data of 1997, 1998 and 1999.

Overall, figure 12 shows ammonia peaks (0.10 - 0.20 mg/L) during the rainy season (winter). These concentrations which are within the natural levels of ammonia in surface water may indicate a contribution of ammonia from runoff. Even though, Friends of Lake Merced (2000) do not know what the sources of the runoff might be, based on literature review and the GEO/ Resources consultants report (1993), it is possible that groundwater and the use of fertilizers and pesticides from Harding golf course are sources. Because groundwater is the largest source of recharge to Lake Merced, its nutrient content (total nitrogen and phosphorus) probably contributes substantially to the growth of algae and phytoplankton in the lake. In 1987 and 1988 the United States Geological Survey (USGS) conducted well water sampling at the San Francisco Golf Club and San Francisco State University, both uphill of the lake. Chemical results indicated dissolved nitrogen (as N) ranges from 7.6 to 12 mg/L (high value). At an annual ground water inflow rate of about 800 acre-feet per year, and an assumed average total nitrogen concentration of 11 mg/L 23,900 pounds of nitrogen per year could enter the lake, or about 88 pounds per acre. This loading rate is of the same order of magnitude as nitrogen application for many agricultural crops. Almost all nitrogen entering Lake Merced is taken up by plants, algae and phytoplankton. The fact that essentially all of the nitrogen is consumed by plants, algae and phytoplankton indicates that it is the limiting factor for those organisms. If other factors are limiting, such as phosphorus or sunlight, higher concentrations would be expected. This means that the population or biomass of these organisms is a function of the nitrogen mass loading rate (GEO/ Resources consultants report 1993).

Figure 12. Nitrogen Ammonia at Lake Merced (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

A nitrogen study developed in 1990 reported by the GEO/ Resources Consultants (1993) for Golden Gate Park and Lake Merced indicated that the relatively high nitrogen concentrations measured in wells within western San Francisco is derived approximately equally from inorganic fertilizers and leaky sewer pipes. Nitrogen influx from these sources would be fairly constant and independent of lake level.

Considering the importance that both phosphorus and nitrogen have for the plants and animals that make up the aquatic food web, it is important to analyze phosphorous data in Lake Merced.

Based on parameters previously analyzed, it is possible to mention that there is not a significant lake-to-lake difference. With the exception of the North Lake in parameters such as plankton count and ammonia. These parameters are correlated to each other, and the use of fertilizers in the golf course might be the main explain the dynamic of the Lake.

Phosphorus

Since phosphorous is the nutrient in short supply in most fresh water bodies, even a modest increase in phosphorus can, under the right conditions, set off a whole chain of undesirable events in a lake including acceleration of plant growth, algae blooms, low dissolved oxygen, and the death of certain fish, invertebrates, and other aquatic animals (Margaleff 1996, Chapman 1997).

There are many sources of phosphorus, both natural and human. These include soil and rocks, wastewater treatment plants, runoff from fertilized lawns and cropland, failing septic systems, runoff from animal manure storage areas, disturbed land areas, drained wetlands, water treatment, and commercial cleaning operations (USEPA 1991).

Phosphorus data from 1997-1999 are analyzed below. The total phosphorus measurements below include all the forms of phosphorus (orthophosphates, condensed phosphates, and inorganic phosphates) (APHA 1992).

 

Figure 13. Total phosphorus at Lake Merced (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

There is a clear seasonality in these data as the highest values are reported during summer while the lowest values are reported during winter time. Comparing figures 12 and 13, it is possible to see how the dynamic of nitrogen ammonia and phosphorus is different.

Droughts during summer increase algae productivity due to the decrease in lake volume and the high concentration of phosphorus in the lake (See Figures 11 and 13). The main phosphorus source might be runoff from fertilizers used on the golf courses (Friends of Lake Merced 2000). After March 1998, there is a significant difference in phosphorus values between North –East lakes and the South Lake. It is possible that different water inputs and chemicals could have increased the phosphorus in the North-East lakes. Also, the volume of each lake could affect the phosphorus values in each water body. South lake has a volume of 2,150 acre/feet, while North and East Lakes have a volume of 860 acre/feet (Yates et al. 1990). Because the amount of water in the South Lake is higher than the North and East Lakes the concentration of phosphorus in the South Lake is dissolved and reduced.

Alkalinity

Alkalinity is a measure of the capacity of water to neutralize acids. Alkaline compounds in the water such as bicarbonates, carbonates, and hydroxides remove H+ ions and lower acidity of the water. Without this acid-neutralizing capacity, any acid added to a lake would cause an immediate change in the pH. Measuring alkalinity is important in determining a water body ability to neutralize acid pollution from rainfall or wastewater. Alkalinity is influenced by rocks and soils, salts, certain plant activities, and certain industrial wastewater discharges (USEPA 1991). Figure 14 shows alkalinity data at Lake Merced.

Figure 14. Alkalinity at Lake Merced (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

Figure 14 shows the lake alkalinity values between 142 and 304 mg/l of CaCO3. Literature reports that a typical freshwater alkalinity value is 150 mg/L, and observed ranges are between 5-250 mg/L (Mays 1996). Comparing these values with the ones reported at Lake Merced, the values in the lake are generally in the range of 150 to 250 mg/L being within the freshwater alkalinity observed ranges. Even though there are no data for the East Lake before 1997, it is possible to identify the differences in alkalinity readings between the North and the East Lake and South Lake. Again the size of the lakes could explain the lake-to-lake alkalinity difference. Some water inputs into the lake, specifically those close or in the North and East Lake, could increase alkalinity readings. Camp Dresser and McKee (1999) identify stormwater overflow from Vista Grande Channel, combined sewer overflow, stormwater from the surrounding watershed, pesticides/herbicides, and unplanned treated water discharge as possible contaminant sources in Lake Merced. All of these inputs may affect and increase alkalinity, but the exact cause remains unclear.

Conductivity

Conductivity is a measure of the ability of water to pass an electrical current. Conductivity in water is affected by the presence of inorganic dissolved solids such as chloride, nitrate, sulfate, and phosphates anions (ions that carry negative charge) or sodium, magnesium, calcium, iron, and aluminum cations. Conductivity is affected by temperature: the warmer the water, the higher the conductivity. Conductivity is also affected by the geology of the area. Discharges can change the conductivity on a lake depending on their make-up. Discharges could raise the conductivity because of the presence of chloride, phosphate, and nitrate. Conductivity is measured in micromhos per centimeter (µmhos/cm) or microsiemens per centimeter (µS/cm) (USEPA 1991). Conductivity of freshwater has a range from 10 to 1,000 µS/cm but may exceed 1,000 µS/cm, especially in polluted waters, or those receiving large quantities of land run-off (Chapman 1997). Figure 15 shows the conductivity at Lake Merced from 1997 until 1999.

Figure 15. Conductivity at Lake Merced  (Source: San Francisco Water Department 1960, 1970, 1980, 1997, 1998, and 1999).

The above figure shows that the conductivity range at Lake Merced during the sampling period has been between the 490 and 902 µmhos/cm range, well within the normal values for freshwater. There is a significant lake-to-lake difference in conductivity values between North-East Lake and South Lake. The North and East lakes have a higher conductivity values (689-902 µmhos/cm), as compared with the South Lake (490-694 µmhos/cm). The size of the lakes could play an important role in the values of conductivity explaining the differences in readings. Overall, the conductivity increases in the lake during winter time. Therefore, it is possible that runoff may be one of the principal sources affecting conductivity. Moreover, other inputs could play an important role in increasing the conductivity readings, for example groundwater, stormwater overflows, combined sewer overflow, and stormwater from surrounding watershed.

Lead

Lead analyses are important at Lake Merced due to the location of the shooting club located on the western side of the South. Ecology and Environment (1993) reported that impacts from the shooting clubs are minimal in regard to emergency water use. In water, lead is most soluble under conditions of low pH, low alkalinity, low organic content, low concentrations of suspended sediment, and low concentrations of salts of calcium, iron, manganese, zinc, and cadmium. These conditions are the opposite of the conditions in Lake Merced. Low lead solubility is supported by measurements made of lead concentrations in lake waters. In 1989, 1990, and 1991, lead in lake water samples was measured at 0.003 mg/L, less than 0.001 mg/L, and less than 0.001 mg/L respectively. Therefore, the lead concentrations measured are below the Drinking Water Standard of 0.05 mg/L (GEO/Resource Consultants 1999).

Ecology and Environment (1993) included in their study sampling and analysis of soils, sediments, plants, and benthic invertebrates. Investigations of lead distribution in soils and sediments was conducted by sampling within 100-foot by foot grid system. The geometric mean of surface soil and sediment samples was 265 ppm and 143 ppm, respectively. Maximum soil and sediment concentrations at the surface layer were 19,000 ppm and 1,200 ppm, respectively. Background surface soil and sediment sample geometric mean was 27.7 ppm and 38.9 ppm, respectively. The highest concentrations of lead was found in the central area and the northwest end of the gun club. Lead in biota was greatest in aquatic plants. A maximum concentration of 222 ppm was found in coontail, a submerged weed. Lead was also elevated in benthic invertebrates at 52.8 ppm in snails attached to the coontail. Although lead occurs above background levels in benthic invertebrates and vegetation, there did not appear to be to be any substantial effects of lead on these species during the Ecology and Environment (1993) report. The ecological impacts of lead which could potentially affect beneficial uses (fish and wildlife habitat maintenance) are considered to be minimal or low for benthic invertebrates, aquatic plants, soil invertebrates, terrestrial plants, and water fowl through the food chain (Ecology and Environment 1993).

Because of their feeding habits, diving ducks are susceptible to pick up lead shot. Lead shot could produce weakened birds possibly causing starvation, affect their reproduction, or even die (Ecology and Environment 1993).

Coliform Bacteria

Members of the Coliform bacteria are used as indicators of possible sewage contamination because they are commonly found in animal and human feces. Although they are generally not harmful themselves, they indicate the possible presence of pathogenic (disease causing) bacteria, viruses, and protozoans that also live in human and animal digestive systems (USEPA 1991).

Considering that Lake Merced is a potential emergency water supply and has several recreational uses, data about total Coliforms are very important. There are weekly records available for total coliform counts since 1958, from the San Francisco Water Department. However, to provide information of total coliforms in Lake Merced, values reported by Camp Dresser and McKee (1999) taken at the surface of the North and South lakes are presented.

Table1. North and South Lake Total Coliforms in Lake Merced

Total Coliforms

North Lake Average

South Lake Average

MPN 100 ml

397

457

MPN: Most probable number.

For recreational purposes, the total coliform values in freshwater should be below 1000 MPN/100 ml (Coffman Comm.per. 2000)Therefore, total coliform values in both lakes are within the recreational limit. However, for drinking water purposes, total coliform values should be 0 MPN/100ml. Thus, both lakes should be treated to reduce total coliform values, if they were to be used to supply drinking water.   The values previously reported are probably due to the incidence of animals in the lake (birds) (Dingman comm. per. 2000).

CONCLUSIONS

1. Pysically, Lake Merced has changed drastically in the recent past, both by natural events (earthquakes and coastal processes) and human alteration (primarily the Spring Valley Water Company and the ground water pumping).

2. The heart of the problem appears to be how much water is in the lake. However, this is not that easy to determine because it is connected to a much larger aquifer that is harder to assess. Trying to make sense of the units that measure the volume, area and elevation of the lake is hard enough and surely boggles the decision makers when it is explained to them.

3. Besides issues associated with water quantity, water quality is of great concern. Lake Merced water quality is governed in part, by characteristics common to many shallow surface water bodies. In part, the formation of algae in Lake Merced is common of many lakes.

4. Lake Merced water quality is alkaline, turbid, with high organic content and more phosphorus than nitrogen. Because conditions are frequently windy and water depth is fairly shallow, the water in Lake Merced is probably well mixed most of the time. In some parameters such as phosphorus, alkalinity, and conductivity, there is a significant difference between North-East lakes and South lakes. Factors such as inputs and the lakes sizes may explain the differences.

5. Balancing the beneficial uses shouldn’t be that hard as they appear to be complimentary. Continuing to supply other uses outside of the immediate lake area is going to be the hard part as they compete with lake uses. Unfortunately, a natural connection to the ocean seems unlikely.

6. The future of the lake’s water will prove to be exciting. AB3030 should ensure that the interested parties continue to work together. Also the Friends and the Committee should provide the necessary public oversight to prevent corrupt or skewed decisions over the use of the water in the lake and below the ground. Regardless, the future is always uncertain, so stay tuned to find out what happens.

Water Quality References

APHA. 1992. Standard Methods for the examination of water and waste water. 18th
edition. American Public health Association, Washington, DC.

Brown, Nate, Pezzetti, Toni, Carlson, Fritz and Dumas, Leslie, 1997. San Francisco Groundwater Master Plan Technical Memorandum TM-18. Prepared by CH2Mhill for the San Francisco Public Utilities Commission.

CH2MHill – AGS, Inc., an association, 1997.   "San Francisco Water Department Groundwater Master Plan, Lake Merced Field Study." Technical Memorandum 17, Vol. 1.  San Francisco Water Department.

Camp Dresser & McKee. 1999. Lake Merced Water Sanitary Survey November 1999 Report.  San Francisco Public Utilities Commission . .

Campo, Jon. 2000. Lake Merced, San Francisco. From the pamphlet created by the San Francisco Recreation and Park District, Natural Areas Program.

Chapman, Deborah. 1997. Water Quality Assessment. A Guide to the Use of Biota, Sediments and water in Environmental Monitoring. Second Edition. E & FN Spon, London.

Coffman, Doug. 2000. Personal Communication. Certified Laboratory Technician. San Mateo County Health Department. (650) 573-2500.

Dingman, Dave. 2000. Personal Communication. San Francisco Public Utilities Commission. Water Quality Bureau Laboratory. (650-872-5960)

Ecology and Environmental. 1993. Environmental Lead characterization. Pacific Rod and Gun Club. Lake Merced, San Francisco, California. Final Report.

Effler, S. W., C. M. Brooks, M. T. Auer, and S. M. Doerr. 1990. "Free Ammonia and Toxicity Criteria in a Polluted urban lake" Journal Water Pollution Control Feb 62: 771-779.

EIP Associates 2000. Significant Natural Resource Areas Management Plan. Prepared for Natural Areas Program, San Francisco Recreation and Parks Department. 

Friends of Lake Merced webpage. 2000. www.lakemerced.org. (April-May 2000)

Geo/Resource Consultants, Inc., 1993. Lake Merced Water Resources Planning Study. San Francisco Water Department in association with Montgomery/Watson, Jones and Stokes Asso., Inc. Public Affairs Management. S.F., CA GRC Project No. 1756-00

Gilliam, Harold, 1967. The Natural World of San Francisco.  Double Day and Company, Garden City, New York.

Holland, Robert F. and the California Department of Fish and Game, 1986. Preliminary Descriptions of the Terrestrial natural Communities of California. October, 1986. State of California, Resources Agency, DFG. Sacramento.

King, Michael J. 1994.  "Westside Basin Field Investigation Program."  Technical Memorandum 8 Vol. 1.  AGS, Inc for the San Francisco Water Department.

Law, Patrick. 2000. Personal Communication. San Francisco Water Department. (650-872-5961).

Ludlow, Lynn, 1994. "Mercy for Lake Merced." San Francisco Examiner, Editorial, February 9, 1994.

Margaleff, R. 1996. Limnology Now. A Paradigm of Planetary Problems. Elsevier, Amsterdam. pp. 220-222.

Mays, L. W. 1996. Water Resources Handbook. McGraw-Hill. New York, p: 9.2

National Research Council. 1992. Restoration of Aquatic Ecosystems. Committee on Restoration of Aquatic Ecosystems. National Academy Press, Washington, D. C.

Pezzetti, Toni and Bellows, Michele. 1998. Feasibility Evaluation of Alternatives to Raise Lake Merced.  San Francisco Public Utilities Commission by CH2M Hill and the Duffey Co., San Francisco, CA.

Shoup, Lawrence H., and Baker, Suzzane. 1981. Cultural Resource Overview: Lake Merced Transport. San Francisco Clean Water Program, SF, CA. SWRCB No. C-06-1102-0:13. Prepared by Archaeological Consultants, 1464 La Playa, San Francisco, California.

San Francisco Public Utilities Commission & San Francisco Recreation & Park Department 1998. Lake Merced Comprehensive Management Plan.

San Francisco Water Department. 1960, 1970, 1980, 1997, 1998, and 1999. Report of Laboratory examination. Millbrae, CA.

The California Water Quality Control Board. 1995. San Francisco Bay Basin Water Quality Control Plan. June 21, 1995.

USEPA. 1991. Volunteer lake monitoring: A methods manual. EPA 440/4-91-002. Office of Water. U. S Environmental Protection Agency, Washington, DC.

Wetzel, R. G. 1983. Limnology. Sounders College Publishing. Harcourt Brace Jovanovich College Publishers, New York.

Yates, Eugene B., Hamlin, Scott N. and Horowitz-McCann, Lisa, 1990. Geohydrology, Water Quality and Water Budgets of Golden Gate Park and the Lake Merced Area in the Western Part of San Francisco, California. U.S. Geological Society, Water Resources Investigation Report 90-4080. Prepared in Cooperation with the S.F. Water Department. Sacramento, CA 1990.