he Sweetwater Reservoir has 

been supplying water to local residents since 1888 and is presently 

operated by Sweetwater Authority (Authority) under the guidance of its 

board of directors. The Authority is the 

third largest water retailer in San Diego 

County and serves residential and commercial customers in Chula Vista, 

National City, and Bonita, California. 

The water is supplied from a combination of sources, including local surface 

runoff, ground water, and imported 

water from the San Diego County Water 

Authority. The surface runoff—and 

when needed, imported water—are 

stored in Sweetwater Reservoir, which 

is located about 9 miles southeast of San 

Diego, California. Runoff from the 

upper watershed is stored in Loveland 

Reservoir, which is located about 19 

miles east of Sweetwater Reservoir, 

near Alpine, California. The Sweetwater 

River watershed contains these two reservoirs and covers an area of 180 square 

miles (fig. 1).

The Sweetwater River watershed is 

undergoing a rapid change in land-use 

patterns as urbanization increases. The 

watershed includes two Indian reservations; part of the Cleveland National 

T

Forest; agricultural land; rural, urban, 

and suburban residential developments; 

mining and industrial land use; and 

commercial recreation and business 

developments. The watershed also 

includes four golf courses. The Authority, very concerned with identifying all 

factors that can degrade water quality, is 

implementing ways to protect the watershed. These efforts include education of 

watershed stakeholders, studies to elucidate the relation between water quality 

and watershed characteristics, and the 

construction of an urban runoff diversion system. These programs help the 

Authority to evaluate the env

health of the watershed by m

changes that may degrade th

the water supply and by app

tional water treatment if nec

report describes a cooperativ

toring study by the Authority 

U.S. Geological Survey (US

designed to assess the impac

ing urbanization on the wate

Sweetwater Reservoir. 

One urban project that has 

tial to affect the water qualit

water Reservoir is the constr

operation of State Route 125 

In 1984, the San Diego Asso

Governments added SR-125 

Regional Transportation Pla

San Diego’s future freeway 

SR-125 alignment connects 

SR-905 (fig. 1). The initial p

be a four-lane tollway that w

expanded to eight lanes (Cal

1996). More than 200,000 v

day, including a high percen

than 10 percent) of heavy di

from both the United States a

are expected to travel the SR

ment when it is completed. T

ment will be upwind of the r

elevated about 100 feet at its 

point, and within 500 feet of 

voir at its nearest point. Beca

factors, the Authority is conc

vehicle emissions, as well as 

might enter the reservoir by at

deposition in concentrations 

governmental health standar

increase the cost of treating t

water. 

USGS Monitoring Progra

In 1998, the USGS began 

monitor the effects of the co

and operation of the SR-125 

on the water quality in Swee

ervoir and to assess the over

the watershed with respect to 

contamination. Three enviro

compartments—air, water, a

ments—in the Sweetwater w

regularly sampled for chemi

nants:

• Air—sampled for volatile 

compounds (VOCs), poly

Water-Quality Monitoring of Sweetwater Rese

Figure 1. Sweetwater and Loveland Reservoirs, the surrounding watershed, and the proximity 

of SR-125 to Sweetwater Reservoir.

Sweetwater Reservoir Study

Objectives

• Assess the chemical impact on water 

quality in the reservoir resulting from 

land-use changes and development in 

the watershed.

• Compare chemical concentrations of 

samples from air, water, and bed sediment, and determine if any changes in 

reservoir water quality are the result 

of atmospheric deposition of organic 

chemicals and metals originating 

from the construction and operation 

of SR-125.

117° 00 116° 30

32° 45

32° 52 30

Alpine

Loveland

Reservoir

Sweetwater

Reservoir 0 2 4 Kilometers

0 2 4 Miles

Sweetwater River

Road

Stream

SR-125 alignment

Watershed boundary

Reservoir

'

' 

'

'

''

to SR-905

S17 125

94

94

79

8

8

8

 Los

Angeles

 San

Diego

 San

FranciscoCALIFORNIA

Study

Area
aromatic hydrocarbons (PAHs), and 

pesticides.

• Water—sampled for VOCs and 

pesticides.

• Bed sediments—sampled for PAHs, 

polychlorinated biphenyls (PCBs), 

pesticides, and major and trace 

elements. 

Study Design

The study is divided into two phases. 

During Phase I, the detection frequency 

and concentration levels of select contaminants in the air, water, and bed sediments are measured. These data will 

establish baseline conditions for the 

target compounds of concern to the 

Authority. During Phase II, sampling 

will continue at the established monitoring sites during and after the construction of SR-125. The chemical 

concentrations from Phase II monitoring will be compared with those from 

Phase I to assess the inputs of organic 

compounds resulting from changing 

land-use practices in the watershed and 

will determine what impact SR-125 has 

on the water quality of Sweetwater 

Reservoir. 

Many of the sampling and analytical 

methods used in this study were developed for the USGS’s National WaterQuality Assessment Program (Gilliom 

and others, 1995). The results are 

entered into the USGS’s National Water 

Information System database so that 

water-quality data from Sweetwater 

Reservoir can be compared with data 

from other water supplies throughout 

the nation.

Methods and Results

Bed Sediment

The bed sediments in the Sweetwater 

Reservoir come from a variety of 

sources. They wash off the surrounding 

land surface and are transported by the 

Sweetwater River during rainstorms and 

water transfers from Loveland Reservoir. They also come from direct deposition of airborne dust and particles. Over 

time, these sediments accumulate and 

form a thick layer on the bottom of the 

reservoir. The chemical composition of 

the deposited sediments changes over 

time, thus reflecting the historical activity in the watershed. An analysis of discrete slices of a sediment core provides 

information on historical water-quality 

conditions (Charles and Hites, 1987). 

Sediment samples from the Sweetwater 

Reservoir were taken using a gravity 

piston corer (fig. 2). Two sediment 

cores, approximately 75 inches in length 

by 2.5 inches in diameter, were 

collected from the lower part of Sweetwater Reservoir on September 24, 1998. 

The sampling site was located in an 

undisturbed area so that the core sample 

would reflect the undisturbed, natural 

sedimentation buildup in the reservoir. 

One core was subsampled at discrete 

1.6-inch intervals, and each segment 

was analyzed for organochlorine insecticides, industrial compounds, the isotope 

cesium-137, major and trace elements, 

grain size, diatoms, and pollens. The 

second was split lengthwise and 

inspected for color, texture, odor, 

organic detritus and benthic invertebrates, and original land-surface material before the Sweetwater dam was 

built.

The age of the core sections can be 

estimated from the concentration of 

anthropogenic compounds, such as 

cesium-137 and DDT, that were introduced into the environment at known 

environment. DDT use bega

and peaked in the 1960s (Ma

Capel, 1995). Its widespread 

ued in the United States unti

banned in 1972. DDT break

two products, DDE and DDD

accumulate in sediment and 

which are also toxic and ver

to further chemical breakdow

and DDD were detected in th

water Reservoir sediments th

deposited beginning in the la

early 1950s (fig. 3). The con

of total DDT (sum of the DD

and DDD concentrations) in 

ment deposits peaks around 

Together, cesium-137 and to

concentrations allow for reli

of the sediment sections from 

the present. Once the core se

been dated, the chemical rec

use activities in the watershe

examined.

PAH compounds are anoth

chemicals that are long-lived 

environment (National Rese

Council, 1983). They are pro

during the combustion of org

stances such as coal, oil, gas

bage, and during brush and s

fires; they also are present in 

bile and diesel exhaust, as w

uncombusted petroleum prod

are present in the air as gase

Figure 2. Bed sediment coring at Sweetwater 

Reservoir.

Figure 3. Total DDT and cesium-1

tions in bed sediment core segme

Sweetwater Reservoir. ~, approx

times. Cesium-137, a radioactive byproduct of nuclear weapons testing, is 

deposited into the watershed only from 

the atmosphere. Measurable concentrations of this isotope first appeared in 

the atmosphere around 1952 with the 

advent of large-scale weapons testing, 

then peaked around 1963 (Pennington 

and others, 1973). The top of the core 

was assigned the sampling date (September 1998). 

Organochlorine insecticides like 

DDT are generally characterized by 

their persistence and toxicity in the 

000–004 

008–012 

016–020 

024–028 

032–036 

040–044 

048–052 

056–060 

064–068 

072–076 

080–084 

088–092 

096–100 

104–108 

112–116 

120–124 

128–132 

136–140 

144–148 

152–156 

160–164 

168–172 

176–180 

Core segment depth, in centimeters

0 5 10 15 20 

DDT conce

(parts per 

0 0.1 0.2 0.3 

Cesium-

(picocurie pe

T

C

~19

1998

~197
and Loveland Reservoirs, th

likely sources of MTBE are 

off and atmospheric depositi

average concentrations of M

the other VOCs detected at b

ervoirs are hundreds of time

than the California primary a

ondary drinking water stand

13 and 5 ppb, respectively (C

Code of Regulations, 1998). 

water quality in Sweetwater 

voir is very good with respec

monitored chemical contami

which are expected to increa

because the dominant source 

chemicals is urban activities

Air 

Many chemicals become a

during their manufacture or 

can be present in gaseous and 

form. The atmosphere plays 

important role in moving the

borne chemicals from their p

origin and depositing them i

Figure 4. Total PAH concentrations in bed sediment core segments from Sweetwater Reservoir.

Figure 6. MTBE water concentrations at Sweetwater 

Reservoirs, and MTBE air concentrations at Sweetwa

and prometon—are frequently detected at 

both reservoirs. The insecticide diazinon 

is also routinely detected, but only at 

Sweetwater. These pesticides are used for 

agricultural and urban purposes. Figure 5 

shows that the concentrations in water for 

these pesticides are extremely low, with 

average concentrations—0.015 parts per 

billion (ppb) simazine, 0.006 ppb prometon, and 0.007 ppb diazinon—many times 

lower than maximum contaminant levels 

and health advisories set by State and 

Federal governments (Majewski and 

Capel, 1995). Sources for these pesticides 

include runoff from agriculture use, 

maintenance of roadways and other 

rights-of-way, home and garden use, and 

atmospheric drift during and after any 

application.

VOCs also are routinely detected in 

the water at both reservoirs. Sources of 

these compounds include industrial solvents, refrigerants, aerosol propellants, 

petroleum production, dry cleaning 

operations, plastic foam production, 

and vehicle emissions. Common 

household products that contain VOCs 

include pesticides, painting supplies, 

solvents, adhesives, cleaners and 

waxes, moth crystals, air fresheners, 

fabric protectors, chlorine bleach, and 

tobacco smoke. The gasoline additive 

methyl tert-butyl ether (MTBE) is classified as a VOC and is routinely 

detected in both reservoirs at an average concentration of about 0.16 ppb 

(fig. 6). MTBE has been detected in 

almost every reservoir in California 

(California Department of Health Services, accessed February 12, 2001). 

Table 1 shows that the MTBE concentrations found in Sweetwater and 

Loveland Reservoirs are many times 

where their presence 

never intended. Dry 

tion of airborne gase

ticulate matter is gen

continual and slow p

Precipitation cleans 

atmosphere of airbor

lutants and rinses the 

face of the accumula

deposition (Majewsk

Capel, 1995). The re

runoff, including eve

has collected from th

sphere and the land s

replenishes the drink

water reservoirs. It is 

in any assessment of 

shed health to monit

atmosphere for the ty

particulate matter, as well as on street 

dust. Although they are not very soluble 

in water, they can accumulate in fish and 

other wildlife that live in and around the 

reservoir. PAHs are ubiquitous in the 

environment and are good indicators of 

urban development. Figure 4 shows a 

steady increase in PAH concentration 

over the past 70 years coincident with 

increasing urbanization over time. 

lower than the concentrations found in other 

southern California reservoirs. When water 

from other reservoirs is 

transferred into Sweetwater Reservoir, however, the chemical 

contaminants also are 

transferred. 

In other state reservoirs, MTBE water 

concentrations are generally associated with 

gasoline-powered 

water craft (Dale and 

others, 2000). Because 

there is no public boating in the Sweetwater 

Water

The water in Sweetwater 

Reservoir also comes from a 

variety of sources including 

direct rainfall, local runoff, 

and imported water. The 

chemical composition in the 

local runoff reflects the 

urban, industrial, rural, and 

natural land-use activities in 

the watershed (Lopes and 

Dionne, 1998). The water is 

sampled on a regular basis at 

various locations within 

Sweetwater and Loveland 

Reservoirs. These samples 

are analyzed for various pesticides and VOCs used today. 

Two herbicides—simazine 

Figure 5. Water concentrations of the herbicides simazine and 

prometon and the insecticide diazinon in the Sweetwater and Loveland 

Reservoirs.

1930 1940 1950 1960 1970 1980 1990 2000

Estimated date of sediment deposition

0

100

200

300

400

500

600

700

800

900

Total PAH concentration

(parts per billion)

Trend line

23 Aug.

22 Sept.

22 Oct.

21 Nov.

21 Dec.

20 Jan.

19 Feb.

21 Mar.

20 Apr.

20 May

19 June

19 July

18 Aug.

17 Sept.

17 Oct.

16 Nov.

16 Dec.

15 Jan.

14 Feb.

15 Mar.

14 Apr.

14 May

13 June

13 July

Sampling date

 0

0.005

0.010

0.015

0.020

0.025

0.030

Concentration

(parts per billion)

1998 1999 2000

Simazine at Sweetwater

Simazine at Loveland

Prometon at Sweetwater

Prometon at Loveland

Diazinon at Sweetwater

23 Aug. 

22 Sept.

22 Oct. 

21 Nov. 

21 Dec. 

20 Jan. 

19 Feb. 

21 Mar. 

20 Apr.

20 May 

19 June 

19 July 

18 Aug. 

17 Sept. 

17 Oct.

16 Nov. 

16 Dec. 

15 Jan. 

14 F

b

Sampling date

0

0.5

1.0

1.5

 Concentation 

(parts per billion)

MTBE in water

MTBE in air

1998 1999 
variety, and concentration of contaminants that are deposited and to assess 

what impact these chemicals have on the 

overall health of the watershed and its 

water quality.

The Sweetwater monitoring program 

includes an atmospheric monitoring station near the Sweetwater Reservoir. The 

air sampling data collected at this station 

establish detection frequency, temporal 

patterns, and ambient levels of selected 

airborne VOCs, PAHs, and pesticides in 

the gaseous or particle phases. This site 

was installed downwind of the proposed 

SR-125 route and upwind of Sweetwater 

Reservoir along a transect of the predominant wind direction. The station is 

currently monitoring for effects from the 

construction of the SR-54/SR-125 

junction and will continue operating 

during the construction and operation of 

has measured in Chula Vista (median of 

1.5 ppb in 1999) (California Air 

Resources Board, accessed Feb. 13, 

2001).

Summary

Sweetwater Authority is concerned 

with the quality of water it provides to 

its customers. Results from the waterquality monitoring study that the USGS 

is conducting in the Sweetwater watershed show that the contaminant concentrations in bed sediments, water, and air 

are reflected in increased urbanization. 

The bed sediments show the most dramatic evidence of this impact with a 

sharp increase of persistent organic 

chemical concentrations over the past 65 

years. Water quality is also affected by 

urbanization in the form of chemicals in 

the runoff water and deposition of airborne chemicals. The concentrations of 

the detected organic chemicals in 

Sweetwater and Loveland Reservoirs 

are all well below the guidance limits set 

by State and Federal agencies to protect 

human health. Many of these compounds are detected only because of the 

sensitive analytical methods used. 

This monitoring program provides the 

Sweetwater Authority with information 

on what monitored chemicals are 

present in the reservoirs, and at what 

concentrations. With this information, 

the Authority can assess the associated 

risks, and consider future water treatment and remediation. These results 

also help focus and support future 

efforts by Sweetwater Authority to 

protect the watershed.

References

California Air Resources Board, Annual toxics 

summary, accessed Feb. 13, 2001, at URL 

<http://www.arb.ca.gov/aqd/toxics/ 

sitepages/mtbechul.html> 

California Code of Regulations, 

Division 4, Chapter 15, Article 

64444 and 64449.

California Department of Health 

MTBE in drinking water; surf

sources, accessed Feb. 12, 200

<http://www.dhs.cahwnet.gov

chemicals/mtbe/surfacewater.

Caltrans, 1996, Fact Sheet—Sta

(From I-905 to State Route 54

Department of Transportation, 

Sacramento, Calif.

Charles, M.J., and Hites, R.A., 1

ments as archives of environm

trends, in Hites, R.A., and Eis

eds., Advances in chemistry se

Sources and fates of aquatic p

Washington, D.C., American C

Society, 558 p.

Dale, M.S., Koch, B., Losee, R.F

E.W., and Davis, M.K., 2000, 

southern California water: Jou

American Water Works Assoc

no. 8, p. 42–51.

Gilliom, R.J., Alley, W.M., and G

1995, Design of the National W

Assessment Program: Occurre

bution of water-quality conditi

Geological Survey Circular 11

Lopes, T.J., and Dionne, S.G., 1

of semivolatile and volatile org

pounds in highway runoff and 

water: U.S. Geological Survey 

Report 98-409, 64 p.

Majewski, M.S., and Capel, P.D

Pesticides in the atmosphere—

trends, and governing factors: 

Mich., Ann Arbor Press, Pestic

Hydrologic System series, v. 1

able from CRC Press]. 

National Research Council, 198

aromatic hydrocarbons—Eval

sources and effects: Washingto

National Academy Press, 476 

Pennington, M., Cambray, R.S., 

E.M., 1973, Observations on l

using fallout 137Cs as a tracer: 

v. 242, p. 324–326.

Table 1. MTBE concentrations in select southern California 

reservoirs

[Current California Primary Standard for drinking water is 

13 ppb (parts per billion)]

Reservoir Concentration 

range (ppb)

Lake Perris (Los Angeles County) 3.6–15.0

Canyon Lake (Riverside County) 7.0–14.0

El Capitan (San Diego County) 1.53–12.0

Miramar Lake 1.07–2.40

Otay Lake 1.82–5.44

San Vicente Lake 2.16–8.04

Study Area

Sweetwater Reservoir 0.044–0.897

Loveland Reservoir 0.043–0.293

Figure 7. Air monitoring station at Sweetwater 

Reservoir.

the SR-125 alignment. The site 

includes a fully instrumented 

meteorological station (fig 7) 

that records hourly averages of 

wind speed and direction, ambient air temperature and relative 

humidity at two heights, rainfall, barometric pressure, and 

atmospheric stability. 

Figure 6 shows the air concentrations of MTBE at Sweetwater Reservoir. Concentrations 

for MTBE range from 0.124 to 

2.43 ppb with a median concentration of 1.11 ppb. These values 

are consistent with those the 

California Air Resources Board 

USGS Fact Shee

August 2001

For more information contact

Michael Majewski

(916) 278-3086

majewski@usgs.gov

U.S. Geological Survey

6000 J Street, Placer Hall

Sacramento, California 95819

http://water.wr.usgs.gov

In cooperation with:

Mary Ann Mann, Director of 

Water Quality

 (619) 475-9047

mmann@sweetwater.org

Sweetwater Authority

P.O. Box 2328

Chula Vista, CA 91912-2328

http://www.sweetwater.org