Figure 2. Sweetwater Reservoir, dam, and intake tower for the Sweetwater Authority’s Perdue Water

Treatment Plant.

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 112–01

2001

Improving Water Quality in Sweetwater Reservoir,

San Diego County, California

Sources and Mitigation Strategies for Trihalomethane (THM)-Forming Carbon

US SG

Sweetwater Reservoir in San Diego

County contains natural organic carbon

that has a high potential to form hazardous disinfection byproducts during

the drinking water-treatment process.

The U.S. Geological Survey and the

Sweetwater Authority, the managers of

Sweetwater Reservoir, are investigating the sources of natural organic carbon to the reservoir and ways to mitigate potential disinfection byproduct

problems. Research results suggest

that relatively simple changes in reservoir-management and water-treatment

strategies may reduce natural organic

carbon concentrations, thereby reducing by 30 percent the formation of a major type of disinfection byproduct,

trihalomethanes (THMs).

Introduction

Drinking water can be adversely affected by certain types of natural organic

material present in the source water.

Sweetwater Reservoir is the water source

for the Sweetwater Authority’s Perdue Water Treatment Plant, which provides drinking water to Chula Vista and National

City, California (figs. 1 and 2). Chlorine,

which must be added as a disinfectant during the drinking water treatment process,

reacts with certain types of natural organic

material and naturally occurring bromide

to produce undesirable disinfection

byproducts, including trihalomethanes

(THMs; figs. 3 and 4). THMs, which

present a human health hazard, are stringently regulated by the U.S. Environmental Protection Agency (EPA). Therefore,

the Sweetwater Authority desires information about the sources of THM-forming

organic carbon and possible means to

lower the concentration of such material in

Sweetwater Reservoir. The U.S. Geological Survey and the Sweetwater Authority

El Cajon La Mesa

National

City

San Diego

Chula Vista

Sweetwater

Reservoir

Loveland

Reservoir

0 2 4 KILOMETERS

0 2 4 MILES

N

8

15

5 805 Wetland

habitat

Perdue Water

Treatment 

Plant

Sweetwater River

San Diego

Los Angeles

San

Francisco

CALIFORNIA

Figure 1. Locations of Sweetwater and Loveland Reservoirs and Sweetwater River.

Wetland

habitat

Loveland

Reservoir

Sweetwater

River

Annual water

transfer Particles

Sweetwater

Reservoir

Flooded

vegetation

THMs

form

Chlorine

added

Perdue Water

Treatment Plant

Public

drinking

water

Annual

water level

variation 

Figure 3. Four potential sources of trihalomethane (THM)-forming natural organic carbon to Sweetwater Reservoir
are cooperatively studying possible watershed management strategies to reduce the

concentration of THM-forming carbon in

Sweetwater Reservoir, and thus, improve

drinking-water quality.

The majority of natural organic carbon

in the water is present as dissolved organic carbon (DOC), with the remainder

as particulate organic carbon (POC), each

with multiple sources in the watershed.

POC and DOC may react with chlorine to

form THMs. This study is examining four

potential sources of THM-forming natural

organic carbon to Sweetwater Reservoir:

1) particles in the water, 2) flooding of

vegetation on the reservoir margins, 3) the

annual transfer of water from an upstream

reservoir, and 4) nearby wetlands (fig. 3).

No imported water was added to the reservoir during the study. The reservoir water

contains particles, including algae, that

contain natural organic carbon. Vegetation

on the reservoir margins is flooded due to

fluctuation in the reservoir water level.

During the year, as water is withdrawn

from the reservoir for use, the water level

gradually drops 4-5 meters (m) [13-17

feet (ft)], exposing almost one-half of the

reservoir bottom as dry land; vegetation

grows in these areas. The reservoir is

filled once a year during the wet season

by a transfer of water from Loveland Reservoir, 27 kilometers (km) [17 miles (mi)]

upstream on the Sweetwater River (fig.

1). Flooding of the vegetation may release

natural organic carbon into the water,

similar to steeping of tea leaves in water

to produce tea. Also, natural organic carbon may be swept out of the river channel

during the water transfer or released from

wetlands along the Sweetwater River into

the reservoir. Wetlands along the river are

important habitat, and the Sweetwater Authority maintains wetlands just east of

Sweetwater Reservoir for the least Bells’

vireo (Vireo bellii pusillus), an endangered bird.

The concentration of DOC in

Sweetwater Reservoir varies on an annual cycle from about 6 milligrams per

liter (mg/L) when the reservoir is filled,

to about 8.5 mg/L when reservoir water

levels drop (fig. 5). DOC concentration

also varies with water depth, in part due

to stratification of the water column.

Only a small fraction of the DOC present

reacts with chlorine to form THMs and

other disinfection byproducts. The

amount of this reactive fraction of DOC

in the reservoir varies by a factor of almost two during the year (fig. 5), ranging

from 400 parts per million (ppm) in the

winter to about 750 ppm in the summer

(a reactivity of 400 ppm means that 400

out of 1 million carbon atoms in the DOC

will react to form THMs during 5 minutes of reaction between added chlorine

and the DOC). The variation in reactivity

is related to the sources of the organic

carbon.

How much THM-forming carbon do

particles contribute to Sweetwater

Reservoir?

In the Sweetwater Authority’s watertreatment process, water is chlorinated

prior to filtration, and therefore particles

containing natural organic carbon are

present in the water (fig. 3). Common

types of particles include algal and bacterial biomass, flocculated humic substances, and sediments. These particles

may contribute substantially to the total

amount of THMs formed.

In an experiment comparing chlorinated filtered and unfiltered water, unfiltered water formed more THMs than the

corresponded filtered water sample (fig.

6), indicating that a significant amount of

the THM-forming carbon in unfiltered

water is derived from particles. For example, in samples collected from near the

intakes to the treatment plant, unfiltered

water formed 4 - 49 percent more of the

THMs than filtered water (fig. 6); the additional THMs formed from natural organic carbon in the particles. Furthermore, because the greatest contribution

from particles occurred during the summer and early fall, when algal populations

were greatest, it is likely algae are the

dominant particles containing THM-forming carbon. SAMPLING DATE

4/1/99 8/1/99 12/1/99 4/1/00 8/1/00 12/1/00 4/1/01 

DISSOLVED ORGAN C CAR I BON, NI MILL GI RA

MS PER LITER 6.0

6.5

7.0

7.5

8.0

8.5

REACT VI I TY, NI PARTS PER MILL ON I

0

250

500

750

1000

GAUGE HE GI HT, NI FEET

55

65

75

85

DOC

Reactivity

Water level

Figure 5. Dissolved

organic carbon (DOC)

concentration and reactivity of DOC to form

trihalomethanes (THMs) in

water samples from the

intake tower in

Sweetwater Reservoir for

the Perdue Water Treatment Plant. Reactivity is

defined as the number of

carbon atoms that react to

form THM per one million

carbon atoms in a sample

during five minutes of

reaction with added chlorine. Reactivity of DOC to

form THMs was measured

using modified versions of

EPA Methods 510.1 and

502.2. Water level at the

gauge in the reservoir is

also shown.

Chlorine added for 

disinfection (Cl2, HOCl, OCl-)

Natural organic carbon in water 

(DOC, dissolved organic carbon 

and POC, particulate organic carbon) THMs

+

CHCl3 chloroform

CHCl2Br bromodichloromethane

CHClBr2 dibromochloromethane

CHBr3 bromoform

Natural bromide 

in water (Br)

+

 Figure 4. Reaction of

natural organic carbon and

bromide with chlorine added

during water-treatment forms

trihalomethanes (THMs).
TRIHALOMETHANES FORMED FROM FILTERED ALIQUOT,

 IN MICROGRAMS PER LITER

30 40 50 60 70 80 90

TR HI ALOMETHANES NI UNFILTERED AL QI UOT,

NI MICROGRAMS PER LITER 40

50

60

70

80

90

100

Treatment plant intake

Other water samples

1:1

2:1 1.5:1 1.25:1

Figure 6. Comparison of

trihalomethane (THM) concentrations produced by

chlorination of filtered and

unfiltered aliquots of

samples from Sweetwater

and Loveland Reservoirs and

the Sweetwater River. The

heavy line labelled 1:1 shows

where samples plot if unfiltered and filtered aliquots

formed the same amount of

THM. Lighter lines indicate

where samples plot if the

unfiltered aliquot forms 1.25,

1.5, and 2 times as much

THM as the filtered aliquot.

WATER DEPTH, IN METERS

0 2 4 6 810 12 14 16

D SI SOLVED ORGANIC CARBON,

NI MILLIGRAMS PER LITER 4

8

12

16

20

24

Sites flooded 

by filling of

Reservoir 

Sites always submerged

Sweetwater Reservoir

February and March 2000

Water level

increase as a

result of annual

water transfer

ing the mowed vegetation as the source

of the higher DOC. Calculations indicate

that 1 month after the annual water transfer filled the Reservoir, DOC from the

degradation of the mowed vegetation

constituted 6 percent of the DOC in the

Reservoir as a whole. Furthermore, this

DOC, which has a reactivity of 1,000-

2,000 ppm, has two to five times more

capacity to form THM than does the average DOC in the reservoir (compare

figs. 5 and 8). As a result, DOC from the

degradation of the mowed vegetation in

the first month after flooding may account for 10 percent of the THM-forming

carbon in the reservoir.

These results suggest that harvesting the

vegetation and removing it from the reservoir margins prior to flooding could improve water quality by reducing THM formation 10 percent. This study did not assess the contribution from the organic-rich

soils in the flooded areas.

How much THM-forming carbon does

the annual transfer of water from

Loveland Reservoir contribute to

Sweetwater Reservoir?

Sweetwater Reservoir is filled annually in early winter with water released

from Loveland Reservoir, which is 27

km (17 mi) upstream on the Sweetwater

River (figs. 1 and 3). During the release,

water flows in the Sweetwater River for

2 weeks; the riverbed is dry most of the

rest of the year. The water in Loveland

Reservoir is of higher quality than that in

Figure 7. Vegetation and organic-rich soils near Sweetwater Reservoir in July

1999. The vegetation was mowed in November 1999 after the soil dried, and the

area was flooded by rising water during filling of the reservoir in February 2000.

Figure 8. Dissolved organic carbon (DOC) concentrations in water from

Sweetwater Reservoir. Samples were collected during the first month following the annual water transfer that fills the Reservoir. Water depth refers to

the total water depth at the sampling site, and samples collected from

different levels in the water column at a site are plotted according to total

water depth. Samples were collected from ten sampling sites in the reservoir.

This comparison indicates that an average of 20 percent reduction in THM formation may be possible by filtering water

prior to chlorination. This study did not

determine the sources of all the particles

containing the THM-forming natural organic carbon.

How much THM-forming carbon does

flooded vegetation contribute to

Sweetwater Reservoir?

The maximum water level in

Sweetwater Reservoir usually is reached

in late winter, after the annual water transfer from Loveland Reservoir and the conclusion of the rainy season. The water

level drops as water is withdrawn for use,

and the minimum water level usually is

reached in late fall. The receding water

level exposes highly fertile sediments

around the margins of the reservoir, particularly at the eastern end, which may

comprise up to one-half of the maximum

reservoir surface area (fig. 3). Vegetation

quickly grows in these areas (fig. 7) and is

dominated by volunteer stands of smartweed and grasses, which may be 2 m (6.5

ft) tall when the reservoir is filled the following year. Currently, the vegetation is

mowed to prevent plant succession and

infilling of the reservoir. As the water

level rises and the margins of the reservoir

are inundated, the newly flooded, mowed

vegetation degrades and releases DOC

into the water.

Water covering the newly flooded areas of the reservoir bottom has consistently higher DOC concentrations than

water from areas of the reservoir that always were under water (fig. 8), implicat-
Printed on recycled paper

Miranda S. Fram, Brian A. Bergamaschi, and Roger Fujii

Sweetwater Reservoir because it contains

lower concentrations of DOC and bromide (a naturally occurring salt ion). To

determine the quantity of DOC and bromide added to the water as it flows from

Loveland to Sweetwater Reservoir,

samples were collected at sites along the

Sweetwater River during the winter 2000

transfer and analyzed for DOC concentration, THM formation potential, and

specific conductivity. For this study, specific conductivity was used as a surrogate measurement for bromide (a linear

relation between specific conductivity

and bromide was observed for a subset of

water samples).

The results indicated that the leading

edge of the transferred water as it entered

Sweetwater Reservoir contained as much

as five times higher DOC and bromide

concentrations than the water in Loveland

Reservoir. After several days of flow, these

high concentrations decreased to levels

close to those in Loveland Reservoir. Calculations of loads indicate that during the

winter 2000 transfer, 60 percent more bromide and 11 percent more DOC were

added to Sweetwater Reservoir than would

be expected for a “clean” transfer of water

from Loveland Reservoir. The additional

DOC was from natural organic debris

swept from the Sweetwater River channel,

and the bromide was from ground water

flushed out of the shallow aquifer beneath

the channel.

These results suggest that it may be possible to prevent some of this THM-forming DOC and bromide from reaching

Sweetwater Reservoir. Transferred water

flowing into the Reservoir within the first

24 hours contains the highest concentrations of DOC and bromide, and this DOC

has a larger reactive fraction than the average DOC in the reservoir (750-1,200 ppm

compared with 400-750 ppm). At the expense of losing about 2 percent of the

transferred water, diverting water into the

existing urban runoff diversion system

around Sweetwater Reservoir during the

first 24 hours of transfer would reduce the

THM formation potential of the entire reservoir by about 2 percent.

How much THM-forming carbon does the

wetland habitat area contribute to

Sweetwater Reservoir?

The eastern end of Sweetwater Reservoir contains cottonwood-willow wetland, open pond, and marsh areas maintained by the Sweetwater Authority for

wildlife habitat, particularly for the least

Bells’ vireo, an endangered species (figs.

1 and 3). In many systems, wetland

habitats can be important contributors to

THM formation because they produce

large concentrations of DOC with a high

capacity to form THM. However, this

study suggests that the wetland habitats

currently are not a major source of DOC

to Sweetwater Reservoir. During the annual water transfer from Loveland Reservoir, DOC concentrations upstream

and downstream of the habitat area were

not appreciably different, as they would

have been if significant amounts of DOC

were added during flow through the wetlands. The extent of the wetlands habitat

is only 6 percent of the river length between the two reservoirs. Although the

wetlands ponds contain high concentrations of DOC with a large reactive fraction, there is negligible overland water

flow from the wetlands into Sweetwater

Reservoir during most of the year, and

thus, minimal wetland DOC seems to

enter the reservoir.

Wetland habitat

INSIGNIFICANT SOURCE

Loveland

Reservoir Sweetwater

River

Annual water transfer

MINOR SOURCE

DIVERT FIRST 24 HOURS

OF WATER TRANSFER

Sweetwater

Reservoir

Flooded

vegetation

IMPORTANT SOURCE

HARVEST VEGETATION

Public

drinking

water

Particles

IMPORTANT SOURCE

ADD FILTRATION PRIOR 

TO CHLORINATION

fewer

THMs

form

Chlorine

added

Perdue Water

Treatment Plant

Summary

The Sweetwater Authority has undertaken a cooperative study with the U.S.

Geological Study to evaluate watershed

improvements that would support its commitment to providing high-quality drinking water to the residents of Chula Vista

and National City, California. This study

provides information about the sources

and processes contributing natural organic

carbon and bromide to Sweetwater Reservoir, which can react with chlorine during

water treatment to produce undesirable

disinfection byproducts, such as

trihalomethanes (THMs). The main results

of this study are:

Three strategies could reduce the average THM formation potential in

Sweetwater Reservoir water by about 30

percent (fig. 9):

• Filtering particulate material from water

prior to chlorination may reduce THM

formation by 20 percent, on average.

• Harvesting vegetation from the reservoir margins that are flooded when

water level rises may reduce THM formation by 10 percent.

• Diverting the first 24 hours of flow

during the annual water transfer from

Loveland to Sweetwater Reservoir

around the reservoir may reduce THM

formation by 2 percent.

Wetlands maintained for species habitat

at the eastern end of Sweetwater Reservoir

currently are not a major source THMforming carbon to the reservoir.

Figure 9. Relative importance of and possible mitigation strategies for the four

potential sources of

trihalomethane (THM)-

forming organic carbon to

Sweetwater Reservoir. Red

type indicates the relative

importance of each source

and green type indicates a

potential mitigation strategy.

Reducing the input of THMforming natural organic

carbon and bromide from

these sources could result in

a 30 percent decrease in the

amount of THM formed

during the drinking water

treatment process.

COOPERATING ORGANIZATION

Mary Ann Mann

Director of Water Quality

Sweetwater Authority

P.O. Box 2328

Chula Vista, CA 91912-2328

http://www.sweetwater.org

FOR MORE INFORMATION CONTACT THE AUTHORS:

Miranda S. Fram (mfram@usgs.gov)

Brian A. Bergamaschi (bbergama@usgs.gov)

Roger Fujii (rfujji@usgs.gov)

U.S. Geological Survey

6000 J Street, Placer Hall

Sacramento, CA 95819-6129

phone number: 916-278-3000

http://ca.water.usgs.gov