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San Joaquin RiverSan Joaquin River

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3132

November 2012 Printed on recycled paper

Understanding Pesticides in California’s Delta

Delta Watershed:

•	 6 million people (2009 estimate).

•	 4 million acres agricultural land.

•	 400 pesticides applied for total of

10 million pounds each year.

Decades of USGS research have provided a good

understanding of when, where, and how pesticides enter

the Delta. However, pesticide use is continually changing.

New field studies and methods are needed so that scientists

can analyze which pesticides are present in the Delta, and

at what concentrations, enabling them to estimate exposure

and ultimate effects on organisms. Continuing research

will provide resource managers and stakeholders with

crucial information to manage the Delta wisely.

Pesticide Sources 

•	 Pesticides are applied in

agricultural and urban areas

to control unwanted insects,

plants, and fungi.

•	 The amount and type of

pesticides used changes as

pesticides are withdrawn

from the market, cropping

patterns and pest pressures

change, and new chemicals

are developed.

•	 Pesticide application,

followed by runoff from

rain or irrigation, creates

well-defined seasonal

patterns of pesticide

concentrations in Delta

water channels.

•	 Waters carrying pesticides

enter the Delta from the

Sacramento and San

Joaquin Rivers, Yolo

Bypass, surrounding creeks,

and within-Delta lands.

•	 Changing water use and

climate affect pesticide

input and transport through

the Delta.

The Sacramento-San Joaquin River Delta (Delta) is the

hub of California’s water system and also an important

habitat for imperiled fish and wildlife. Aquatic organisms

are exposed to mixtures of pesticides that flow through

the maze of Delta water channels from sources including

agricultural, landscape, and urban pest-control applications.

While we do not know all of the effects pesticides have

on the ecosystem, there is evidence that they cause some

damage to organisms in the Delta.
WINTER

Orchards

Rainfall

SPRING

Alfalfa

Rainfall & Irrigation

SPRING-SUMMER

Rice

Field Water Release SUMMER

Row/Truck Crops

Irrigation

How do Pesticides Move into the Water?

Resulting in Distinct Seasons of Pesticide Occurrence

PESTICIDES ARE 

APPLIED

THEN TRANSPORTED BY

WATER AND SEDIMENTS 

INTO WATERWAYS

ALL YEAR

Urban Use

Rainfall & Irrigation
How Are Living Organisms Affected?

Pesticide Concentration

Short time at high concentration

Long time at low concentration

Time

EXPOSURE = CONCENTRATION * TIME

CH2 Cl

S

O

N

CH3

H2

C

H3C

H3C

CH2CH3

N

CH3

CHCH2OCH3

COCH 2CI

CH3

N

S

O

CH2

CH3

PESTICIDES

Causes mortality or decreases growth

Changes quantity or quality of food

Direct Effects

Indirect Effects

ORGANISMS LIVING IN THE 

DELTA ARE EXPOSED TO 

PESTICIDES

EFFECTS CAN BE DIRECT OR 

INDIRECT

Both pesticide concentration and the length of time that

an organism is exposed to a pesticide determine the

eventual effect it has on the organism. Some pesticides

move through the Delta quickly (hours to days), but

are present at high concentrations. Other pesticides are

present for longer periods (weeks to months), but at

much lower concentrations. Both types of exposure can

affect an organism, so it is important to know specific

environmental pesticide concentrations and durations.

The most obvious effect of pesticide exposure on

organisms is death, usually, as a result of direct exposure

to high concentrations of pesticides over a short time. But,

exposure to lower concentrations over a longer time can be

just as deadly. Decreased growth, impaired reproduction,

and inhibition of swimming performance can all diminish

population size. Pesticides in the environment can also

have indirect effects, which result in problems such as

reduction in availability or quality of food.

USGS Pesticide Fate Research Group

Water is analyzed because this is where most animals live. Sediments are

important for organisms that live in, or at the bottom of, streams. Pesticides

can also “move up the food chain” as organisms eat animals or plants

that have already accumulated pesticides. To gain a more complete view

of environmental effects of pesticides on organisms, we analyze water,

sediments, and animal tissue.

Effective pesticide analysis requires persistent method

development and research. The USGS Pesticide Fate

Research Group is a leader in current-use pesticide

science. Researchers frequently add new compounds

to USGS methods to keep up with changing pesticide

use and new environmental concerns. Laboratory

methods are sensitive enough to precisely and

accurately measure pesticide concentration levels that

are very low but that still may affect living organisms.
Authors: Kathryn M. Kuivila, kkuivila@usgs.gov, and

James L. Orlando, jorlando@usgs.gov

Contact: California Water Science Center;

6000 J Street, Placer Hall; Sacramento, CA 95819

http://ca.water.usgs.gov/ 

For More Information: http://ca.water.usgs.gov/projects/toxics/

Designed by Yvonne Roque

Reference: Kuivila, K.M. and Hladik, M.L., 2008. Understanding the occurrence and transport of current-use pesticide in the San Francisco Estuary Watershed, San Francisco Estuary and Watershed Science, v. 6 (3), article 2.

Science to Help Resource Managers

Summarizing Our Current Understanding of Pesticides in the Delta

Four major seasonal patterns of riverine inputs of pesticides to the estuary were identified on the basis of application

timing and transport mechanism. In contrast, urban inputs took place throughout much of the year. Identified data

gaps included the lack of analysis of many pesticides (or degradates), changing pesticide use, limited information on

pesticide transport within the Delta, and an incomplete understanding of the transport and persistence of sedimentassociated pesticides (Kuivila and Hladik, 2008).

State-of-the-Art

Analytical

Laboratory

Proven Field 

Sampling

Methods Understanding 

of Delta 

Hydrology

Knowledge of 

Pesticide-Use 

Practices

Knowledge of

Chemical

Processes

Continuing to Conduct Focused Research on Pesticides

ADAPTING TO CHANGING USE

As pesticide use changes and new

chemicals come on the market, the

timing and concentrations of

pesticides entering the Delta will

change. Routine monitoring and

advanced methods to analyze new

pesticides provide current information

on the complex mixture of pesticides

present in the ecosystem.

EVALUATING PESTICIDE FATE

Pesticides entering the Delta can be

dissolved in the water or attached to

sediments, and can also degrade to

new compounds of potential concern.

Focused field and laboratory studies

provide information on the factors

controlling these processes, which

in turn influence the transport and

persistence of pesticides.

ASSESSING FISH EXPOSURE

In the complex Delta, it is difficult

and expensive to adequately

characterize fish exposure by

measuring pesticide concentrations

in water and sediments. A different

approach is to “ask the fish” by

directly analyzing levels of pesticides

and their metabolites in fish tissue

with sensitive analytical methods.

Through ongoing collaborations with toxicologists, ecologists, and fisheries biologists, we 

can address questions of concern such as “how do pesticides affect native fish populations in 

the Delta and the ecosystem as a whole?”

Using Our Expertise to Address Data Gaps