Sampling for Pesticide Residues in California Well Water
1995 Update of the Well Inventory Data Base. EH95-06.
For Sampling Results Reported From July 1, 1994 to June 30, 1995
D. Bartkowiak, K. Newhart, M. Pepple, J Troiano, D. Weaver
About The The Pesticide Contamination Prevention Act
The pesticide contamination prevention Act (PCPA) was enacted
in 1985 to prevent further pesticide pollution of the State's
- The PCPA requires:
- The Department of Pesticide Regulation (DPR) to maintain a
statewide database of wells sampled for active ingredients of
- Agencies (government and private) to report the results of
any well sampling for the active ingredients of pesticides.
- DPR to review findings of pesticide contamination and undertake
- DPR, in consultation with the California Department of Health
Services (CDHS) and the State Water Resources Control Board (SWRCB),
to annually make this report to the Legislature, the CDHS, the
State Office of Environmental Health Hazard Assessment, and the
The Well Inventory Database
- The well inventory database was developed by DPR (then a division
of the California Department of Food and Agriculture) in 1983,
before the passage of the PCPA.
- The purposes of the database were to centralize information
on the occurrence of nonpoint source contamination of ground water
by the agricultural use of pesticides and to facilitate graphical,
numerical, and spatial analyses of the data.
- To meet the requirements of the PCPA, sampling results from
both point source and nonpoint source contaimiation are included
in the database.
What happens when detections are reported to DPR
- When a pesticide is found in ground water a well-defined process
established by the PCPA is triggered. This process allows for
comprehensive review of the detection.
- DPR refers detections to the SWRCB if the pesticide is:
- not currently registered for use;
- registered for other than agricultural, outdoor industrial,
or outdoor institutional uses; or
- found in ground water and determined not to be due to legal
- DPR attempts to verify the detection of pesticides that are
currently registered for agricultural use by conducting a well sampling study. There are
specific criteria for verification of a detection. If a detection
is verified, a determination is made as to whether the contamination
occurred because of legal agricultural use of the chemical.
- Detections may not be verified for one of several reasons,
- Follow-up sampling has not yet been completed by DPR, or follow-up
sampling was not conducted by DPR. The detection may have been
referred to the SWRCB, there may be no wells available for sampling,
or permission to sample could not be obtained from the well owner.
- Analyses of all other samples taken by DPR in response to
the positive sample were negative for the compound under investigation.
General Information about Sampling Results in the Well Inventory
- A summary of the data in the database by report year is given
in Table 1.
- The data can be used to:
- Display the geographic distribution of well sampling.
- Display the geographic distribution of pesticide residues
in sampled wells.
- Identify areas potentially sensitive to contamination by the
legal agricultural use of pesticides.
- There are limitations on interpreting the data, including:
- The data indicate which pesticides are present in well water
among those pesticides for which analyses were performed. They
do not represent a complete survey of ground water quality throughout
the state nor do they represent sampling for all pesticides.
- Sampling by agencies other than DPR is not necessarily related
to suspected agricultural sources of contamination.
The Data In This Report
This is the tenth report, and the third update to the 1992 cumulative
report on the entire contents of the database.
- Data were submitted to DPR from July 1, 1994 to June 30, 1995.
- Data are the results of 49 studies conducted by three agencies.
- Data are from studies that were conducted from 1993 to 1995.
Summary of Data
- 93,236 records (chemical analyses) were added to the database
for this report.
- 3,322 wells were sampled in 47 counties.
- 166 pesticide active ingredients and breakdown products were
- 27 compounds were reported with positive detections.
Detections Referred to the SWRCB
Detections of eight chemiclas, including three chemicals where
historical agricultural applications are considered by DPR to
be the source of residues in ground water were reported the the
SWRCB. The three chemicals and the number of wells with detections
1,2-dibromo-3-chloropropane (DBCP): 304 wells
1,2-dichloropropane (1,2-D): 11 wells, and
ethylene dibromide (EDB): 15 wells.
Summary of Verified Detections
- Six herbicides had verified detections:
atrazine diuron prometon
bromacil hexazinone simazine
- Three breakdown products of active ingredients had verified
deethyl-atrazine deisopropyl-atrazine (DIPA)
- Verified detections were made in 213 wells in 17 counties
(See Section I, Figure I-1).
- Detections were found in three types of wells
private drinking-water wells (174)
public drinking water wells (18)
non-drinking-water wells (21)
First-time Verified Detections Were Made for the Following
Chemicals and Counties:
- Atrazine in Contra Costa, San Bernardino, and San Joaquin.
- Bromacil in Contra Costa and Stanislaus.
- Simazine in Contra Costa.
- Prometon in Colusa, Contra Costa, Los Angeles, and Solano.
First-time Verified Detections of Hexazinone in California
- Tulare County (four wells): Determined by DPR to be due to
a possible point source contamination originating from the Teapot
- Solano County (one well): Only one of six sampled wells had
a verified detection of hexazinone, and no further action will
Legal Agricultural Use Determinations
- After well sampling and land use surveys are completed a determination
is made as to whether the detection of the pesticide residues
in ground water could have been due to legal agricultural use.
Specific criteria must be met for this determination to be made.
- Six compounds were found in ground water as a result of legal
atrazine diuron simazine
bromacil prometon DIPA
- 122 wells had contamination due to legal agricultural use.
- Nine counties had contamination due to legal agricultural
Fresno Orange Tehama
Los Angeles Riverside Tulare
Merced San Bernardino Yolo
- First-time determinations were made for:
Prometon in Los Angeles County.
Diuron in both Merced and Tehama counties.
Pesticide Management Zone (PMZ)
- A PMZ is a land area where a pesticide has been detected in
ground water, and where it has been determined that the contamination
was due to legal agricultural use. PMZs are established in regulation
to prevent further contamination of ground water. The use of certain
chemicals is restricted in these areas. PMZs exist for atrazine,
bromacil, diuron, prometon, and simazine.
- After evaluation of data, a total of 92 PMZs will be established
in regulation in the following counties:
Fresno Orange Tehama Ventura
Los Angeles Riverside Tulare Yolo
Merced San Bernardino
- Three previously adopted PMZs will be removed from regulation.
It was determined that these PMZs had been established based on
One atrazine PMZ in Stanislaus County.
One bromacil PMZ in Tehama County.
One bromacil PMZ in Tulare County.
Factors That Contribute To Ground Water Contamination
DPR environmental scientists continue their work to understand
the factors that contribute to ground water contamination by pesticides
used in agriculture. They conduct field studies on pesticide movement,
investigate contaminated wells, compile extensive databases, and
review the work of other scientists. The knowledge gained from
these activities is used to develop pesticide use practices that
prevent further ground water contamination. For the past several
years, EHAP scientists have been developing an approach that integrates
climatic, soil, and geographic data in analyses of their combined
influence on the movement of pesticides to ground water.
During the past year, EHAP scientists conducted well monitoring
studies and field investigations in Fresno and Tulare counties
as they continued to examine this method of identifying areas
that are vulnerable to ground water pollution by the agricultural
use of pesticides. This method may provide a basis for development
of regional agricultural management practices to reduce ground
water contamination by pesticides.
The State And Regional Water Boards
The SWRCB and nine RWQCBs are responsible for protecting the beneficial
uses of water in California and for controlling all discharges
of waste into waters of the State. Actions taken by the SWRCB
to prevent economic poisons from migrating to ground water include:
- Development and implementation of water quality plans.
- Cooperating with DPR in areas relating to pesticides and water
- Consultation and collaboration with various agencies and groups
on studies and workshops relating to pesticides, water quality,
and ground water.
- Submittal of a workplan to U.S. Environmental Protection Agency
pursuant to the Clean Water Act for 1995 funding for pesticides
and ground water-related work.
- Adapting the Pesticide Use Retrieval System database queries
of 1990 and 1991.
- Staff of the nine RWQCBs perform site contamination assessment
investigations, development and implementation of remediation
plans (including soil and ground water clean-up), and monitoring.
Table 1. Summary of well sampling results
included in the Department of Pesticides Regulation's well inventory
database, by report year, for data reported through June 30, 1995.
Figure 1. California counties with verified detections of pesticide
residues in ground water that were reported to the Department
of Pesticide Regulation during the period July 1, 1994 through
June 30, 1995.
For a copy of this report, please contact:
Department of Pesticide Regulation
Environmental Monitoring and Pest Management Branch
1020 N Street, Room 161
Sacramento, CA 95814-5624
or phone (916) 324-4100
Please refer to the 1995 Well Inventory Report.