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Watershed Restoration Efforts Reduced Malathion Levels in Colusa
Basin Drain
Waterbody Improvsd ^se Pestic'c'e rnalathion resulted in elevated concentrations
that were toxic to aquatic invertebrates and exceeded water
quality standards in Colusa Basin Drain. As a result, Colusa Basin Drain was placed on California's
Clean Water Act (CWA) section 303(d) list of impaired waters for malathion in 1998. The California
Central Valley Regional Water Quality Control Board (Central Valley Water Board) initiated the
irrigated Lands Regulatory Program in 2002. Outreach and education efforts were coordinated to
promote ways to address the problem. Growers implemented best management practices (BMPs)
to manage the use of malathion and other pesticides. Malathion concentrations have decreased,
resulting in the Central Valley Water Board's recommendation to remove Colusa Basin Drain from
the impaired waters list for malathion during the 2018 Integrated Report listing cycle.
Problem
The Colusa Basin watershed is in Northern California,
mainly in Colusa County, with portions spanning
to Glenn and Yolo counties. The watershed covers
approximately 1,045,445 acres and drains into the
Sacramento River at Knights Landing through the
Colusa Basin Drain (Figure 1). The Lower Colusa Basin
Drain drainage area is 36,624 acres and includes
31,472 irrigated acres (non-rice).
In the late 1990s, levels of the pesticide malathion
in Colusa Basin Drain exceeded applicable quality
standards. As a result, the segment was placed on
California's 1998 CWA section 303(d) list of impaired
waters for malathion. Levels of malathion con tinued
to be high over the next '15 years. Exceedances of
the aquatic life criterion (0.028 micrograms per liter
[[ig/L]) were observed in March 2010 and August 2011,
with a subsequent exceedance in March 2013. The
Sacramento Valley Water Quality Coalition developed
a management plan in 2012 as a requirement for the
Irrigated Lands Regulatory Program (ILRP); the plan
identified BMPs needed to reduce the offsite move-
ment of malathion to surface water.
in addition, an evaluation of potential sources
contributing to the malathion detections was com-
pleted. Based on this evaluation, agriculture is likely
a contributing source. The use of malathion pesticide
ft
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Colusa Basin Dra
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Story Highlights
Malathion in Colusa Basin Drain
The Sacramento Valley Water Quality Coalition's
Colusa Glenn Subwatershed Program and the county
agricultural departments mailed exceedance notices
to their growers that were using malathion. Workshops
were held for growers to discuss implementation of
BMPs and regulations associated with and alternatives
to malathion use.
The subsequent absence of Malathion exceedances
related to agricultural applications were attributed to
changes in practices as a result of increased awareness
of the growers and applicators, which resulted from
outreach efforts of the Colusa Glenn Subwatershed
Program and the local resource conservation districts,
Farm Bureau offices and agricultural departments.
Management practices that were implemented
included improved pesticide application practices,
irrigation practices (e.g., microirrigation systems,
improved irrigation management practices), and prac-
tices intended to manage sediment and erosion (e.g.,
vegetative buffer strips). Three contracts for financial
assistance with management practice implementation
have been funded in Colusa County, supporting the
implementation of 17 total practices across 674.5
acres.
Results
Outreach and education to growers in the area,
along with improved irrigation systems and irrigation
management, has been shown to prevent or reduce
discharges of malathion from agricultural opera-
tions—and thus prevent or reduce the discharge of
pesticides to surface waters. Specific efforts include
reducing applications of malathion during conditions
with high risk of drift or runoff, increasing the number
of practices that minimize applications of malathion
and alternative pesticides, and managing irrigation to
eliminate tail water runoff.
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Figure 2. Malathion concentrations in the Colusa Basin
Drain. Note that nondetectable samples were assigned
the method detection limit of 0.003 |ig/L.
Malathion in Colusa Basin Drain was delisted based on
water quality results that were well below the 0.028
|ig/L aquatic life criteria. Since March 2013, malathion
concentrations have been below the aquatic life crite-
ria (Figure 2). The Central Valley Water Board adopted
the delisting of the Colusa Basin Drain for malathion,
which was documented in the final staff report titled
"Section 303(d) 2018 Impaired Waters List Updates for
the Central Valley Region."
Partners and Funding
Implementing relevant practices occurred in 2016
through the U.S. Department of Agriculture Natural
Resources Conservation Service (NRCS) Environmental
Quality Incentives Program (EQIP) financial assistance
program and additional special funding with priorities
to address water quality in Colusa and Yolo counties
(Bay Delta Initiative for Water Quality Improvement in
Lower Colusa Basin Drain Representative Area). The
Colusa Glenn Subwatershed Program, in partnership
with the Colusa office of the NRCS, received $473,872
in EQIP funding during fiscal year 2015 to assist local
growers in addressing water quality issues related to
irrigation systems and irrigation management.
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U.S. Environmental Protection Agency
Office of Water
Washington, DC
EPA 841-F-20-001T
October 2020
For additional information contact:
Holly Grover
Central Valley Regional Water Quality Control Board
916-464-4747 • holly.grover@waterboards.ca.gov
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