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Diazinon and Pesticide-Related Toxicity in Urban Creeks TMDL



In the 1990s, San Francisco Bay Area urban creeks were found to exceed water quality standards for aquatic toxicity, primarily due to runoff of the common insecticide diazinon. Diazinon was commonly used throughout the Bay Area to manage a broad spectrum of pests, such as ants and grubs. Although only a small fraction of the diazinon applied outdoors reached surface water, that fraction was sufficient to result in diazinon concentrations that were toxic to test organisms. The Diazinon and Pesticide-Related Toxicity in San Francisco Bay Area Urban Creeks TMDL and supporting documents examine this water quality problem, identify sources of diazinon, and set forth actions that will lead to a solution.

As the TMDL was being developed in the early 2000s, it was evident that other pesticides with equal or greater aquatic toxicity would replace diazinon in the marketplace. Therefore, the TMDL addresses pesticide-related aquatic toxicity in general, regardless of which pesticide causes the toxicity or when the toxicity is discovered.

In 2010, Kirker Creek in Contra Costa County was found to be impaired by toxicity caused by pyrethroid pesticides, which are a category of pesticides that have replaced diazinon in the marketplace. The toxicity found in Kirker Creek is being addressed through implementation of this TMDL.

UC Davis IPM Program Drawing

TMDL Approval

The Diazinon and Pesticide-Related Toxicity in Urban Creeks TMDL was adopted by the Water Board on November 16, 2005 and State Water Resources Control Board on November 15, 2006. The TMDL was approved by the State Office of Administrative Law and became effective on January 28, 2007. On May 21, 2007, the U.S. Environmental Protection Agency (U.S. EPA) approved a Basin Plan amendment incorporating a TMDL for Diazinon and Pesticide-Related Toxicity in Urban Creeks and an implementation plan to achieve the TMDL.

TMDL Implementation:

To address pesticide-related toxicity in urban water bodies, the TMDL contains a comprehensive implementation strategy. Federal, State, and local agencies, and others, are called on to take actions to reduce the potential for pesticides to runoff into waterways. Implementation actions focus on three areas:

1) Regulatory programs: Use regulatory tools to ensure that pesticides are not applied in a manner that results in discharges that could harm the quality and threaten beneficial uses of urban creeks.

The following groups work with the California Department of Pesticide Regulation (DPR) and the U.S. EPA to bring the potential threats to water quality to the forefront during pesticide evaluation and registration processes:
    • Water Board staff submit comment letters when pesticides with the potential to harm water quality are under review by the U.S. EPA. These letters provide monitoring data, request specific analysis of the fate and transport of the pesticide to surface water, and/or comment on U.S. EPA’s modeling inputs, for example. Staff submitted nearly 50 letters on U.S. EPA pesticide registration actions from mid-2011 through 2015.
    • Municipal Stormwater Permittees’ (see Municipal Regional Permit below) work collectively through the California Stormwater Quality Association (CASQA) to participate in pesticide regulatory actions.  Recent annual summaries of outcomes of this work are presented here and here.
    • Wastewater dischargers work collectively through the Bay Area Clean Water Agencies (BACWA) in much the same way as the groups above. Click the diagram below for an informative flyer describing these actions.
    • In 2012 DPR adopted regulations to protect surface water from urban uses of certain pyrethroid pesticides. In addition, DPR now reviews the potential impacts of pesticides on water quality when a pesticide product is submitted for registration. Further information on DPR’s actions to mitigate surface water contamination from pesticide use can be found here.

2) Education and outreach programs: Attempt to decrease demand for pesticides that threaten water quality, while increasing awareness of alternatives that pose less risk to water quality.

Research shows that pesticides applied around homes according to label instructions can and do lead to toxicity in local water bodies. Education and outreach initiatives funded by State grants, wastewater and stormwater dischargers, and others promote the behavior change necessary to reduce this threat of pesticide-related toxicity in our creeks. These initiatives include:

    • Our Water - Our World provides materials, including fact sheets displayed at Bay Area hardware stores and a helpful free app, developed to assist consumers in managing home and garden pests in a way that helps protect water quality.
    • Got Ants? Get S.E.R.I.O.U.S provides easy-to-use information about controlling ants indoors and out, without using toxic sprays.
    • EcoWise Certified is an independent, third-party certification program that distinguishes knowledgeable, leading-edge licensed pest management professionals who practice prevention-based pest control.  
    • BayWise.org, hosted by Wastewater and Stormwater Dischargers, provides useful information on preventing all types of pollution where we live, work, and play, including how to find a certified pest control professional near you.



3) Research: Fill information gaps and monitor to measure TMDL implementation progress. Some research and monitoring information is summarized below.

Municipal Regional Permit
The NPDES Municipal Regional Permit (see Provision C.9), adopted November 19, 2015, requires municipalities in Alameda, Contra Costa, San Mateo and Santa Clara Counties and the cities of Fairfield-Suisun and Vallejo to take TMDL implementation actions. These Permittees must adopt and enact Integrated Pest Management policies for property and buildings they own or operate. They also must encourage residents to avoid using pesticides that pose a threat to water quality (see Education and Outreach above). To see annual reports in which Permittees summarize their recent efforts, click here. Water quality reports required by the permit are here.

Integrated Pest Management (IPM): As stated in the San Mateo Countywide Water Pollution Prevention Program’s IPM policy, IPM is an ecosystem-based strategy that focuses on long-term prevention of pests or their damage through a combination of techniques such as biological control, habitat manipulation, modification of cultural practices, and use of resistant varieties. Pesticides are used only after monitoring indicates they are needed according to established guidelines, and treatments are made with the goal of removing only the target organism. Pest control materials are selected and applied in a manner that minimizes risks to human health, beneficial and nontarget organisms, and the environment.
IPM techniques could include biological controls (e.g., ladybugs and other natural enemies or predators); physical or mechanical controls (e.g., hand labor or mowing, caulking entry points to buildings); cultural controls (e.g., mulching, alternative plant type selection, and enhanced cleaning and containment of food sources in buildings); and reduced risk chemical controls (e.g., soaps or oils).

Permittees also are sampling their creeks for toxicity and pyrethroids in both the water column and creek sediment, to which pesticides often adhere. If samples exhibit toxicity, Permittees will conduct studies to determine whether the toxicity is caused by pesticides or by another source. The data will be used to determine how well the TMDL is being implemented, and whether additional actions are needed.

Staff Updates
Periodically, staff updates the Water Board on the status of these implementation actions:


The following links are for your information and do not indicate an endorsement by the Regional Water Quality Control Board.

Pesticide Regulation

Pesticide Information

Integrated Pest Management & Less Toxic Alternatives


For more information contact:

Jan O’Hara
Water Resources Control Engineer
Planning and TMDLs Division