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Project: Pesticides from Residential Land Uses

Background and Objectives

Diazinon and chlorpyrifos are commonly used organophosphorus (OP) pesticides in agricultural and urban watersheds. Significant concentrations of OP pesticides have been measured in receiving waters downstream from applications of OP pesticides. Unlike agricultural watersheds, though, OP pesticides are not well-regulated in urban watersheds. The lack of regulation is due, in part, to an incomplete understanding of their sources, runoff characteristics, and fate in urban environments.

The goal of this study was to quantify the contribution of diazinon and chlorpyrifos in wet and dry weather runoff from residential land uses. In addition, two sub-objectives were identified:

• To quantify the effectiveness of public education versus a technology-based best management practice (BMP) for controlling OP pesticide contributions from residential land uses during dry weather; and
• To quantify the relationship between rainfall characteristics and OP pesticide concentrations from residential land uses during wet weather runoff.


This project was completed in 2003.


The concentration of OP pesticides in wet and dry weather runoff was quantified in three replicate residential neighborhoods. Each of the neighborhoods was relatively similar in age, size, orographics, and imperviousness. Runoff from the neighborhoods could be isolated and sampled at a single point from within their municipal separate storm sewer system, enabling each neighborhood to be considered a separate treatment.

In order to evaluate the effectiveness of potential “best management practices” (BMPs), one of the neighborhoods received an intensive public education campaign, the second neighborhood received public education and a technology-based BMP, and the third was used as a control. The education BMP consisted of an information packet, plus monthly reminders of ecologically sound irrigation, herbicide, and pesticide practices. The technology-based BMP consisted of remotely-controlled sprinkler timers that varied the timing and frequency of irrigation based on local weather data.

Dry weather samples were collected three days a week, every other week, from January 2000 to June 2001. The BMPS were then implemented and sampling continued at the same pace through November 2002. A total of four storm events were sampled via collection of 10 grab samples per event at intervals of 15-30 minutes, representing the rising, peak, and receding flows of the hydrograph. In all, 465 dry weather and 120 wet weather samples were taken.


Main results and conclusions were:

• Mean concentrations of OP pesticides were typically greater during dry weather than wet weather at all three neighborhoods. Mean dry weather concentrations ranged from 20 to 572 ng/L chlorpyrifos and from 1,031 to 1,726 ng/L diazinon. Mean wet weather concentrations ranged from 6 to 156 ng/L chlorpyrifos and from 685 to 1,812 ng/L diazinon.
• Temporal plots of OP pesticide concentrations during dry weather did not indicate any seasonal patterns. There was no noticeable change in concentration following the initiation of the technological or educational BMPs.

Monthly average OP concentrations in dry weather discharges from three residential neighborhoods in Orange County, CA

• There was substantial variability in OP pesticide concentrations both within and among storm events at all three neighborhoods. Event mean concentrations ranged from 3 to 250 ng/L chlorpyrifos and from 183 to 28,640 ng/L diazinon. At least part of the variability in diazinon concentrations was correlated to rainfall quantity and duration.

Rainfall characteristics versus diazinon concentration

• Mass emissions of diazinon and chlorpyrifos during wet weather exceeded emissions during dry weather by roughly an order of magnitude. This was a function of substantially more volume during storm events than during dry weather.
• Concentrations of OP pesticides from the test neighborhoods appeared high relative to water quality thresholds recommended by the California Department of Fish and Game.
• Diazinon and chlorpyrifos are currently being phased out for residential uses. This study began prior to the onset of the phase out and can serve as a benchmark to assess reductions in these OP pesticide concentrations over time.


Irvine Ranch Water District
Orange County Water District
California Department of Pesticide Regulation
Orange County Public Facilities and Resources Department
Santa Ana Regional Water Quality Control Board

For more information on Pesticides from Residential Land Uses, contact Eric Stein at (714) 755-3233.
This page was last updated on: 7/2/2014