Citation:

Duirk, S E. AND T W. Collette. ORGANOPHOSPHATE PESTICIDE DEGRADATION UNDER DRINKING WATER TREATMENT CONDITIONS. Presented at American Water Works Association Annual Conference, San Francisco, CA, June 12-16, 2005.

Impact/Purpose:

Conditions for treatment of DW vary widely. However, most all processes involve some form of conventional treatment (filtration, etc.), and some form of disinfection. Also, systems sometimes use various other treatments, including softening by the addition of a base. Treatment processes can have profound effects on the pesticides and toxics that occur in DW sources. For example, hydrophobic chemicals may be partially removed by conventional treatment, however, percent removal can vary significantly depending on conditions. On the other hand, conventional treatment generally has little or no effect on hydrophilic chemicals.

If pollutants are not removed by conventional treatment, they may be altered by other treatment processes. For example, disinfection can transform some chemicals via oxidation; however, little is known about the identity of products formed by this process. Limited information shows that disinfection can yield products that are more toxic than the parent. Also, some chemicals are transformed via base-catalyzed hydrolysis during the softening process. The nature and extent of transformations vary greatly depending on treatment conditions.

EPA Program Offices recognize that treatment often has a large effect on pesticides and toxics that occur in DW sources; and they have articulated a need to incorporate these effects into risk assessments. This task will provide regulators with methods, tools, and databases to forecast the fate of pesticides and toxics during DW treatment. The early task outputs will be chemical-specific information from bench-scale studies that simulate disinfection and softening. However, all task efforts will be focused on the long-range goal of providing predictive models for chemical removal and transformation that cross chemical class and treatment conditions. Early experiments will provide information to elucidate transformation mechanisms. Next, we will investigate effects of varying treatment conditions and chemical speciation. This strategy will lead to broadly applicable tools for forecasting fate for a wide range of chemicals. Finally, we envision that the output of our predictive fate tools will be used as input into models developed under the ORD Computational Toxicology Initiative. In this fashion, the final contaminants and concentrations predicted by our models to occur in finished DW can then be considered for toxic potential. This will provide Program Offices with an integrated system for risk assessment and management for the pesticides and toxics in drinking water.

Description:

The Food Quality Protection Act (FQPA) of 1996 requires that all tolerances for pesticide chemical residuals in or on food be considered for anticipated exposure. Drinking water is considered a potential pathway for dietary exposure and there is reliable monitoring data for the source water. Approximately 95% of the U.S. population obtains potable water from community water systems. Conventional water treatment processes (e.g. coagulation/flocculation, sedimentation, and filtration) do not appear to remove or transform hydrophilic pesticides. However, disinfection and softening have been shown to chemically oxidize or catalyze hydrolysis of certain pesticides. Over 90% of all community water systems use free chlorine as both a primary and secondary disinfectant, and approximately 30% of the United States' drinking water plants serving populations of 100,000 or greater use up-flow lime softening. Therefore, base catalyzed hydrolysis and chemical oxidation can potentially transform the parent pesticide to either a less or more toxic stable end-product in potable water. Chlorine reacting with organophosphate (OP) pesticides that contain the thiophosphate functionality (P=S) often results in the formation of oxons (P=O) [5-7]. The resulting oxons are expected to be more toxic than the parent compound. However, the kinetics and mechanisms of these transformations have yet to be fully investigated and modeled. We chose chlorpyrifos (CP) as a representative pesticide to study and model the reaction of free chlorine (TOTOCl = HOCl + OCl-) with OP pesticides. The overall goal of this research is to assess the potential exposure of potable water consumers to pesticides and their transformation products after raw water has undergone water treatment (e.g. chlorine disinfection and water softening).

Record Details: