National Exposure Research Laboratory Research Abstract Fate of Pesticides and Toxic Chemicals During Drinking Water Treatment: Organophosphate Pesticide Degradation Under Drinking Water Treatment Conditions


I ii i (I'd Slides limimnmenlal Pro loci ion .\»enc\	Office of Research ;iikI l)o\ olopmoiil

National Kxposure Research Laboratory
Research Abstract

Government Performance Results Act (GPRA) Goal #4
Annual Performance Measure #244

Significant Research Findings:

Fate of Pesticides and Toxic Chemicals During Drinking Water
Treatment: Organophosphate Pesticide Degradation Under
Drinking Water Treatment Conditions

Scientific	The Food Quality Protection Act of 1996 (FQPA) requires that all pesticide

Problem and	chemical residuals in or on food be considered for anticipated exposure. This

Policy Issues	includes understanding drinking water exposures, a potentially major dietary

exposure pathway. Reliable monitoring data for pesticide residuals in source
waters is available, however, there is a relative dearth of information on the
occurrence of pesticide residuals and pesticide metabolites in finished drinking
water. Conventional surface water treatment (i.e., coagulation, flocculation, and
sedimentation) has been shown to remove some pesticides. Water softening and
chlorination processes can degrade or transform the parent pesticide compound
with the transformation products sometimes being more toxic than the parent
compound. The need exists to understand how the various drinking water
treatment processes affect the removal/transformation of pesticides and their
transformation products. Improved understanding of the fate of the pesticide
transformation products in community water systems is also needed.

This research was designed in response to a high-priority need of the
Environmental Fate and Effects Division (EFED) of the EPA Office of Pesticide
Programs (OPP) articulated in an OPP Science Policy document entitled, "The
Incorporation of Water Treatment Effects on Pesticide Removal and
Transformations in Food Quality Protection Act (FQPA) Drinking Water
Assessments," dated October 25, 2001. This work addresses GPRA Goal 4,
GPRA Objective 4.5, and GPRA Sub-objective 4.5.2 Healthy Communities and
Ecosystems and was developed in consultation with the OPP/EFED staff.

Research	The purpose of this research was to identify drinking water treatment processes

Approach	responsible for the transformation of pesticides. Individual goals of this research

are to: 1) provide chemical-specific information on the effects of water treatment
for high-priority pollutants, 2) provide physicochemical parameters for
transformation products, and 3) develop predictive models for forecasting
treatment effects that cross chemical class and treatment conditions.

Based on the literature, two conventional treatment processes (chlorination and
lime softening) were identified as processes that could potentially transform
pesticides to either a more toxic or a benign product. As a "proof of concept," the

-------
organophosphate (OP) class of pesticides was chosen for study due to the fact that
they exhibit a common toxic response by inhibiting the function of
acetylcholinesterase, an enzyme necessary for proper function of the nervous
system. Chlorpyrifos (CP) was chosen as a model OP pesticide for this study due
to its historical widespread use and the frequency that it has been detected in
drinking water sources.

The chlorination studies were conducted in the presence of excess chlorine.
Observed first-order rate coefficients were obtained for CP loss in the presence of
increasing chlorine concentrations over the pH range of 6.3-11. The reaction order
for both CP and free chlorine was then determined. Then, from the apparent
second-order rate coefficients, the intrinsic rate coefficient for hypochlorous acid
(HOC1) reacting with CP was calculated. Since the aqueous stability of the
chlorpyrifos oxon (CPO) was relatively unknown, hydrolysis experiments were
conducted over the pH range of 1-11 and in the presence of free chlorine over the
pH range of 4-11. These experiments resulted in the elucidation of the
degradation pathways for CP and CPO in the presence of free chlorine. A model
was developed from the proposed degradation pathways that is capable of
predicting the loss of CP and CPO as well as the formation of
3,5,6-trichloro-2-pyridinol (TCP), which is the final degradate observed for these
reactions. TCP is commonly reported as a biomarker of human exposure to CP in
studies such as the Centers for Disease Control and Prevention (CDC) National
Health and Nutrition Examination Survey (NHANES).

Results and The loss of CP in the presence of free chlorine in laboratory prepared aqueous

Impact solutions was observed and found to be first-order with respect to both the CP

concentration and the free chlorine concentration. A total of two transformation
products was observed with near-complete mass balance - (CPO) and (TCP). The
transformation of CP to CPO is via oxidation by HOC1. This reaction is rapid, and
increasingly so as the pH decreases, due to the pKa of HOC1 at 7.5. This
observation is relevant for drinking water disinfection, which is often achieved
using chlorine near neutral pH. This finding is very important since the oxon
degradates of OP pesticides are typically much more toxic than the parent
compound.

Both CP and CPO are observed to hydrolyze to TCP, particularly at higher pH.
The most prominent form of hydrolysis is base-catalyzed (i.e., alkaline
hydrolysis). Also, for the first time, we report a hydrolysis pathway for both CPO
and CP that is assisted by OC1" when chlorine is present under alkaline conditions.
These observations are relevant for drinking water softening, which is often
achieved by significantly raising the pH (by adding lime or soda ash) in order to
precipitate minerals. Hydrolysis products such as TCP are generally less toxic
than the parent OP pesticides or their oxon tranformation products.

These experimental results demonstrate that the change in risk (associated with
anthropogenic chemicals in source waters) due to drinking water (DW) treatment
is a complex issue. For example, while oxidation of OP pesticides by chlorine
below neutral pH leads very rapidly to a more toxic form, hydrolysis (assisted by
chlorine) above neutral pH leads to a less toxic form. Because of this complexity,
we have chosen to develop screening-level models that forecast the concentrations

-------
of all reaction products as a function of pH, chlorine dose, OP pesticide
concentration, and time after chlorine dosing. To test this approach, we have
determined intrinsic rate coefficients for all relevant pathways of transformation
for CP in chlorinated water. Using these simple models and intrinsic rate
coefficients, we demonstrate that the concentrations of CP, CPO, and TCP can be
adequately predicted under a variety of scenarios that are similar to DW treatment
conditions. The work reported here serves as the first steps - and the
"proof-of-concept" - for the goals of this task.

Research This research was conducted solely by the research staff at Ecosystems Research

Collaboration and Division in Athens, GA. Current products of this work are:

Research

Products Duirk, S.E., and Collette, T. W. "Organophosphate Degradation Under Drinking Water Treatment

Conditions: Modeling Perspectives." Annual Conference, American Water Works Association,
San Francisco, CA, United States, June 13-16, 2005.

Duirk, S.E., and Collette, T.W. "Organophosphate Degradation Under Drinking Water Treatment
Conditions: Modeling Perspectives." Annual Conference, American Water Works Association,
THU7: Distribution Systems, page 1 - page 13, 2005.

Duirk, S E., and Collette, T.W. "Degradation of Chlorpyrifos in Aqueous Chlorine Solutions:
Pathways, Kinetics, and Modeling." Environmental Science and Technology, accepted for
publication..

Cherney, D.P., Duirk, S.E., and Collette, T.W. "Monitoring the Speciation of Hypochlorous

Acid from pH 1-11 with Raman Spectroscopy to Determine the Active Oxidant Species at low
pH." Submitted to Applied Spectroscopy.

Future Research Research has been initiated to apply the experiments and models to other OP
pesticides that were judiciously selected to reflect the full range of chemical
structure variability in this class. Our objective is to develop predictive models
that associate relative rates of reactivity to structural variability. This will allow
decision makers to rank and prioritize chemicals according to potential risk. Also,
we are now in the process of identifying natural aqueous matrix components that
most significantly affect the rate and pathways of transformation of anthropogenic
chemicals under drinking water treatment conditions. The goal of these
experiments is to incorporate these most important matrix effects into our models
so that they can be applied to actual drinking water treatment scenarios.

Questions and inquiries can be directed to:

Stephen E. Duirk, Ph.D., E.I.T.

U.S. EPA, Office of Research and Development
National Exposure Research Laboratory
960 College Station Road
Athens, GA 30605
Phone: 706/355-8206
E-mail: duirk.stephen@epa.gov

Funding for this project was through the U.S. EPA's Office of Research and
Development, National Exposure Research Laboratory and was conducted at the
Ecosystems Research Division.

Contacts for

Additional

Information

-------