I. INTRODUCTION

A. Ground Water Dating

Previous well sampling conducted by the Environmental Hazards Assessment Program (EHAP) of the Department of Pesticide Regulation was designed to determine if contamination in a section of land was limited to that section or if contamination existed over broad areas. These data were used in a statistical analysis that identified the occurrence of residues in broad categories of climate and soil conditions (Troiano et al., 1994). Although the previous studies were instrumental in providing a geographical framework for further regulatory activities, they do not provide information on the lag times between farming practices at the land surface and changes in quality to the underlying ground water (Spurlock and Troiano, 1995).

EHAP is conducting a program to mitigate existing ground water contamination in Fresno and Tulare counties through the voluntary adoption of modified farm management practices. Growers are reluctant to change their current farming practices when the extent that these practices contribute to existing contamination is unknown. A question frequently asked by the regulated community is if current farming practices contribute to ground water contamination or alternately, whether the present contamination is associated with historical practices. One way to answer this question is by using ground water dating. Improved methods for dating ground water using chloroflourocarbons (CFC) have recently been developed (Spurlock, 1995). The CFC concentration of a well water sample can be used to estimate the time since that water entered the aquifer. If herbicide-containing ground water samples are determined to have recently entered the aquifer, this is evidence that recent recharge is contributing to the presence of herbicides in ground water ­­­ hence, that current practices are making a contribution to ground water contamination.

B. Simazine Degradates

The most frequently detected preemergent herbicide in Fresno and Tulare County wells is simazine, accounting for about 40 per cent of herbicide detections in the two counties. Typical of chloro-s-triazine herbicides, the two most important degradation routes for simazine are (1) N-dealkylation, which yields deethyl simazine (DES) followed by diaminochlorotriazine (DACT) in a stepwise fashion, and (2) hydrolysis, to give hydroxysimazine. Although hydroxysimazine is persistent, it binds tightly to soil, is immobile (Smith et al., 1982), and is considered relatively unimportant with respect to plant and animal toxicities (Barrett, 1996). In contrast, the N-dealkylated simazine degradates DES and DACT are mobile in soil. While the N-dealkylated degradates have not been fully characterized with regard to mammalian toxicity, they are generally considered to be the most important primary degradates for plant and animal toxicities (Barrett, 1996). Owing to uncertainties in mammalian toxicity and the demonstrated persistence and occurrence of the N-dealkylated chlorotriazine degradates such as DES and DACT, health advisory levels set for triazine parents are sometimes applied to the total concentration of parent plus degradates [Wisconsin Ground-Water Act 410 (1983), Rule under the Law, Enforcement Standard, Chapter NR 140, Wisconsin ADM CODE (1991)].

The N-dealkylated metabolites of simazine, DES and DACT, have not been routinely determined in most California well sampling studies. However, a recent EHAP study of Fresno and Tulare County wells determined reported both DES and simazine (Troiano and Nordmark, 1995). That data showed (1) generally higher concentrations of DES relative to simazine in the positive samples, and (2) higher frequencies of detection of DES in wells as compared to simazine. Preliminary data from a well monitoring study in Fresno County indicates similar results for both N-dealkylated products of simazine: concentrations and frequencies of detection of DES and DACT were comparable to the parent (Ciba-Giegy Corporation, 1996). Finally, high frequencies of detection of N-dealkylated atrazine breakdown products relative to atrazine are typical for wells located in other areas of the United States (Balu et al., 1996). Atrazine is a chloro-s-triazine that is similar in structure to simazine.

California data for simazine, DES, and DACT concentrations in well waters are needed to determine the occurrence and environmental significance of simazine=s N-dealkylated degradates in California well waters. This protocol describes a study to (1) age-date well waters, and (2) determine concentrations of simazine, DES, and DACT in Fresno and Tulare County wells that have previously had confirmed detections of simazine.

II. OBJECTIVES

A. Primary Objectives

The primary objectives of this study are to (1) measure the effective recharge age of well waters containing pre-emergent herbicide residues using environmental chloroflourocarbon tracers, and (2) measure concentrations of simazine, DES, and DACT in wells in coarse and hardpan vulnerable soil clusters that have previously had confirmed detections of simazine.

B. Secondary Objective

A secondary objective of this study is to determine if herbicides used as substitutes for detected ground water contaminants can also be detected in ground water. This objective depends on the availability of resources.

III. PERSONNEL

This study will be conducted by the Environmental Hazards Assessment Program (EHAP) under the general direction of Don Weaver, Senior Environmental Research Scientist. Key personnel are listed below:

Project Leader and Field Coordinator: Frank Spurlock

Senior Staff Scientist: John Troiano

Laboratory Liason/Quality Assurance: Nancy Miller

Experimental Design/Data Analysis: Terri Barry

Authorship of final report should include, but not be limited to Frank Spurlock, Terri Barry, and John Troiano.

Questions concerning this monitoring program should be directed to Peter Stoddard at (916) 324-4078 and FAX (916) 324-4088.

IV. STUDY DESIGN

The wells selected for sampling will be located in Fresno and Tulare Counties. These counties were chosen because they contain an extensive history of well sampling and have been evaluated for the presence of geographic areas vulnerable to ground water pollution by pesticides (Troiano et al., 1994). Previous analysis of the geography of the Fresno-Tulare County area indicated that contaminated wells occurred in two predominant soil types. One was a coarse soil condition where leaching may be a predominant process for pesticide movement to ground water and the other a hardpan soil condition where dry wells are constructed to aid in drainage of runoff water from these poorly-drained soils.

Wells will be selected based on previous sampling history, well construction data, and depth to ground water in the vicinity of the well. Greater priority will be given to wells that:

1) had the highest concentrations of simazine or deethyl simazine (DES)

2) have been sampled since 1993

3) are screened in areas containing shallow ground water because these areas should reflect the minimum time for transport between the surface and ground water.

4) had mutiple residues in wells. In the coarse soil cluster these residues will include simazine, DES, and diuron; in the hardpan soil cluster simazine, DES, diuron, and bromacil will be considered.

5) were recently constructed with reliable construction information obtained from logs or from well owners.

The initial set of candidate wells will be identified using the Department's Well Inventory Database. The wells will be sorted by the date of the last sample and triazine residue in the sample which will be the sum of simazine and DES. Average depth to ground water for the section where the well is located will be determined from a database for sections in the lower San Joaquin Valley that was generated from a map published by the U.S. Bureau of Reclamation (1989) indicating depth from surface to ground water for spring of 1990. Estimates of the average depth to ground water were manually determined for each section. The wells will be prioritized so that greater weight is given to wells that have the highest concentrations of a single triazine, the most recent sampling dates, and the shallowest sectional ground water. Next, information on well construction will be determined either from well logs, if they exist, or from data collected from the owners in previous studies, or from further contact with well owners. Finally, candidate wells will be required to have a minimum distance from each other of 500 feet to ensure statistical independence of the well sampling results.

Once the candidate wells have been prioritized, fifteen wells will be sampled in each cluster to assess the current concentration of analytes in the wells. Samples will be analyzed using a Gas Chromatography (GC) screen developed for detecting simazine, DES, DACT, bromacil, and diuron by ALTA Analytical Laboratory.

Based on the triazine concentrations in the initial samples, ten wells in each cluster will be sampled a second time for ground water dating analytes (CFC-11, CFC-12, and inert gases) in addition to repeat sampling for the herbicide residues.

Five sets of three proximate wells each will also be sampled for CFCs and herbicides to provide information on the vertical distribution herbicide residues and CFCs (i.e., ground water recharge dates). These sets of wells will either consist of monitoring well clusters or monitoring well pairs co-located with a nearby positive domestic well. The purpose of these samples is to investigate herbicide concentrations and recharge ages with depth.

Further studies that target other pesticides that substitute for the use of simazine active ingredients will depend on availability of resources.

V. CHEMICAL ANALYSIS / QUALITY CONTROL

Alta Analytical will analyze water samples for simazine, DES, DACT, bromacil, and diuron using a chemical screening method under development. Ground water dating analytes will be analyzed by the U.S. Geological Survey or their contractors subject to DPR approval.

The pesticide quality control program will include the following: A solvent blank and a matrix spike will be analyzed with each extraction set. Results of matrix spikes will be compared to the warning and control limits currently established. Samples from ten percent of the wells will be analyzed by liquid chromatography for confirmation. Two blind spikes for each chemical will be submitted with each quarter's samples.

VI. DATA ANALYSIS

The CFC concentrations will be used to determine effective recharge dates for the ground water parcels that the well samples represent. For the five sets of three proximate wells, recharge age and herbicide concentrations will be investigated as a function of depth. One-dimensional transport modeling may be used to estimate effective time between herbicide application and entry of residues in the water table, alternately, water balance using ET, total crop water applied and rainfall can be used to estimate transport time through the unsaturated zone. Data for the 20 domestic well samples taken in the coarse and pan clusters will be evaluated for relationships between recharge age and total triazine residue concentration, triazine degradate as a fraction of total triazine residue in sample, average sectional depth to ground water, or cluster variable (pan vs. coarse).

VII. TIMETABLE

Initial Sample Collection May - June 1996

Herbicide Chemical Analysis June 1996

Second Sampling July 1996

Chemical Analysis

Herbicides July - August 1996

CFCs, inert gases July - December 1996

Data Analysis December 1996 - January 1997

Final Report March 1997

VIII. REFERENCES

Balu, K., P.W. Holden, and L.C. Johnson. 1996. Summary of Ciba/State Groundwater Monitoring Study for atrazine and it=s major degradation products in the U.S. Presented at the 211th American Chemical Society National Meeting, Agrochemical Division, March 1996. New Orleans, LA.

Barrett, M. 1996. Impact of triazine degradates in groundwater in relation to relevant regulatory endpoints. Presented at the 211th American Chemical Society National Meeting, Agrochemical Division, March 1996. New Orleans, LA.

Ciba-Giegy Corporation. 1996. Unpublished well monitoring data from Fresno and Tulare Counties, California.

Smith, A.E., D.C.G. Muir, and R. Grover. 1982. Chapter 3: The Triazine Herbicides. IN: Analysis of Pesticides in Water, Volume III. A.S.Y. Chua and B.K. Akghan eds. CRC Press, Boca Raton Fl.

Spurlock, F. 1995. Determining sources and ages of shallow ground waters, with potential application to studies of pesticide transport to California ground water. Memorandum to Kean Goh, Ag. Program Supervisor IV, Environmental Monitoring and Pest Management, Department of Pesticide Regulation.

Spurlock, F., and J. Troiano. 1995. Evaluation of ground water quality trends based on temporal variation of measured pesticide concentrations in well water. October 10, 1995 memorandum to John Sanders, Branch Chief, Environmental Monitoring and Pest Management, Department of Pesticide Regulation.

Troiano, J., and C. Nordmark. 1995. Summary of Results for a Study to Identify Areas of Ground Water Contamination by Pesticides in California. August 7, 1995 memorandum to Don Weaver, Senior Environmental Research Scientist, Environmental Monitoring and Pest Management, Department of Pesticide Regulation.

Troiano, J., B. Johnson, S. Powell, and S. Schoenig. 1994. Use of cluster and principal component analyses to profile areas in California where ground water has been contaminated by pesticides in California. Environmental Monitoring and Assessment, 32:269-288.

U.S. Bureau of Reclamation. 1989. Thirty-ninth water supply report. Fresno Office (CVP), Fresno, CA.