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
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
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.
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
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
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
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).
Initial Sample Collection May - June 1996
Herbicide Chemical Analysis June 1996
Second Sampling July 1996
Herbicides July - August 1996
CFCs, inert gases July - December 1996
Data Analysis December 1996 - January 1997
Final Report March 1997
Balu, K., P.W. Holden, and L.C. Johnson. 1996.
Summary of Ciba/State Groundwater Monitoring Study for atrazine
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,
U.S. Bureau of Reclamation. 1989. Thirty-ninth
water supply report. Fresno Office (CVP), Fresno, CA.