WRIR 02-4304

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Lietz, A.C., 2003, Occurrence and Distribution of Pesticides in the St. Lucie River Watershed, South-Central Florida, 2000-01, Based on Enzyme-Linked Immunosorbent Assay (ELISA) Screening: Water Resources Investigations Report 02-4304, 35 p.


The St. Lucie River watershed is a valuable estuarine ecosystem and resource in south-central Florida. The watershed has undergone extensive changes over the last century because of anthropogenic activities. These activities have resulted in a complex urban and agricultural drainage network that facilitates the transport of contaminants, including pesticides, to the primary canals and then to the estuary. Historical data indicate that aquatic life criteria for selected pesticides have been exceeded. To address this concern, a reconnaissance was conducted to assess the occurrence and distribution of selected pesticides within the St. Lucie River watershed.

Numerous water samples were collected from 37 sites among various land-use categories (urban/built-up, citrus, cropland/pastureland, and inte-grated). Samples were collected at inflow points to primary canals (C-23, C-24, and C-44) and at control structures along these canals from October 2000 to September 2001. Samples were screened for four pesticide classes (triazines, chloroacetanilides, chlorophenoxy compounds, and organophosphates) by using Enzyme-Linked Immunosorbent Assay (ELISA) screening.

A temporal distribution of pesticides within the watershed was made based on samples collected at the integrated sites during different rainfall events between October 2000 and September 2001. Triazines were detected in 32 percent of the samples collected at the integrated sites. Chloroacetanilides were detected in 60 percent of the samples collected at the integrated sites, with most detections occurring at one site. Chlorophenoxy compounds were detected in 17 percent of the samples collected at the integrated sites. Organophosphates were detected in only one sample.

A spatial distribution and range of concentration of pesticides at the 37 sampling sites in the watershed were determined among land-use categories. Triazine concentrations ranged from highest to lowest in the citrus, urban/built-up, and integrated areas, respectively. The highest median triazine concentration was found in the cropland/pastureland area. Chloroacetanilide concentra-tions ranged from highest to lowest in the citrus, integrated, urban/built-up, and cropland/pastureland areas, respectively. Chlorophenoxy compound concentrations ranged from highest to lowest in the urban/built-up, integrated, citrus, and cropland/pastureland areas, respectively. The maximum concentrations of triazines, chloroacetanilides, and chlorophenoxy compounds were 0.63, 1.0, and 14 micrograms per liter, respectively. Organophosphate was detected once at an integrated site at a concentration of 0.20 microgram per liter.

Currently, the U.S. Environmental Protection Agency has no aquatic life guidelines for atrazine and metolachlor. However, assuming that all triazine and metolachlor concentrations from ELISA and gas chromatography/mass spectrometry (GC/MS) analyses were the result of atrazine and metolachlor detections, no concentrations exceeded the Canadian aquatic life guidelines for atrazine and metolachlor. One organophosphate detection (0.2 microgram per liter) did exceed the U.S. Environmental Protection Agency aquatic life guideline for chlorpyrifos.

The deethylatrazine/atrazine ratio (DAR) is an important indicator of atrazine transport in the environment. The DAR ranged from 0.25 to 0.33, indicating that postapplication runoff was the most likely source of atrazine to the environment at the time of sampling. Deisopropylatrazine is a metabolite of atrazine and structurally similar compounds, such as simazine and cyanazine. The deisopropylatrazine/deethylatrazine ratio (D2R) is an indicator of nonpoint sources of deisopropylatrazine to the environment. The ratio ranged from 1 to 3 in this study, indicating simazine was an important source of deisopropylatrazine to the environment at the time of sampling, as opposed to atrazine alone. Confirmation analyses by GC/MS for triazines detected by ELISA indicated the presence of atrazine, simazine, deethylatrazine, and deisopropylatrazine. Metolachlor, however, or any of the pesticides for which the ELISA test is cross reactive (acetochlor, butachlor, propachlor, or alachlor) were not detected by GC/MS analysis. The chloroacetanilide detections may be the result of antibody cross reactivity with two metolachlor metabolites; namely, metolachlor ethane sulfonic acid (ESA) and metolachlor oxanilic acid (OA). Metalaxyl, a fungicide also used in the watershed, cross reacts with metolachlor antibodies at low concentrations.

Quality assurance efforts included the collection of duplicate samples and equipment blanks for every 10 and 20 environmental samples collected, respectively. All equipment blank test results showed concentrations below the method detection limit (MDL) for triazines, chlorophenoxy compounds, and organophosphates. Chloroacetanilide concentrations ranged from the method detection limit of 0.05 to 0.07 microgram per liter. Further results indicated statistically significant differences between triazines by ELISA and atrazine by GC/MS when ELISA results are converted to the MDL. There was, however, no statistically significant difference between triazines determined by ELISA and atrazine determined by GC/MS when ELISA results were converted to one-half the MDL.


Purpose and Scope
Previous Studies
Description of Study Area
Environmental Setting
Hydrologic Setting
Methods of Investigation
Immunoassay Screening
Sampling and Laboratory Methods
Analytical Methods
Occurrence and Distribution of Pesticides in the Watershed
Factors Affecting Occurrence of Pesticides
Variability of Pesticides
Temporal Variability
Spatial Distribution
Herbicide/Metabolite Ratios and Geochemical Transport
Comparison of Detected Pesticide Concentrations to Water-Quality Standards 30
Quality Assurance Criteria
Selected References

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