Chemical Risk Assessment of pesticide Atrazine in pond water using GENEEC

Mathew Nkem Ikediashi

The effects of some agrochemicals to specific non target organisms remain a problem. This report looks at the chemical risk assessment of a known pesticide that is widely used and predicts the environmental fate of the aquatic living organisms within proximity using a tier one risk assessment model GENEEC. This report discusses the potential exposure to non-target species living in water bodies in proximity to sites where a pesticide is applied and also analyses the model used, looking at how different physical and chemical parameters affect the concentrations over a given period of time.

Introduction

USEPA office Division of Pesticide Programs for Environmental Fate and Effects uses a tiered system of pesticide exposure modelling to assess the risk of a pesticide product to the environment GENEEC (GENeric Estimated Environmental Concentration) model is a tier one computer program designed by USEPA as a simple screening tool used to assess the potential adverse effects of pesticides by estimating the conservative pesticide concentrations in surface water from a few basic chemical parameters. The Tier 1 Risk Assessment (RA) is intended to be a qualitative screening process using a collation of information, the purpose of the Tier 1 RA is to determine two main points: Whether there is a potentially complete pathway

between the contaminant of concern and potential receptors

If contaminant concentrations exceed benchmark or guideline values for relevant receptors or media of concern

If the pathways are assessed as incomplete, or if contaminant concentrations do not exceed benchmark criteria for ecological or human health values, the RA process may be suspended without proceeding to Tier 2 assessment. A risk management decision may be made, based on the Risk Characterizations, to:

Do nothing further  Perform an iteration  Undertake a Tier 2 RA, or  Undertake remedial work

A Tier 1 RA assesses contaminants of concern against published assessment criteria (guideline values or benchmark criteria).These criteria have been developed to help the assessor undertake risk assessments and incorporate in one step many of the factors involved in receptor identification, toxicity and exposure assessment.

In developing these criteria, specific receptors have been identified, certain contaminant pathways have been assessed, and specific contaminants of concern identified. Tier 1 is used to screen chemicals to determine which ones potentially pose sufficient risk to warrant higher level modelling. Chemicals failing to pass this program, move on to the tier two modelling. GENEEC calculates acute as well as longer-term Estimated Environmental Concentration (EEC) values. It considers reduction in dissolved pesticide concentration due to adsorption of pesticide to soil or sediment, degradation in soil before wash off to a water body, direct deposition of spray drift into the water body, and degradation of the pesticide within the water body. It assumes one single large rainfall/runoff event occurs that removes a large quantity of pesticide from the field to the water all at one time. Longer-term, multiple-day average concentration values are calculated based on the peak day value and subsequent values considering degradation processes.The model uses a chemical's label application information, its soil/water partition data and its degradation kinetics to estimate high level exposure values (USEPA, 2009).A risk assessment of the triazine herbicide atrazine using GENEEC is conducted by first analysing the toxicity database and subsequently estimating exposure, by inputting some parameters.

AtrazineThe triazine herbicide atrazine (2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine) is one of the most used pesticides in North America. Atrazine is principally used for control of certain annual broadleaf and grass weeds, primarily in corn but also in sorghum, sugarcane and to a lesser extent, other crops and landscaping (Solomon et al, 1995). Atrazine is found in many surface and ground waters allowing non-target living organisms to be exposed to its toxicity.

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Problem Identification

Exposure Assessment

Toxicity Assessment

Receptor Characterisation

Risk Characterisation

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Analysis shows that atrazine is responsible for abundance of larval trematodes (parasitic flatworms) in the declining northern leopard frog Rana. Reports also show that the effects of atrazine are consistent across trematode Taxa (Jason R et al, 2008).

Physical/chemical properties ( WSSA, 1989)

Chemical name: 2-chloro-4-ethylamino-6-isopropylamino-s-triazineCommon name: atrazine

Trade Names: Aatrex, Aatram, Atrasol, Crisazana, Gesaprim, Primatol, ZeazinEmpirical Formula: C8H14N5ClMolecular Weight: 215.7Physical State: white crystallineMelting Point: 173-177oCIonization constant: (pKa) 1.68Density: 1.187 g/cm3

Stability: Stable in neutral, slightly acidic/basic solutions; hydrolyzed by alkali/ mineralacids at higher temps.Vapour Pressure: 40 cPa (3 x 10-7 mm Hg) at 20o CSolubility: water ~33 ppm, methanol 18,000 ppm, chloroform 52,000 ppm, dimethyl sulfoxide 183,000 ppm.Henry's Law Constant: 2.6 x 10-9 atm m3 mol-1

Octanol/Water Coeff: 418 ± 14 at 20oC; logP = 2.6 The scientific approach of assessing risk is applied to assess the potential risk of Atrazine to pond (Figure 1).

Hazard identificationAn aquatic risk assessment of atrazine is being conducted to evaluate the significance of the toxicity demonstrated in various studies. The averaged concentrations of exposure with similar time interval in some aquatic organisms reveal 96 hours for fish to be acute and 21 to 56 days for invertebrate as being chronic (Pennington et al 2001). Atrazine levels as low as 0.1 ppb result in the feminization process in male frogs (Hayes et al, 2002).

Exposure assessmentThe physical, chemical and Environmental fate input parameters are put into GENEEC and a predicted concentration is generated, if the derived risk ratio is greater than stipulated comparing to real life scenario, a

requirement of a second more thorough assessment would be needed. The principal crop use of atrazine in this scenario is wheat. Applications are made from the air and these are not considered seasonal. Exposure assessment in this case aims to determine the concentration of the pesticide in an environmental compartment for comparison with the effects concentration (i.e. the concentration at which adverse effects may occur to some organism).

Figure 1: Structure for risk assessment of atrazine in aquatic ecosystems.

Risk characterisationThe risk characterization process consists of calculating a margin of exposure (MOE) by dividing the critical acute or chronic NOEL value for a specific toxicological endpoint; A high Level of Concern (LOC) is presumed if the ratio of the exposure and effect concentration is (for both acute and chronic effects) greater than 1, concentrations to receptors should not in any case be remotely close, this is referred to as the point of criticality (USEPA, 2009). "Passing" a given assessment tier indicates that there is a low possibility of risk to the aquatic environment. "Failing" an assessment tier however, does not mean the chemical is likely to cause environmental problems, but that the assessment should continue on to the next higher assessment tier.GENEEC is used to generate concentrations of atrazine in pond water for different time intervals following application, and comparing findings with the toxicity tests (i.e. EC50 values [Effective Concentration to adversely impact 50% of the test organisms]) conducted by Pennington and Scott (2001). Whereby an ecological risk quotient (ErQ) is given by:ErQ = exposure concentration (or dose) / toxicologically relevant concentration i.e.EC50 which leads to a tier 2 risk assessment.

MethodologyInformation sheets provided include:

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Aqueous solubility and partitioning information (i.e. Kd [soil/soil-water distribution coefficient]) on the physical-chemical properties of atrazine by (Mackay et al, 1992).

Toxicologically relevant concentration i.e.EC50 is 147 mg/l (Pennington et al, 2001).

Koc (organic carbon/water partition coefficient) is used as a substitute for Kd because the organic carbon normalized soil/water equilibrium partition coefficient (Koc) may be preferred for pesticides for which there is a strong positive correlation between the Kd value and the organic carbon content of the soils on which the adsorption tests were performed. If there is correlation, the multiple Koc values will be less variable than the multiple Kd values and the Koc

is likely to be a more accurate estimator. If there is no correlation, use of the Kd is preferable. The Kd / Koc

conversion is based on an organic matter content of 2 % and an organic carbon content of 1.16 %. If neither the Kd

nor the Koc is available, it is recommended to use 0.35 times the Kow value (Burns, 2001).

Summary of Model Structure and MethodsGENEEC is a FORTRAN based meta-model. The user answers consecutive questions presented by user on the screen and output is displayed on the screen and to a printable file.GENEEC simulates pesticide runoff, spray-drift, adsorption and degradation/dissipation from a treated,ten-hectare field to an adjacent, static, one-hectare water body. Pesticide in runoff from the field ranges from 10 % to 0.01 % of the applied pesticide depending on the fraction of the pesticide that is adsorbed to field soil and to water body bottom sediments. Model output concentrations are designed to exceed field-monitored values at most sites most of the time based on comparisons with measured data (USEPA, 2009).

Input parametersToxicologically relevant concentration i.e. EC50 is 147 mg/L (Pennington et al, 2001).The following criteria (or label recommendations) areused in the model:Application rate: 2.25 pounds per acre (or ~40 mg/m2)No. of applications per year = 2Minimum interval between applications = 10 daysMode of application = Aerial‘Wetting in’ is not required = NSoil Kd = 100Solubility (ppm) = 30Buffer = 0Metabolic (field) = 90Photolysis =28Metabolic pond = 90

All information provided is imputed into the model and GEEC concentrations are generated (Table 1).

Table 1. The input parameters for GENEEC selected from an environmental fate data sheet.

Figure 3. GEEC Concentration output over different time interval

The risk quotient in this scenario is given by ErQ = exposure concentration (or dose) / toxicologically relevant concentration i.e.EC50

Where Peak EEC= 34.97 ppm

EC50 i= 147 mg/L (Pennington et al, 2001).

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34.97 ppb/147 mg/L = 0.24 less than 1 therefore thepesticide would not require further screening. Though for the purpose of some other organism like the frog further work should be carried out to investigate remedial action to help the organism.

Figure 2 Structure of how GENEEC works.

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The KOC of choice, when two or more are available, is usually the smallest numerical value. This will result in the highest (most conservative) exposure value, indicating an estimate of the adsorbed or dissolved fraction of the chemical (USEPA, 2009).A sensitivity analysis for Kd was also carried out by, running the model several times but selecting different Kd

values each time (from the physical-chemical property sheets), all other parameters will be kept fixed. This is to show how sensitive the model is to varying Kd values. Results show that for lower Kd values the higher the EEC concentration (Figure 3).The results are not entirely generalized as some parameters would not affect the rate of concentrations if varied more than others; some are likely to have no effects or negligible effects like pesticide solubility. Results also show differences in GEEC concentrations when other parameters are changed but not significant.

Table 2. GEEC Concentrations ppb

GEEC Conc ppm

Kd values Kd anti log values

93.9 1 1047.75 1.7 5034.97 2 10027.75 2.18 15022.72 2.3 20018.99 2.4 25016.14 2.48 25012.21 2.6 400

Figure 3. Sensitivity Analysis for Kd

Sensitivity analysis for several other chemical parameters:If there is a range of half-lives, the aerobic aquatic metabolic half-life entered is usually the longest one, which would result in the highest exposure. Sensitivity

analysis was run for the aquatic metabolic half-life (Figure 5).

Figure 5. Sensitivity Analysis for Metabolic half-life

Prior to rainfall/runoff the dissolved pesticide concentration is reduced by degradation in the soil the largest half-life is entered if there is a wide range available, signifying the longest persistence which would yield the highest, most conservative exposure. Figure 6 shows a soil aerobic metabolic half-life sensitivity analysis.

Figure 6. Sensitivity Analysis for Metabolic half-life

The longest photolysis half-life would result in the highest exposure concentration. The effective photolysis half-life will be higher than the one entered in the program due to light attenuation in the pond. A sensitivity analysis for photolysis is carried out (Figure 7).

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Figure 7. Sensitivity analysis for metabolic half-life

GENEEC is no doubt a good model for the role it plays but it also has some flaws which if corrected, the results of the model would be more practical. The key limitations to this model include the following:

1. GENEEC predictions of pesticide concentrations in surface water are designed to be higher than most concentrations from field sampling as well as most concentrations predicted by other, more complex models most of the time.

2. The model does not indicate the effects of the chemical over time and space, the results are vague and over conservative taking into account the worst case scenarios.

3. Certain factors such as interception of spray, foliar dissipation prior to rainfall, or a pesticide wash-off fraction are unaccounted for.

4. There are only a few, readily available input parameters.

5. GENEEC is a generic program, differences in climate, soils, topography or crop type are not considered in estimating potential pesticide exposure (USEPA, 2009).

Conclusions

GENEEC is an easy model for risk assessment used in predicting the ecological fate and transport of pesticides The risk quotient of atrazine in this report is 0.24 indicating that the site is safe for aquatic organisms taking into concentration that GENEEC generates the estimates of worst case senerios.

References

Burns, L.A. 2000. Exposure Analysis Modeling System (EXAMS), Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.

Hayes, T., Haston, K., Tsui, M., Hoang, A., Haeffele, C., and Vonk, A. (2002). ''The feminization of male frogs in the wild.'' Nature, 419, 895-896.  Mackay, D.; Shui, W-Y.;Ma, K-C Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals. Vol V. Pesticides. Lewis publishers. 1995.Pennington P, L and Geoffrey SI, (2001) Toxicity of atrazine to the estuarine phytoplankter pavlova sp. (prymnesiophyceae): increased sensitivity after long-term, low-level population exposureSolomon KR, Baker DB, Richards RP, Dixon KR, Klaine SJ, La Point TW, Kendall RJ, Weisskopf CP, Giddings JM, Giesy JP, Hall LW, Williams WM (1996) Ecological risk assessment of atrazine in North American surface waters. Environ Toxicol Chem 15:31–76Rohr, J. R.; Schotthoefer, A. M.; Raffel, T. R. et al. (2008) Nature 455: 1235-1240.http://www.epa.gov/oppefed1/models/water/geneec2_description.htmhttp://contamsites.landcareresearch.co.nz/risk_assessment_tiers.htm#ERA Level 1

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