## Use of PGW studies for testing leaching models

Michael Barrett and Ian Kennedy

26 July 2010

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 1 / 28

## Introduction

The datasets from prospective groundwater (PGW) monitoring studies, required by EPA’s Office of Pesticide Programs (OPP) for registration of pesticides thought likely to leach to groundwater, are being used by EPA and Health Canada’s Pest Management Regulatory Agency (PMRA) to evaluate and develop ground-water exposure models.

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## Use of Existing PGW Data

PMRA’s Environmental Assessment Directorate (EAD) and OPP’s Environmental Fate and Effects Division (EFED) examined and modeled a number of completed PGW study reports for the utility of the datasets for use in testing leaching models. We will discuss a number of factors which limit the use of these data.

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## Uncertainties in pesticide and degradate fate properties.

PGW studies usually do not have site-specific degradation and sorption data Oxamyl – hydrolysis rate varies with pH leading to a high variability in persistence and difficulty in predicting site-specific fate properties Acifluorfen – high site to site variability in mobility and persistence. Fenamiphos – PGW study enhanced by site specific pesticide and degradate fate data collection. DCPA – very predictable degradation and sorption properties of parent and acid derivatives.

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## Field vs. Laboratory data

Laboratory and field degradation rates are not clearly correlated OPP and PMRA usually consider field dissipation data too unreliable for use in modeling Degradation can vary with depth, depending on Organic carbon and pesticide KOC Microbial populations Temperature Many pesticides and degradates are much more persistence with depth

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## Range of Kd 

## values for fenamiphos

Soil Kd KOC Standard study Sand 2.86 543 Sandy loam 0.96 166 Silt loam 3.46 226 Clay loam 1.98 171 PGW specific GA 0-6” 1.64 202 GA 18-24” 0.80 345 CA 0-6” 1.10 190 CA 18-24” 1.00 171

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## Data for more complex processes
For modelling, we currently use a linear, equilibrium sorption model. Real processes may be more complex, but more complex models require more parameters Parameters may depend on soil, temperature, moisture Estimating more parameters requires more data Freundlich sorption – two parameters Also, Freundlich is an empirical equation Difficulty extrapolating at low concentrations. Is meaningless without both parameters K f has ugly units, value is unit-dependent Time-dependent sorption – three or more parameters

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 7 / 28

## Temperature measurments

Temperature measurments in PGW studies are usually not taken below 2 ft. No measurement of the attenuation of temperature fluctuations. Difficult to verify simulation temperatures at deeper depths Average soil temperature is set by more than air temperature Insulation from snowpack Radiation Geothermal heat Temperature is easier to simulate than water or chemical movement.

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## Temperature in the Thiamethoxam PGW study (MI)

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## Data censoring

Reporting/detection limits which are set too high to accurately establish breakthrough patterns with depth for the pesticide or degradates. This also can be an issue for the conservative tracer. Installation of monitoring equipment can introduce bromide

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## Data censoring examples

Acetochlor – Parent leaching is at very low levels Cannot be characterized well without extremely sensitive methods. Oxanilic acid breakthrough difficult to characterize at some sites because of detection limit issues Ethanesulfonic acid – very easy to characterize breakthrough at most sites. Bromide breakthrough better characterized at some sites than others Choice of tracer application rate needs consider bromide background levels in vadose zone and ground water at the site Methods with appropriate detection limits are needed.

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## Data censoring examples (continued)

Thiamethoxam and flumetsulam studies Failure to establish breakthrough in vadose zone in spite of widespread ground water detections. In-furrow application of thiamethoxam Fenamiphos Parent leached, but at concentrations too close to the LOQ – difficult to pick up with depth Degradates leached at higher concentrations, yeilding better defined breakthrough curves

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## Comparison of application rate to water detection limit in PGW studies

Used the nominal (“target”) rate Assumed a porosity of 0.4 (bulk density of 1.6) and a water depth of 1m when calculating total target rate dissolved in GW. This gives a total of 4 × 106 L/hect, but converting kg to ug introduces another factor of 1 × 109 , so the formula uses 0.004.

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## Application rates and LOQ’s Study Appl. (kg/ha) LOQ (ug/L) Ratio (%) Cloransulam-methyl (3) 0.0615 0.1 0.65 Flumetsulam IN 0.078 0.058 0.30 Sulfentrazone 95% 0.18 0.1 0.22 Sulfentrazone 75% 0.22 0.1 0.18 Flumetsulam NC 0.15 0.058 0.15 Imidacloprid MI 0.386 0.1 0.10 Thiamethoxam MI, GA 0.193 0.05 0.10 Oxamyl (IN) 4.5 1.0 0.089 Imidacloprid CA 0.504 0.1 0.079 Fenamifos FL 6.2 0.1 6.5e-3 Acetochlor IA 3.54 0.05 5.6e-3 Fenamifos GA 7.4 0.1 5.4e-3 Fenamifos CA 8.24 0.1 4.9e-3
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## Only transformation product has a well-defined breakthrough
Have to simulate the formation of the transformation product Often the parent is short lived and only a transformation product reaches groundwater This adds complexity to the task of simulating the study Adds parameters Those parameters change with depth, water content, temperature, etc. More uncertainty in model results because of the more complex processes Laboratory studies often inadequately characterize the transformation process.

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## Insufficient irrigation or precipitation Conducting studies under conditions where precipitation or irrigation were insufficient to promote ground water recharge Often associated with a high rate of soil-pore water sample collection failure. 1,3D (WI) sampling missed during breakthrough (illustrated on next slide) Acifluorfen WI study shows that with sufficient irrigation (every 3 days for much of the study, normall practice at that site) all samples are successful Normal weather at some sites may not result in recharge for long periods of time – if supplemental irrigation is not used, the duration of the study would have to be extended beyond the time period originally envisioned for the study. A balance is often needed between normal irrigation practice and additional irrigation sufficient to ensure data are adequate for model calibration for the pesticide of interest Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 16 / 28

## Missing data example – 1,3-D

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## Study was ended too early
Some studies ended before a complete breakthrough curve was observed. Leaching of persistent pesticides / degradates with K d greater than 2 or 3 (or KOC greater than 150 or 200) occurs significantly more slowly, especially in medium textured or (relatively) high organic matter soils. In some cases PGW studies are conducted in medium textured soils and the rate of movement is very slow Acetochlor and some unregistered pesticide studies in Nebraska silt loam. These studies were terminated after several years because of a lack of a sufficiently long-term commitment to see possible breakthrough into ground water.

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## Example – Imidacloprid

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## Example – Acetochlor, NE

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## Temporal and spatial variability in the soil matrix is often high
The sampling frequency and sampling locations may be insufficient to track water and solute movement. Poor site characterization Variability in the soil profile (presence or absence of restrictive layers), Preferential flow, which is not necessarily predictable from the type of soil profile data available. Examples Imidacloprid Thiamethoxam – In-furrow application Acetochlor Indiana bromide and ESA data

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## Example – Thiamethoxam

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## And a transformation product

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## And another transformation product

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## Acetochlor PGW studies – Bromide at 9-ft

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## Use of pipe lysimeter data for model testing

Pipe lysimeters have a zero tension bottom boundary Wetter than a field soil All other factors equal, should leach more slowly Wall effects Ability to measure water flux gives excellent mass balance Complete recovery of leachate More of an artificial environment than a field More difficult to crop in a way consistent with field cropping

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## Comparison of Pipe and Suction lysimeter data Three foot pipe lysimeter compared to the three foot suction lysimeter

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## Other issues

1 Presence and quality, completeness of on-site weather data.
2 Methods of and detail of soil characterization. Full details of soil analyses are not always reported. Two points (-1500kPa and -33kPa) are insufficient to define the van Genuchten retention curve
3 Preparation of site, performance testing before study start.

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