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.

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 2 / 28
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.

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 3 / 28
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.

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 4 / 28
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

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 5 / 28
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

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 6 / 28
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

Kf 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.

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 8 / 28
Temperature in the Thiamethoxam PGW study (MI)

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 9 / 28
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

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 10 / 28
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.

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 11 / 28
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

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 12 / 28
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.

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 13 / 28
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

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 14 / 28
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.

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 15 / 28
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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Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 17 / 28
Study was ended too early

Some studies ended before a complete breakthrough curve was observed.

Leaching of persistent pesticides / degradates with Kd 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.

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 18 / 28
Example – Imidacloprid

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Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 19 / 28
Example – Acetochlor, NE

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Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 20 / 28
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

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 21 / 28
Example – Thiamethoxam

Date

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Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 22 / 28
And a transformation product

Date

CGA.322704 0.1

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Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 23 / 28
And another transformation product

Date

NOA.459602 0.06

0.08

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Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 24 / 28
Acetochlor PGW studies – Bromide at 9-ft

DAT

Concentration.ESA 0.5

1.0

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2.0

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IA

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Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 25 / 28
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

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 26 / 28
Comparison of Pipe and Suction lysimeter data

Three foot pipe lysimeter compared to the three foot suction lysimeter

Day

Concentration

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50

100

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● Pipe ● Suction

Michael Barrett and Ian Kennedy () Use of PGW studies for testing leaching models 26 July 2010 27 / 28
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.

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