A Method for the Assessment of Ground Water Contamination

18 A Method for the Assessment of Ground Water Contamination Potential Using a Pesticide Root Zone Model (PRZM) for the Unsaturated Zone

Downloaded by UNIV OF ARIZONA on April 2, 2017 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch018

M. N. Lorber and Carolyn K. Offutt Office of Pesticide Programs (TS-769C), U . S . Environmental Protection Agency, Washington, DC 20460

The PRZM model was used to evaluate the potential of aldicarb to leach through soil and contaminate ground water in three use sites: tobacco grown on a sandy loam soil in North Carolina and potatoes grown on a sandy loam and a loamy sand in Wisconsin. Calibration with the use of field data on these sites allowed field values of aldicarb decay rate and partition coefficient to be estimated. Long term simulations then permitted evaluation of the effect of soil type, date of application, and irrigation on the leaching potential of aldicarb in these use sites. Results showed little to no potential for leaching on the sandy loam soils, but significant potential on the loamy sand, with between 1 and 19% of applied to leach below 2 meters, and six-month plume solution concentrations as high as 103 ppb at two meters. Applying later in the season reduced leaching by about one-half, and irrigation increased leaching 3-5 times. The findings i n 1979 of 1,2-dibromochloropropane i n ground water in five states and aldicarb in ground water in Long Island, New York, began an intense effort on the part of the Office of Pesticide Programs in the U.S. Environmental Protection Agency (EPA) to evaluate the potential for pesticides used i n agriculture to leach through soils and cause contamination of ground water ( 1). A critical component of this effort i s the use of mathematical models, which can predict the fate and transport of pesticides. An early model used was called PESTANS, the Pesticide Analytical Solution model ( 2 ) , which employed a steady state solution to the analytical equation describing solute transport in a homogenous s o i l . However, the transient aspect of the wetting and drying cycle i n the root zone, and the inhomogenous character of soil necessitated a model which would consider variations in weather patterns and allow for temporal and vertical reassignment of model parameters. The Pesticide Root Zone Model (3.,£) was developed at This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

Garner et al.; Evaluation of Pesticides in Ground Water ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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the US EPA Environmental Research Laboratory in Athens, Georgia, to meet this need. The ability to predict the transport of pesticides leaching ttirough soil allows for an evaluation exercise known as an "exposure assessment". Strictly defined, an exposure assessment would result in an estimation of the concentrations of pesticide in drinking water. However, PRZM and related models such as PESTANS only predict the mass flux and the concentration in the "vadose", or unsaturated zone, which includes the root zone, and some defined distance below the root zone. Although the science and mathematics of predicting transport in the saturated zone exist, and aldicarb transport in the saturated zone has been modeled (5,6) > there are s t i l l several practical difficulties in modeling this process. Quantifying the dispersion process and estimating the rate of pesticide decay in the saturated zone are the two main problems in parameter estimation. As well, there are no adequate field data bases available to test a model which i s capable of predicting the movement of pesticide from the point of application to and through an aquifer. Although also a complex phenomena, movement of water and solute in the unsaturated zone has been studied more extensively. Several field site data bases, including the three used to calibrate PRZM in this paper (7-9) , are becoming available to use in conjunction with unsaturated models. Quantification of pesticide movement in the unsaturated zone can be accomplished effectively with an adequate number of soil cores taken at regular intervals following pesticide application ( 1 0 ) . In contrast, wells penetrating the aquifer either above or below the pesticide plume, or well samples taken before or after the plume has passed the well will result in false negative findings. A major advantage to models such as PRZM or PESTANS i s that they are transportable: they can simulate a variety of situations with simple changes in weather input and parameters. More Importantly, however, is the fact that in most situations, 90$ or more of applied pesticide would have runoff, volatilized, been taken up by the plant, or otherwise decayed before any of i t leaches below the root zone. It makes sense, therefore, to develop the capability to predict the fate of pesticides in the root zone, and hence determine the potential for pesticides to contaminate ground water. The purpose of this paper is to present an assessment exercise of a leaching pesticide using the PRZM model. The assessment begins with a calibration of PRZM for the pesticide aldicarb applied to tobacco in North Carolina and potatoes in Wisconsin. Following these calibrations, long term simulations are performed using these same calibration scenarios. Examination of key PRZM output indicates the "potential" for aldicarb to contaminate ground water in the scenarios modeled. Description of PRZM As the name of the model implies, PRZM models the unsaturated zone, which includes the root zone and a user-specified depth below the root zone within the "vadose" zone. The simulation uses a daily


344

E V A L U A T I O N O F P E S T I C I D E S IN G R O U N D W A T E R

time step, and mass balances of water and pesticide are maintained in "zones" of finite depth, usually 5 cm. Model parameters can vary as a function of both space and time. Complete details of model theory, including equations, sensitivity analysis, and other applications are presented elsewhere (3.>i0. The water balance algorithm i s based on the Soil Conservation Service Curve Number approach ( 1 1 ) , which estimates daily runoff as a function of the antecedent moisture condition (wetness of the s o i l profile prior to a storm) and a curve number determined from f i e l d conditions (soil type, crop type, etc). Simply put, rainfall is partitioned into runoff and infiltration - that which doesn t run off, infiltrates. Following a storm, the soil drains by gravity to field capacity in one day. A "slow drainage" option allows the accumulation of water above field capacity, which then drains over the next several days. Between storms, water is extracted from the root zone via évapotranspiration, which can be determined from daily pan evaporation, i f i t is available, or from an empirical equation based on average daily a i r temperature. The parameters for the soil water model include: runoff curve numbers, s o i l field capacity and wilting point, soil bulk density, crop planting, maturity, and harvest dates, root depth, and crop surface area coverage. Mass balance equations of pesticide fate and transport are developed for the surface and subsurface zones i n PRZM. In the surface zone, avenues of loss include soluble loss in runoff, percolation to the next zone, sorbed loss in erosion, and decay i n both phases. In the subsurface zones, losses include plant uptake and percolation i n the soluble phase, and decay in both phases. A backward difference, implicit numerical scheme i s used to solve the partial differential solute transport equations, with a time step of one day and a spatial increment specified by the user. The important assumptions for the pesticide model are: instantaneous, linear, reversible adsorption described by an adsorption partition coefficient, K^, and first-order decay described by an overall decay rate, k. Parameters for the pesticide model include: universal soil loss equation parameters ( i f erosion loss i s to be modeled), pesticide application information (rate, date, and method of application), K^, k, and a dispersion coefficient.


f

Calibration of PRZM The appropriate means to "test" a model i s dependent on the biases of the model tester and the purposes of his exercise. Words such as calibration, validation, and verification have been used to describe a model testing procedure. For this study, PRZM was calibrated to three f i e l d sites to determine appropriate parameters for longer term simulations. These long term simulations employed the same parameters as the calibration simulations, and their purpose was to examine trends i n pesticide leaching as expressed by PRZM output. PRZM was calibrated to field data on the pesticide aldicarb applied to two potato sites i n Wisconsin and one tobacco site i n North Carolina ( 7 - 9 ) . Field data required for the calibration exercise include: soil physical parameters, crop cultural



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information, aldicarb rate and date of application, depth of incorporation, on-site weather data including daily rainfall and average a i r temperature (or pan evaporation i f available), and field observations of pesticide leaching over time. Specifically, these observations were soil cores taken at several dates following application and measured for total aldicarb residues. Aldicarb degrades rapidly in soil to aldicarb sulfoxide and aldicarb sulfone. In this paper, aldicarb refers to the sum of parent aldicarb and dégradâtes sulfoxide and sulfone. The physical parameters for the three sites are given in Table I. The purpose of the calibration exercise was to determine appropriate parameters which would result in a best-fit match between model simulations and the soil core field observations. The f i r s t step in the procedure was to assign values to field-measured, physically-based, parameters which include a l l parameters for the water balance and crop development portions of the model. The second step was to calibrate chemically and biologically based parameters which are not easily measured i n the field. These i n clude the adsorption partition coefficient, K^, and the first-order rate of decay, k, of aldicarb. Using reasonable ranges of and k as defined by the literature, a trial-and-error method was used until model predictions matched field observations. In Table II are the values of the aldicarb parameters which were calculated from field data. The calculated was determined assuming a K for aldicarb of 36 (1), a ratio of organic matter to organic carbon of 1.7, and field data for soil organic matter for the different soil layers. The decay rate was assumed uniform within the soil profile. The range of "observed" decay rates was determined from the field observations made on different dates. They were calculated by estimating mass of aldicarb remaining on the observation date (based on soil concentrations and soil bulk density), and then applying the first-order equation of pesticide decay. In estimating the decay rate from field observations, the important assumption is made that the residues measured represent the fate of a l l aldicarb applied, i.e., that no aldicarb was lost in runoff, leached below the depth of sampling, or was taken up by the crop. Table III summarizes the several calibration scenarios, which are now described. oc

Site 1 - North Carolina tobacco. This site i s located in Hertford County, North Carolina. The soil is classified as a sandy loam, with 0.85% organic matter in the top 0 . 3 m of s o i l . The mechanical analysis for the top 3 meters i s as follows: 55-69% sand, 18-38% s i l t , and 7-16% clay. Aldicarb was incorporated to a depth of 10 cm at a rate of 3.36 kg/ha at tobacco transplanting on May 1 3 , 1983. Pre treatment samples taken prior to aldicarb application insured that the soil was free of aldicarb residues. Daily rainfall and pan evaporation for this site were obtained from a nearby weather station. Soil cores were obtained by bucket auger to a depth of 3 meters, separated into increments of 0 - 0 . 3 m, 0 . 3 - 0 . 6 m, 0 . 6 - 1 . 2 m, 1 . 2 - 1 . 8 m, 1 . 8 - 2 . 4 m, and 2 . 4 - 3 . 0 m.

The plot

was

divided into four subplots, and four samples were taken per subplot and composited at each date of observation, resulting i n a total of four samples per date per core depth. These four samples were


346


Table I. Summary of PRZM soil and crop parameters for calibration and exposure assessment exercises for North Carolina and Wisconsin (parameters developed from data i n 7-9).


Parameter Description - Soil Classification - Runoff Curve Number Assumptions

sandy loam row crops straight row good condition "B" soil group residue after harvest

- Water Holding Capaci ties, cnP/cnP field capacity 0-30 cm 0.18 30-300 cm 0.24 wilting point 0-30 cm 0.06 30-300 cm 0.10 i n i t i a l soil water 0-30 cm 0.16 30-300 cm 0.24 - Bulk Density, gm/crrP 0-30 cm 1.47 30-60 cm 1.34 60-300 cm 1.56 - Zone Depths, cm core 300 root 45 evaporation 20 storage 5 - Erosion not considered - Crop Grown tobacco - Year 1983 - Crop Development emergence May 20 maturity Aug 1 harvest Nov 1 - Rain interception, 0.20

Wisconsin Hancock

Wisconsin Cameron

North Carolina

loamy sand sandy loam row crops row crops straight row straight row good condition good condition B soil group "A soil group residue after residue after harvest harvest ff

,f

!t

0.38 0.20

0.11 0.08

0.20 0.08

0.05 0.03

0.20 0.20

0.10 0.08

1.46 1.54 1.54

1.48 1.54 1.54

300 30 15 5 not considered potatoes 1982 June 4 July 15 Nov 1 0.15

300 30 15 5 not considered potatoes 1982 1983

May 28 June 21 July 15 Aug 1 Nov 1 Nov 1 0.15

cm


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Assessment of Ground

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347

Table II. Summary of PRZM calibration parameters for aldicarb use in North Carolina and Wisconsin (calculated values for K(3 and k from f i e l d site data are given i n parentheses besides calibrated values).


Parameter Description

North Carolina

Adsorption Partition Coefficient, K^, ml/gm 0 - 15 cm 15 - 30 cm 30 - 60 cm 60 - 300 cm First-Order Decay Rate, k, day" planting, 0-300 cm

0.50 0.15 0.01 0.01

(0.18) (0.18) (0.04) (0.01)

Wisconsin Cameron

1.00 0.50 0.05 0.01

(0.22) (0.22) (0.04) (0.01)

Wisconsin Hancock

1.00 0.50 0.05 0.01

(0.16) (0.16) (0.01) (0.01)

1

emergence, 0-300 cm

0.016 (0.018-0.026)

0.010 (0.011) 0.015 (0.016)

0.010 (0.013-0.019) 0.015 (0.014-0.049)

Equivalent h a l f - l i f e , days 43 (27-39)

planting, 0-300 cm emergence, 0-300 cm Date of Application 1982 planting emergence 1983 planting emergence

69 (63) 46 (43) May 15 June 4

Rate of Application, kg/ha 1982 & 1983 planting 3.36 emergence 2.24

.36 .24

Depth of Incorporation, cm

10

10

Chemical

May 19 May 28 May 10 June 21

May 13

American

69 (36-53) 46 (14-49)

3.36 2.24

Society

Library 1155 16th St.,

N.W.

Garner et al.; Evaluation of Pesticides in Ground Water Washington, D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

10

348


Table III.


Run #

Summary of PRZM calibration runs.

Location

Crop

Soil

Irr*

Date/Rate

1

North Carolina

tobacco

sandy loam

5/13/83; 3.36

Ν

2

Cameron, Ws.

potato

sandy loam

5/15/82; 3.36

M

3

Cameron, Ws.

potato

sandy loam

6/4/82;

2.24

M

4

Hancock, Ws.

potato

loamy sand

5/19/82; 3.36

M

5

Hancock, Ws.

potato

loamy sand

5/28/82; 2.24

M

6

Hancock, Ws.

potato

loamy sand

5/19/82; 3.36

H

7

Hancock, Ws.

potato

loamy sand

5/28/82; 2.24

H

8

Hancock, Ws.

potato

loamy sand

5/10/83; 3.36

M

9

Hancock, Ws.

potato

loamy sand

6/21/83; 2.24

M

* Irrigation regimes practiced: Ν = no irrigation; M irrigation schedule; H = high irrigation schedule

= medium

averaged to represent "observed" results. Observations were taken on June 6 , July 1 4 , Sep. 1 5 , and Nov. 3 0 , and these are represented by the dashed lines shown in Figure 1. Further details on this f i e l d site are given i n Jones et a l . ( 7 ) . The simulated results are shown by the solid line curves in Figure 1. The observations are matched against predictions for the Nov. 30 date in tabular form in Figure 1. Table IV summarizes the simulated water balance, and Table V s u m m a r i 2 e s the aldicarb fate and transport for this North Carolina calibration site. Site 2 - Cameron, Wisconsin potatoes. This field site is located on a commercial potato farm in northwestern Wisconsin i n Cameron. The soil i s an Onamia sandy loam, with 1.03% organic matter in the top 0 . 3 m of s o i l . The mechanical analysis for the top three meters of soil i s as follows: 83-99% sand, 0-13% s i l t , and 1-8% clay. The experimental field was divided i n half, with one-half receiving 3.36 kg/ha aldicarb incorporated at potato planting on May 1 5 , 1982, and the other half receiving 2.24 kg/ha at emergence on May 2 8 . Soil cores were taken prior to application to insure that the profile was free of aldicarb. The procedure i n North Carolina for post-treatment sampling including subplots, composites, and soil coring was also followed in Wisconsin. In addition, 11.7 cm of irrigation was applied between May 15 and Dec. 8 to meet the evapotranspirative demands of the crop when rainfall was insuffi cient. This irrigation was directly input as part of the rainfall


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record and as such, was treated as rainfall by PRZM. Daily totals of rainfall were recorded on-site ( 8 ) . Evapotranspiration was estimated on-site using Penman* s equation between June and September ( 8 ) , and was directly input to PRZM. Otherwise, daily average a i r temperature was used to estimate evapotranspirative demand. Model predictions are compared with observations on June 8 and Dec. 8 . Samples were also taken on July 13 and Sep 2 2 , but only the f i r s t and last were chosen for illustration in this study. Additional details on this field site are given in Wynan et a l ( 8 ) . Figure 2 shows the planting and emergence simulations vs. observations, including tabular summaries for the Dec. 8 date. Table IV summarizes the water balance for the Cameron calibration, and Table V summarizes aldicarb fate and transport. Site 3 - Hancock, Wisconsin potatoes. Aldicarb was applied i n 1982 and 1983 at the University of Wisconsin Experimental Farm at Hancock in central Wisconsin. The soil at this site is a Plainfield loamy sand. The organic matter i n the top 0 . 3 m i n this soil i s 0.77%. The mechanical analysis for the three meter core depth sampled was: 89-97% sand, 0-6% s i l t , and 1-9% clay. Like the Cameron site, there was a planting (May 1 9 , 3.36 kg/ha) and an emergence (June 4 , 2.24 kg/ha) application of aldicarb in 1982, and two similar applications i n 1983 (May 10, 3.36 kg/ha; and June 2 1 , 2.4 kg/ha). There were two irrigation regimes practiced i n 1982, and one in 1983. One irrigation scheme was similar to the irrigation in Cameron in that i t sought to meet evapotranspirative demand. This was called the "medium schedule". The second irrigation scheme, which was only practiced in 1982, was termed the "heavy schedule" i n that 60% more water than estimated to meet the evapotranspirative demand was applied with identical timing as the medium schedule. In summary, there were a total of six separate scenarios at Hancock: four in 1982 which included permutations of application date (and rate) and irrigation strategy, and two i n 1983 having different application dates. Daily rainfall was obtained on-site for 1982 and 1983. Actual évapotranspiration was estimated on-site for 1982 with Penman's equation and directly input to PRZM, while pan evaporation was available for 1983. The methodology for soil coring was similar to the Cameron and North Carolina sites. Additional details on the Hancock site can be found in Wynan et al ( 8 , 9 ) . Figures 3-5 summarize the Hancock calibration results, and Tables IV and V summarize the water balance and aldicarb fate and transport for the Hancock scenarios. Discussion of Calibration Results Table IV summarizes the water balance results of a l l the calibration scenarios. A trend that can be seen from this data is that évapotranspiration demand i n Wisconsin is consistantly around 43 cm for the summer months between May and September, regardless of irrigation added or soil type. Recharge was higher for the loamy sand soil in Hancock, 35 cm, than for the sandy loam i n Cameron, 26 cm. Recharge increased to 53 cm when 19 cm of extra irrigation water was added to the sand s o i l , indicating that this extra irrigation was not needed by the crop. Evapotranspiration i n North Carolina


350


R U N #1 Concentration, ppb

Depth, cm


3.36 kg/ha Aldicarb Applied May 13,1983

J U L Y 14

JUNE 6

100 I

200

300

I

_1_

500 I

200 -JL_

Depth

300

400

Obs

cm

Pred ppb

0-30

S E P T E M B E R 15

500

-J

_1_

7

5

30-60

4

16

60-120

4

4

120-180

ND

0

180-300

ND

0

N O V E M B E R 30

Figure 1. Calibration results of PRZM, scenario 1 (see Table 3): concentration-depth profiles for predicted (smooth curves) vs. observed (dashed lines) aldicarb applied to tobacco i n North Carolina (observed data from 7).


18.

Assessment

LORBER A N D OFFUTT

RUN

of Ground

Water Contamination

Potential

351

#2

Concentration, ppb 300

100

I

Depth

Depth, cm



ppb

0-30

47

30-60

10

19

60-120

12

10

120-180

ND

2

180-240

ND

0

240-300

ND

0

JUNE 8

ι

#3

100

200 I

300 I

L_

Depth

June 4 , 1 9 8 2

JUNE 8

Pred

Obs

cm

2.24 kg/ha Aldicarb Applied

45

DECEMBER 8

RUN

100

Pred

Obs

cm

ppb

0-30

13

13

30-60

ND

6

60-120

2

3

120-180

ND

1

180-240

ND

0

240-300

ND

0

DECEMBER 8

Figure 2. Calibration results of PRZM, scenarios 2 & 3 (see Table 3): concentration-depth profiles for predicted (smooth curves) vs. observed (dashed lines) aldicarb applied to potatoes i n Cameron, Wisconsin (observed data from 8).


352


was higher than in Wisconsin, at 50 cm for the period of testing between May and November. Recharge was only a small fraction of rainfall during this period, equalling 14 cm. Table IV.

Summary of water balance results for calibration scenarios ( a l l quantities in cm/yr).

Description 1


Dates

May-Nov 1983

2^3

Calibration Run # 4,5

May-Nov 1982

May-Sep 1982

May-Sep 1982

May-Sep 1983

Precipitation 71.4

86.4

44.7

43.4

59.3

Irrigation

0.0

11.7

31.7

50.8

24.4

Runoff

6.9

2.9

0.9

1.0

3.6

Evapotranspiration

50.4

44.6

41.4

41.9

44.4

Recharge

14.0

25.6

35.1

52.5

37.0

0.0

24.8

0.0

0.0

0.0

Change in Soil Storage +0.1

+0.2

-1.0

-1.2

-1.3

Snow Storage

The calibrated half-life for aldicarb i s longer than the halfl i f e which was calculated based on field data (Table II). This occurs because runoff loss of aldicarb as well as leaching below the depth of sampling are not accounted for in field-calculated half-lives, which are calculated based only on aldicarb remaining at each sampling date. An additional possible avenue of loss i s plant uptake of aldicarb. However, the total amount of uptake was not estimated in the f i e l d , nor was i t simulated in PRZM. As such, i t can be considered that plant uptake loss was "lumped" in the calibrated (and calculated) half-lives. Table V summarizes the fate and transport of aldicarb in the calibration scenarios. In North Carolina, the simulations predict that 4.2% of applied aldicarb was lost via runoff and none leached below the depth of sampling. The runoff result cannot be verified since field data of runoff were not taken. However, since the s o i l was a loam s o i l , some water runoff would be expected, and a fraction of the soluble aldicarb present in top zone on the date of runoff would also run off. As a result, the calibrated halfl i f e , 43 days, is higher than the range calculated, 27-39 days. In the Cameron, Wisconsin simulations, aldicarb did not leach below the depth of sampling for the planting or emergence appli-


18.

Table V.

of Ground

Water Contamination

Potential

353

Summary of fate and transport results for aldicarb calibration scenarios ( a l l results expressed i n percent of applied).

Description Applied

1

2

Calibration Run # 3 4 5

6

7

8

9

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Decay


Assessment

LORBER A N D OFFUTT

91.9

73.5

92.9

73.8

83.9

67.9

78.5

75.9

77.7

Runoff

4.2

2.1

1.3

0.3

0.4

0.2

0.3

0.0

0.0

Leached Below 3 meters

0.0

0.0

0.0

1.5

1.3

15.2

10.9

2.1

1.3

Remained in profile

3.9

24.4

5.8

24.4

14.4

16.7

10.3

22.0

21.0

cation. However, a small percentage of applied was found to runoff for both application dates, 2.1 and 1.3%. As a result, the calibrated half-life, 69 and 46 days for planting and emergence applications, respectively, were only slightly higher than the calculated half-lives, 65 and 43 days. In Hancock, where there was a loamy sand s o i l , leaching below the depth of sampling was simulated with a l l scenarios, particul a r l y the scenarios of high irrigations. Approximately 15 and 11% of applied aldicarb leached below 3 meters i n these high irrigation scenarios (runs # 6 & 7). The field-calculated half-life for these intense irrigation scenarios is i n the neighborhood of 35 days; the calibrated half-life was 69 days. As noted earlier, the extra irrigation did not add to the consumptive use of water. Therefore, the field calculated half-life was misleading for these scenarios of intense irrigation. Between 1.3 and 2.1% of applied aldicarb was simulated to leach below 3 meters for the scenarios of medium irrigation, again resulting in higher calibrated halflives than were calculated. One other important observation that i s evident from both field observations and simulations i s that the half-life decreases with later application dates. This occurs because the aldicarb is applied during warmer weather and i s hence subject to a healthier environment for microbes, which can degrade i t more quickly. As seen i n Table V, the amount of leaching i n Hancock decreased from planting applications, 1.5, 15.2, and 2.1% of applied (runs 4,6,8), to emergence applications, 1.3, 10.9, and 1.3 (runs 5,7,9). A more appropriate simulation approach might be to make the rate of decay a function of a i r or soil temperature, or to simulate a step-wise change in rate of decay as a function of time. Nonetheless, this more rapid rate of decay for later applications results in less leaching for later applications. In order to force the simulated profiles to match the observed


354


RUN

500 600

#4

750

300 I

—L

400 I

500

—I

2*7 Depth cm


α ν Q


JUNE 9

Obs

Pred ppb

0-30

37

63 28

30-60

14

60-120

20

19

120-180

12

17

180-240

12

12

240-300

3

7


RUN # 5

300

1 2.24 kg/ha Aldicarb Applied May 28,1982

JUNE 9

400

I

500

I

Depth cm

Obs

I

Pred ppb

0-30

28

28

30-60

8

12

60-120

5

8

120-180

7

7

180-240

2

5

240-300

ND

3


Figure 3 . Calibration results of PRZM, scenarios 4 & 5 (see Table 3 ) : concentration-depth profiles for predicted (smooth curves) vs. observed (dashed lines) aldicarb applied to potatoes i n Hancock, Wisconsin (observed data from 8 ) .


18.

LORBER A N D OFFUTT

Assessment

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Water Contamination

Potential

355

RUN # 6

Concentration, ppb

300

400

I

\

400

500

L_

I

I

Depth

Obs

500 600 1300

200



Pred ppb

cm 0-30

16

18

30-60

ND

11

60-120

2

9

120-180

17

13

180-240

4

15

240-300

5

15

JUNE 9


RUN

#7

300 I

100

—I—

400 I

IT Depth

Obs

0-30

2.24 kg/ha Aldicarb Applied May 28, 1982

JUNE 9

Pred ppb

cm ND

30-60

2

60-120

ND

120-180

ND

180-240

ND

240-300

ND


Figure 4 . Calibration results of PRZM, scenarios 6 & 7 ( see Table 3 ) : concentration-depth profiles for predicted (smooth curves) vs. observed (dashed lines) aldicarb applied to potatoes i n Hancock, Wisconsin (observed data from 8 ) .


356


RUN

#8

Concentration, ppb 300

500

400

I

1 Depth

Obs

Pred

cm

ppb


0-30

45

53

30-60

6

24

60-120

3

16


120-180

2

15

May 10,1983

180-240

ND

12

240-300

ND

7


J U N E 23

R U N #9

300

400

Depth

June 21,1983

J U N E 23

I

Obs

cm


500

I

—J

Pred ppb

0-30

74

50

30-60

37

17 10

60-120

11

120-180

5

8

180-240

3

5

240-300

ND

3


Figure 5. C a l i b r a t i o n r e s u l t s o f PRZM, s c e n a r i o s 8 & 9 (see Table 3 ) : concentration-depth p r o f i l e s f o r p r e d i c t e d (smooth c u r v e s ) v s . o b s e r v e d (dashed l i n e s ) a l d i c a r b a p p l i e d t o p o t a t o e s i n Hancock, W i s c o n s i n ( o b s e r v e d d a t a from 9 ) .


18.

LORBER A N D OFFUTT

Assessment

of Ground

Water Contamination

Potential

profiles, a higher adsorption partition coefficient, K^, was required for the top zone than would be calculated based on aldicarb K and soil organic matter. In North Carolina, the required for the top 15 cm was 0.50, and in Wisconsin, was 1.00 for the top 15 cm and 0.50 for the 15-30 cm layer. These are the highest assumed for aldicarb as compared to other published modeling efforts on aldicarb, which range in from 0.0-0.3 (3,5*12,13) The reasons for this discrepancy are not known. Several possibilities exist which include: 1) PRZM may be overestimating the amount of water to percolate from the top zones to lower zones. This would occur i f the water holding capacities measured in the field and assigned in PRZM are too low, resulting in less évapotranspiration and more percolation. This could also occur with inacurrate estimations of runoff - i f more runoff occurs than i s being simulated, then percolation will be overestimated. 2) It may be connected to the granular formulation of aldicarb. The outer protective layer dissolves upon contact with moisture, releasing the active ingredient, aldicarb. Furthermore, granules are incorporated, leading to the possibility of pockets of high granular concentration. If this is the case, a wetting event might not dissolve the protective coating of a l l the granules, leaving some partially dissolved granules. However, i n several aldicarb test plots, high chloride concentrations (where chloride was applied as a tracer) also remain near the surface. This would seem to indicate that the granular formulation is not the cause of high surface aldicarb concentrations (14). 3) I t is possible that evaporation demand at the soil surface results i n water translocating upward from shallow depths,

A Method for the Assessment of Ground Water Contamination

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