Effect of soil conditions on model parameters and atrazine mineralization rates

~

Wat. Res. Vol. 28, No. 5, pp. 1199-1205,1994 Copyright © 1994ElsevierScienceLtd Printed in Great Britain.All rights reserved 0043-1354/94$6.00+ 0.00

Pergamon

EFFECT OF SOIL CONDITIONS ON MODEL PARAMETERS A N D ATRAZINE MINERALIZATION RATES DHILEEPAN R. NAIR and JERALDL. SCHNOOR* Hazardous Substances Research Laboratory, 116 Engineering Research Facility, Department of Civil and Environmental Engineering, University of Iowa, IA 52242, U.S.A. (First received November 1992; accepted in revised form July 1993)

Abstract--Transformation of pesticides is dependent on soil environmental conditions and knowledge of this is necessary to improve subsurface fate and transport pesticide models. Laboratory experiments were performed using 14C ring and isopropyl side-chain labeled atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) applied to three Iowa soils incubated in batch reactors under different environmental conditions. Mineralization of both the ring and isopropyl side chain carbons was proportional to the organic matter content of the soils and oxygen content. Atrazine ring carbon mineralization also increased with soil water content. Oxygen limitation in soils reduced the bio-transformation rate of atrazine, and mineralization was much slower under denitrifying conditions. Empirical models were developed to represent the mineralization rate of atrazine ring carbon and isopropyl side-chain carbon for varying soil organic matter, soil water content, temperature, and oxygen partial pressure. Key words--atrazine, pesticide, soil, bio-transformation, I4C labeled, electron acceptor, mineralization, empirical models

INTRODUCTION Existing pesticide transport and fate simulation models such as CMIS, GLEAMS, LEACHMP, MOUSE, RUSTIC, PESTAN, PRZM and SESOIL use pesticide disappearance kinetics which are independent of soil environmental conditions. These models are usually used for relatively long simulations, during which environmental conditions in soil change significantly, so usage of time and environment invariant pesticide disappearance kinetics is not realistic for modeling purposes, although this is the most convenient way to describe pesticide disappearance if there is a paucity of field and laboratory data. Kaufman and Kearney (1970) stated that environmental conditions such as temperature, soil type, soil water content, aeration and supplemental energy sources influence the capacity of microbial systems to mineralize pesticides like s-triazines and attributed these to the differences reported in the research literature on s-triazine biotransformation and mineralization. For example, Armstrong et al. (1967) found that the atrazine disappearance rate was higher in soils with low soil pH, higher organic matter content, and clay content. McCormick and Hiltbold (1966) found that atrazine mineralization and transformation in soils were increased with higher temperatures and were dependent on soil characteristics. Skipper *Author to whom all correspondence should be addressed.

and Volk (1972) observed that mineralization of atrazine from two soils were the same, although the soils varied significantly in organic matter content, clay content and bacterial counts. Hurle and Kibler (1976) found that atrazine disappearance in sandy loam soil was affected by soil water content and that the half-life of atrazine increased with drier soil. Goswami and Green (1971) did not detect mineralization of atrazine in anaerobic systems, while mineralization of atrazine in aerobic soils has been widely reported in the research literature (Kuhn and Suflita, 1989). Nair and Schnoor (1992) showed that mineralization of atrazine under denitrifying conditions was much slower compared to aerobic conditions. The laboratory batch studies reported here were performed to determine mineralization kinetics for a widely used triazine herbicide, atrazine, under different soil environmental conditions, which could then be used in model formulations to improve predictions. The different soil environmental conditions which were simulated included soil type/organic carbon content, soil water content and soil oxygen content. The experiments were set-up to simulate conditions in deeper unsaturated soils where air diffusion from the surface will be limited.

MATERIAI~ANDMETHODS Three different soils were used for the studies. Soil 1 was taken from the top 150 cm of the surface (organic and upper soil horizons), from an experimental plot just adjacent to Lily Lake, Amana, iowa. The soil is classified as of the

1199

1200

DHILEEPAN R, NAIR and JERALD L. SCHNOOR Table 1. Soil characterization of three Iowa soils (Nodeway-Ely series) used in laboratory experiments Soil 1

Soil 2

Soil 3

Location Horizon Texture pH (1:1) soil:d.w. Organic matter content

Lily Lake, Amana A (0-150 cm) silt-loam 6.3 2.20

Near Visitors Center, Amana B (0.6--1.0 m) C (1-2 m) loam silt-loam 6.8 7.4 1.0% 2.4%

Particle size distribution % sand (0.05-2.0 mm) % silt (0.002-0.05 mm) % clay ( < 0.002 mm)

12.5 67.5 20

40 42.5 17.5

15 65 20

Chemistry Total organic carbon Total Kjeldahl nitrogen Total phosphorus Calcium Potassium Magnesium Sodium CEC (mequiv/100 gm soil)

12500 ppm 1182 ppm 326 ppm 3100 ppm 160 ppm 660 ppm 33 ppm 23.5

3510 ppm 619 ppm 314 ppm 2600 ppm 150 ppm 450 ppm 20 ppm 17.2

7710ppm 1165 ppm 510 ppm 2200 ppm 80 ppm 400 ppm 13 ppm 14.6

Nodeway-Ely series and is of silt-loam texture. Soils 2 and 3 were from an agricultural field only 200 m away. Soil 2 was taken from a depth o f 0.6-1.0 m below the surface (B-horizon) and was o f loamy texture. This soil formed a lense in a predominantly silt-loam soil profile. Soil 3 was from a depth of 1.0-2.0 m (mineral soil horizon) and was of silt-loam texture. Table 1 lists the characteristics of the three soils. All three soils were air-dried, pulverized and passed through a 2 m m sieve. The soils were then homogenized and 500 g portions of the whole soil were added to reactors. Three types of reactors were used for the batch studies. Wide-mouth 1000ml Erlenmeyer flasks were sued for aerobic bio-reactor studies, while 50 ml serum bottles were used for the microcosm reactor studies. For the microcosm reactors, 30 g of soil were added and sealed with gas-tight caps. The bio-reactors were then sealed with neoprene stoppers with two glass tubing inserts and were sealed air tight with paraffin wax. Controls were prepared using glass beads and sterilized soil as media. Soil for the sterile controls was first autoclaved in trays for i h at 250°F and 14 psi. The autoclaved soil was then transferred to the flasks, stoppered and autoclaved again for 15 min each at 250°F and 14 psi (Mihelcic and Luthy, 1988). All bio-reactors were prepared either in triplicate or duplicate, and four, six or nine replicates of the microcosm reactors were prepared. Uniform ~4C ring labeled atrazine (2-chloro-4-ethylamino-6-isopropylamino-l,3,5-triazine) (specific activity 19.4 #Ci/mg) and t4C isopropyl side chain labeled atrazine (specific activity 4.9/zCi/mg) (Ciba-Geigy, Greensboro, N.C.) were used for the soil mineralization experiments. Chemical purity was 95 and > 9 9 % , and radiochemical purity was 98.9 and 98.4%, for the ring labeled and isopropyl side chain labeled atrazine respectively. The atrazine was dissolved in distilled water and applied to the soil in the batch reactors at concentrations normally found in agricultural fields, 0.3-0.8 ppm ( p g / g soil). Deionized water was added after the atrazine application to bring the soil to a range of soil water contents, which were from 15% field capacity (water holding capacity) to 100% field capacity. The reactors were sealed air-tight and specific gas mixtures (100% N2, 60% N2/40% 02, air or 100% 02) were passed through the inlets o f specific bio-reactor sets and microcosm reactor sets until reactors were filled with only the respective gases. The bio-reactors were individually wrapped with aluminium foil to shield off light, and the microcosm reactors were placed in boxes shielded from light. All the reactors were kept under a fume hood. The temperature in the laboratory was in the range of 20-25"C. At regular intervals, the air in the reactors was removed by a vacuum p u m p from one outlet and fresh gas was passed

into the reactors from the other. The gases evacuated were trapped in CO2 traps consisting of glass tubes and 0.2 N N a O H (prepared with CO2 free deionized water). The fresh gas was passed through a flask of CO2-free deionized water to moisten the gas before it refilled the reactors. Recovery of ~4CO2 in the CO2 traps was greater than 95%. The samples from the traps were then pipetted into 20 ml polypropylene scintillation vials and a scintillation cocktail, Scintiverse E (Fisher Chemicals) was added at twice the volume of the sample and the vials were counted using a Beckman Model LS 6000IC liquid scintillation counter. Samples were counted until a fractional error o f + 0.5 % was obtained at the 95% confidence level, subject to a m a x i m u m allowable counting time of 15 min. Counting efficiency for this sample-cocktail mixture was over 90%. LABORATORY RESULTS AND DISCUSSION Soil type F i g u r e 1 s h o w s t h e v a r i a b i l i t y in a t r a z i n e m i n e r a l i z a t i o n r a t e as a r e s u l t o f soil o r g a n i c m a t t e r c o n t e n t .

15

D

o Soil 1 (2.2% OM) A Soil 2 (1 .0% OM) [] Soil 3 (2.4% OM) o

10

•~

5

T "1- T ~ ~T.~l..

L

r,.)

50

100

150

200

250

300

Time (days) Fig. 1. Variability of atrazine ring mineralization rate with soil type and organic matter (OM) content. ~4C ring labeled atrazine applied (applied at 0.77 ppm) to the soils incubated in bio-reactors maintained under aerobic condition and at 60% field capacity soil-water content. Data points are average results of replicate reactors. Error bars show standard errors for the data.

Modelling atrazine mineralization rates It compares cumulative percent mineralization of ~4C ring labeled atrazine in three soils at various locations (Nodeway-Ely Soil Series) in bio-reactors maintained under aerobic conditions. Set AI9 (Soil 3, silt-loam) shows similar percent mineralization when compared to A9 (Soil 1, silt-loam), although A9 was taken from the top soil at another location. However, soil used for set AI9 had similar organic matter content and was from an agricultural field where atrazine was applied during the previous growing seasons, so it contained indigenous microorganisms which were acclimated to atrazine, while the soil used for set A9 was from a plot with no agricultural use and without a long history of atrazine application. Set A8 (Soil 2, loam) showed much lower mineralization. Microbial population and activity was expected to be lower in this loamy soil, with a low organic carbon content. Higher amounts of clay and organic matter provide more adsorption sites in the soil for hydrolysis, microbial attachment and growth. A sterile soil bioreactor showed negligible mineralization of the atrazine ring. This is in agreement with previous findings by other researchers, that mineralization of the triazine ring involves mainly microbial transformation reactions (Skipper et al. 1967; Wolf and Martin, 1975). Figure 2 compares the mineralization of ~4C isopropyl side chain labeled atrazine in the microcosm reactors with different soils incubated under aerobic conditions. Reactor set SAI0 (Soil 3) with silt loam soil from a depth of 1-2 m below an agricultural field showed higher mineralization amounts of the isopropyl side chain carbon, than reactor SA4 (Soil i)

10

-

8 -

B 15% field capacity A 30% field c a p a c i t y o

"~ N

1201

6 -

7"

100% field c a p a c i t y

. l~¢/] ~

× 60% field c a p a c i t y

"r"~/[-'~-I - -

E

L)

2

~2 0

50

100

150

200

Time (days) Fig. 3. Increasing atrazine mineralizations with soil water content. ~4Cring labeled atrazine applied at 0.4 ppm to Soil 1 incubated in microcosm reactors. Data points are average results of replicate reactors. Error bars show standard errors for the data.

with top soil of a non-agricultural plot. One hypothesis for this finding is that Soil 3 has a greater indigenous population of acclimated microbes capable of dealkylating and mineralizing the isopropyl side chain carbons of atrazine than Soil 1, which does not have a history of atrazine application. Soil 2 showed the least mineralization and this confirms the earlier finding with ring labeled atrazine that soils with lower organic matter content and low clay content transform atrazine at a much slower rate. Figures 1 and 2 lead to similar conclusions that the rate of mineralization is directly related to organic carbon content of the soil. Soil water content

8 --

6' L)

o

Soil 1 (2.2% O M )

A

Soil 2 (1.0% O M )

../-

6

"~

4

"3 ~ 2 ",N

.o O ~

0

20

t

t

I

I

40

60

80

100

Time (days) Fig. 2. Variability of atrazine isopropyl side-chain mineralization rate with soil type and organic matter (OM) content. 14Cisopropyl side-chain labeled atrazine applied (applied at 0.37 ppm) to the different soils incubated in microcosm reactors maintained under aerobic conditions and 60% field capacity soil-water content. Data points are average results of replicate reactors. Error bars show standard errors for the data.

The effect of different soil water contents on atrazine ring carbon mineralization in microcosm reactors maintained under aerobic conditions is shown in Fig. 3. Moisture is necessary for microbial activity and other soil processes including diffusion of gaseous oxygen into water. However at higher moisture contents, air diffusion within soil pores will be restricted and with higher biological activity, dissolved oxygen deficit may occur with insufficient diffusion of air into soil pores. Reactor set A13 with a dry soil (15% field capacity) showed a very low rate of mineralization. Microbial activity is expected to be extremely low in dry environments. The saturated soil reactor set A l l , showed the highest mineralization rate. Since the volumetric ratio of soil to gas was small in this reactor, there was sufficient diffusion of air into soil water. Reactor sets A4 (60% of field capacity) and Ai2 (30% of field capacity) showed nearly similar mineralization rates. Note that the mineralization amounts were negligible between days 0 and 60, for all the reactors. These reactors were filled with air after each sampling, for the first 60 days. For the rest of the experimental period the reactors were refilled with 60% N2/40% 02 gas mixture. So, oxygen could have become limiting in

1202

DHILEEPAN R. NAIR a n d JERALD L. SCHNOOR

L~ ~"

7 --

o

0 % oxygen

6 --

0

20% oxygen

~

,~

assuming that substrate concentration is low and microbial concentration is constant with time (Valentine and Schnoor, 1986), a first order mineralization model can be written as,

T

A 40% oxygen 4

o

3

O t. m O O.=

2

~

1

dP dt

--

-k

(I)

x P

where,

I

0

=

20

40

60

80

100

120

Time (days) Fig. 4. Effect of different oxygen contents in the headspace of microcosm reactors on mineralization of isopropyl side chain labeled atrazine. Atrazine applied at 0.4 ppm to Soil 1. Soil at 60% field capacity for aerobic reactors and 100% for anoxic reactors. Data points are average results of replicate reactors. Error bars show standard errors for the data. between sampling times resulting in low mineralization amounts in the first 60 days. Oxygen content Figure 4 shows the effect of oxygen concentration in microcosm reactors on mineralization of t4C isopropyl side chain labeled atrazine. Reactor sets SA3, SA4, SA5 and SN4 had initial (after each sampling) oxygen concentrations (in the headspace of the reactors) of 100, 40 20 and 0% respectively. The results seem to demonstrate an oxygen limitation in the SA5 reactors at 20% oxygen partial pressure. The reactors were not aerated continuously as the reactors were meant to simulate deep unsaturated soil environments, so oxygen levels could have been lowered in this reactor set between sampling, causing a lower mineralization rate. Parr and Reuszer (1959) found that carbon dioxide evolution, from silt-loam soil amended with a wheat straw, was decreased by 87% when nitrogen gas was used in the decomposition chamber instead of air. Oxygen levels of 2.5 and 5.0% decreased carbon dioxide evolution by 30 and 14%, respectively. Reactor sets SA3 (100% oxygen partial pressure) and SA4 (40% oxygen partial pressure) did not have this oxygen limitation problem and showed higher rates of mineralization than the SA5 (20% oxygen partial pressure) reactor sets. Oxygen limitation at deeper soil depths will retard atrazine transformation and mineralization as the soil environment becomes more anoxic. Also at deeper soil depths microbial activity will be much lower with lower organic carbon, thus retarding the biotransformation processes even more. MINERALIZATION

KINETICS

AND MODELS

TO describe atrazine mineralization for these different environmental conditions in soil, a number of empirical models were developed. Using Monod kinetics for microbial substrate (pesticide) utilization and

P = pesticide concentration (/~g/g soil) k = pseudo-first order mineralization rate constant (l/day) t = time (day) The mineralization data was fitted to obtain atrazine mineralization rates under different environmental conditions. The results of the model fits are shown in Table 2 together with the coefficient of determination (r2). A zero order model showed comparable fits with the first order model for this data. The first order model, however, is usually preferred and widely used in the research literature to reflect the dependency of the reaction rate on organic concentration over wide ranges of concentration. The first order model results in Table 2 allow quantitative comparison of the kinetics of mineralization for the different environmental conditions. Soil 1 had 3.1 and 2.8 times higher mineralization rates than Soil 2 for ring and isopropyl side chain carbons, respectively. Comparing reactors with different oxygen contents, the reactors with 100% 02 in the headspace had about 20 and 10 times higher mineralization rates than the anoxic reactors (0% 02) for ring and isopropyl carbon mineralization, respectively. Apparently, isopropyl mineralization under anoxic conditions is not as slow as ring mineralization under anoxic conditions. Mineralization of the isopropyl side chain carbon was 2.1 times faster than mineralization of the atrazine ring.

Table 2. Mineralization rates of atrazine under different environmental conditions (n = number of replicates) Reactor code and environmental variable

First order mineralization rate (10-S/day)

r2

n

14C ring labeled atrazine A9 (Soil 1) A8 (Soil 2) A 19 (Soil 3)

Bio reactors 49.4 15.9 59.4

0.99 0.98 0.97

3 3 3

AI3 (15% FC) A12 (30% FC) AII 0 0 0 % FC) A4(60% FC, 40% 02) A3 0 0 0 % 02) N4 (Anoxic, 0% 02)

Microcosm 2.5 25.7 52.2 29.1 29.9 1.5

reactors 0.90 0.98 0.98 0.97 0.96 0.96

6 6 6 6 4 4

5.9 64.0 62.4 15.7 22.5 68.5

0.91 0.99 0.94 0.97 0.99 0.97

4 4 4 4 4 4

t*C isopropyl side chain labeled atrazine SN4(Anoxic, 0% 02) SA3 (Aerobic, 100% 02) SA4(Soil I, 40% 02) SA5 (Aerobic, 20% 02) SA8 (Soil 2, 40% 02) SAI0(Soil 3, 40% 02)

Modelling atrazine mineralization rates Comparing the bio-reactor and microcosm reactor ring carbon mineralization rates, one can observe that the rates were lower in the microcosm reactors. The mineralization rates for these microcosm reactors were determined for the second experiment period as there was negligible mineralization in the first 60 days as mentioned previously. Yet, the rates were lower and this was attributed to the difference in reactor geometry. The bio-reactors had a higher gas headspace volume per g of soil compared to the microcosm reactors and thus higher oxygen levels. The rate constants (mineralization rates) were plotted against each of the environmental variables, that is, soil water content, organic carbon content and initial oxygen content of the reactor gas headspace. The last variable (oxygen content) includes the two different electron acceptors. The plots were then fitted with models to simulate the effect of each environmental variable on the mineralization rates. Figure 5 is a plot of atrazine ring mineralization rate versus soil organic matter content under aerobic conditions. The predicted line was determined with a linear least-squares model (equation 2) as written below, k = 2 3 . 4 x 10 3 × ( f O M ) (2) where, k = mineralization rate constant of the atrazine ring (day- i ) fOM = fraction organic matter content of soil Results suggest that the organic matter content of the soil serves as the electron donor for microorganisms that also cometabolize atrazine. Thus, the greater the organic matter content of the soil, the greater the rate of mineralization of atrazine. Figure 6 shows the plot of mineralization rate of atrazine ring carbon versus soil water content for Soil 1 (2.2% organic matter) incubated under aerobic conditions. The predicted line was determined with a linear least-squares model (equation 3) as shown below: k = 0.5 x 10 -3 x (SWC) (3)

70 =,-, O

60

1203

m

o -a

Measured rate constant

T

Predicted rate constant

~/

I ~ ~ 20 O

O

10 0

Soil

I

I

I

1

1

I

20

40

60

80

100

120

water content

(% field

capacity)

Fig. 6. Comparison of measured and predicted atrazine ring carbon mineralization rates for soils with different soil w a t e r content. The predicted rates were determined with equation 3. The bars show the 95% confidence intervals.

where, k = mineralization rate constant of the atrazine ring carbon (day- l ) SWC = soil water content (mass fraction of field capacity) Results suggest that there is no microbial activity in a totally dry soil and that microbial mineralization is directly proportional to SWC. Differences in soil:water partition coefficients as the soil water content increases may also result in atrazine being more available for microbial mineralization. Figure 7 shows a plot of atrazine ring carbon mineralization rate versus reactor headspace 02 content. The data were fitted with a modified M o n o d equation as shown below, which was adapted from Borden and Bedient (1982), and it describes oxygen limitation for atrazine mineralization. k

kmax [O21"~ 1.5-1- ~ T [ - ~ 2 ] J x 10 -5

=

=

{ 1 . 5 + 0 ._:5 to2] 1+[O2])x10-5

(4)

80 -

:°70_

o Measured rate

.2~

• Predicted rate constant

E ~

60 --

T/

constant

•,~ eS

~s '~ 30 N

•~- ~

20

"~

10

E~

40

f

30

0 Measured

o ~ 20 .~ a. ~-

~0

P:°

iO

0

t

t

] 1

2

Organic matter content

3 (%)

Fig. 5. Comparison of measured and predicted atrazine ring carbon mineralization rates for soils with different organic matter contents. The predicted rates were determined with equation 2. The bars show the 95% confidence intervals.

~" 0

rate constant

" Predicted rate constant

20

I 40

I 60

I 80

J 100

Oxygen content (%) Fig. 7. Comparison of atrazine ring carbon mineralization rates for Soil I with different o x y g e n contents. The predicted rates w e r e d e t e r m i n e d with equation 4. The bars show the 95% confidence intervals.

DHILEEPAN R. NA|R and JERALDL. SCHNOOR

1204

where,

~,

k =mineralization rate constant for atrazine ring carbon (day- J) kmax = maximum mineralization rate constant (day-t) [O:] = fractional oxygen content in reactor headspace in atm (pO2) Ko = half-saturation rate constant in atm (pO2). Valentine and Schnoor (1986) outlined a number of empirical equations used to calculate biodegradation rate constants at different temperatures. One equation commonly used is, kz = k20 x q(r-20)

(5)

where, T = temperature at which rate constant is required (°C) q = empirical factor for the particular pesticide k20 = rate constant determined at 20°C Figure 8 is a plot showing the predicted values based on equation 5 and a linear least-squares fit of atrazine mineralization rate with temperature. The value of q used for this model was 1.045, and was developed from research literature data on atrazine degradation in soil (Zimdahl et al. 1970; Roeth et al. 1969). A master equation was developed incorporating the effects of soil water content, oxygen content, temperature and organic matter content on atrazine ring mineralization rates. The equation developed is shown below, k = 4.7 x 1.045 ~r- 20). [02] 0.1 + [02] x fOM x SWC x 10 -2

(6)

where, k = mineralization rate constant for atrazine ring carbon (day-l); fOM, SWC, T and [02] are as previously described. Figure 9 compares this model equation predicted ring mineralization rates with the observed mineralization rate constants determined in the laboratory. 60 - o Predicted temperature r , / .3~

50 - ~

R2==0.96

.= ~ 40 o

30

,~ ~ 20 0

10

20

30

40

Temperature (oC) Fig. 8. Comparison of atrazine ring carbon mineralization rates at different temperatures. The predicted data were with equation 5 (developed from Zimdahl et al., 1970 and Roeth et al., 1969).

60 -

[]

R2 = 0.96

,o-

"~

40

"~ ~ 30

~ ~

10 I

0

10

20

30

40

50

60

Predicted mineralization rate constant (x lO-5/day)

Fig. 9. Comparison of predicted and observed atrazine ring mineralization rates (equation 6). SUMMARY These laboratory studies support the hypotheses that atrazine mineralization and transformation rates depend strongly on soil environmental conditions. Consideration of soil environmental conditions is essential to improve model predictions. Under aerobic conditions, atrazine mineralizations and transformation rates increased with increasing soil water content (provided oxygen was not limiting) and with increasing soil organic carbon content. Soil oxygen content affected both ring carbon and isopropyl side chain carbon mineralization. Oxygen diffusion limitation in deep and saturated soils is expected to reduce dramatically transformation rates of atrazine and its metabolites. Higher soil water content in the upper soil profile will enhance microbial activity and transformation rates as long as there is sufficient oxygen in the soil. Dealkylation of the atrazine isopropyl side chain was faster than ring cleavage and mineralization. Erickson and Lee (1989) compared the degree of reductance (equivalents of 02 required per mole of C to oxidize the molecule to final end products) of atrazine which was 3.75, to that of the dealkylated species (the amino and hydroxy metabolites of atrazine which were zero) and explained that there was bioenergetic incentive for dealkylating atrazine aerobically. Skipper and Volk (1972) showed that mineralization of the ethyl side chain was about eight times faster than the isopropyl side chain. Dealkylation of the ethyl side chain and formation of dcethylatrazine is fast, but dealkylation of the isopropyl side chain of atrazine and deethylatrazine are slow and may explain why deethylatrazine is found frequently in unsaturated soils and tile drainage (Muir and Baker, 1976; Adams and Thurman, 1991). Both ring mineralization and isopropyl side chain dealkylation were similarly affected by the different soil conditions used in this study including soil water content, partial pressure of oxygen, organic matter content, and temperature. The empirical relationships (equations 2-6) developed here can be used to improve existing pesticide fate models. Although these

Modelling atrazine mineralization rates equations were developed for atrazine ring and isopropyl side chain carbon mineralization, the model concepts may be applicable to other organic pesticides with similar fate characteristics.

Acknowledgements--We thank our colleagues Professor James Osborne and Joel Burken for their assistance in the conduct of this research. The research was supported by the U.S. EPA Hazardous Substances Research Center, Region 7 and 8, at Kansas State University. It was initiated with seed funding from The Center for Health Effects of Environmental Contamination and completed with additional support from the Environmental Health Sciences Research Center, NIEHS, at The University of Iowa. No endorsement by granting agencies should be inferred. Our thanks to Ciba-Geigy for providing radiolabeled atrazine.

REFERENCES

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