Soil-Pan Method for Studying Pesticide Dissipation on Soil - ACS

Chapter 22

Soil-Pan Method for Studying Pesticide Dissipation on Soil

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: June 20, 1991 | doi: 10.1021/bk-1991-0465.ch022

B. D. Hill, D. J. Inaba, and G. B. Schaalje Agriculture Canada, Lethbridge, Alberta T1J 4B1, Canada

To predict the amount of pesticide that could leach through the soil and contaminate groundwater requires information about the residue levels at the soil surface over time. A soil-pan method has been developed to estimate surface residues and their dissipation rates. An indoor spray chamber is used to apply the pesticide to soil contained in metal flats, the treated flats are moved outdoors and set into a field, and the soil is sampled over the season by taking four cores per flat. Using this method, it was determined that the emulsifiable concentrate formulation of deltamethrin dissipated faster than the Flowable formulation. When the soil-pan method was compared with a field-plot method, the dissipation of lambda-cyhalothrin was faster in the soil pans. Monitoring the soil temperature and moisture indicated that both were slightly higher in the soil pans than in the adjacent field plots. At present, the soil-pan method is best suited for the direct comparison of different treatments. The amount of pesticide that p o t e n t i a l l y can leach through the s o i l is a function of the amount of residues at the surface. Therefore, when determining the potential for groundwater contamination, a f i r s t consideration is to estimate the amount of surface residues. Surface residues vary with the amount of pesticide i n i t i a l l y deposited and the rate of surface d i s s i p a t i o n . The methods available for studying the surface d i s s i p a t i o n of pesticides on s o i l range from large f i e l d plots with a e r i a l a p p l i cation (2), small f i e l d plots with ground-rig application (I), and outdoor microplots with pipet application (1-5) to indoor and laboratory incubations (5-7). Ideally, these methods should meet the following c r i t e r i a : 1. an even and accurate pesticide a p p l i c a t i o n , 2. representative and precise sampling with a manageable number of samples, 0097-6156/91A)465-0358$06.00/0 Published 1991 American Chemical Society

In Groundwater Residue Sampling Design; Nash, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

22. HILL ET AL

Soil-Pan Method for Studying Pesticide Dissipation


3. convenient sample handling and sample preparation for analysis, 4. results should be representative of actual f i e l d s i t u a t i o n s . With f i e l d studies, i t is d i f f i c u l t to achieve c r i t e r i a 1-3. Multi-nozzle spray boom applications are usually uneven and extensive sampling with subsampling is required. With the indoor and laboratory incubations, there are always concerns about whether c r i t e r i o n 4 has been met. The outdoor microplot method, o r i g i n a l l y developed by Smith (2,3) for soil-incorporated herbicides, uses an accurate pipet a p p l i c a t i o n , whole-plot (20 χ 20 cm) sampling to reduce error, and is conducted i n an actual f i e l d s i t u a t i o n . However, we found that for surface-applied p e s t i c i d e s , the high water volume from pipet application washed the pesticide into the s o i l and altered i t s d i s s i p a t i o n compared with nozzle a p p l i c a t i o n ( I ) . In an attempt to r e c t i f y the pipet a p p l i c a t i o n problem and yet r e t a i n the advantages of the microplot method, we developed a s o i l pan method for studying surface residues. This method features a single nozzle spray a p p l i c a t i o n , f i e l d d i s s i p a t i o n , a reduced sample size with no subsampling, and minimum sample handling. This paper describes the soil-pan method and compares d i s s i p a t i o n results from i t with results from f i e l d plots for the pyrethroid insecticides deltamethrin [(S)-a-cyano-3-phenoxybenzyl (lR 3R)-cis-2,2-dimethyl3-(2,2- dibromovinyl)cyclopropanecarboxylate] and lambda-cyhalothrin (formerly PP321), a 50:50 mixture of [(S)-a-cyano-3-phenoxybenzyl (lK,3R)-cis-2,2-dimethy1-3- (2-chloro-3,3,3-trifluoroptop-1enyl)cyclopropanecarboxylate] and [(R)-a-cyano-3-phenoxybenzyl (lS,3S)-cis-2,2-dimethy1-3- (2-chloro-3,3,3-trifluoroprop-1enyl)cyclopropanecarboxylate]. Deltamethrin i s widely used i n western Canada to control grasshoppers, cutworms, flea beetles and a l f a l f a weevil; lambda-cyhalothrin i s currently under development to control the same insects. t

Materials and Methods S o i l Type. The Lethbridge sandy clay loam (Typic Haploboroll, fine loamy, mixed, mesic) contained 24.21 c l a y , 20.5Z s i l t and 55.3X sand with CEC-20.1 meq 100 g" , 2.2Z organic matter and pH*7.9. 1

S o i l Pan Method. The s o i l pans (50 χ 35 χ 9 cm metal f l a t s with drainage holes i n the bottom) were prepared as follows: S o i l from the 0-9 cm layer of a fallow f i e l d was transferred into the pans and loose-packed by tapping the bottom and sides of the pan. The s o i l was l e v e l l e d off at the top of the pans and the pans temporarily set into the fallow f i e l d . The s o i l pans were then equilibrated for 7-10 days under f i e l d conditions. I f no s i g n i f i c a n t rainfall occurred, i r r i g a t i o n (2-3 cm) was used to further s e t t l e the s o i l i n the pans and to re-crust the surface. Pesticide treatments were applied by transferring the pans (without disturbing the s o i l ) to an indoor spray chamber equipped with a single, t r a v e l l i n g nozzle. The spray chamber was calibrated and optimized so that the spray pattern, as indicated by water-sensitive paper ( β ) , was even across the 35-cm pan width. After spraying, the s o i l pans were immediately


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returned outdoors and set into the fallow u n t i l l e v e l with the f i e l d surface. To sample, four strata were v i s u a l l y i d e n t i f i e d across the length of the pan and one core sample (0-2.5 cm χ 2.38 cm i . d . ) was taken at random within each stratum. The holes l e f t by sampling were not f i l l e d i n , but were marked with a small stake so that they could be avoided on subsequent samplings. The s o i l pans were l e f t i n the f i e l d u n t i l the end of the experiment, at which time the pans and the remaining treated s o i l were e a s i l y removed. 1986 Soil-Pan Experiment. An i n i t i a l , d i r e c t comparison of two treatments was conducted to establish that the soil-pan method was viable. The d i s s i p a t i o n of two deltamethrin formulations, an emulsifiable concentrate (EC) and a Flowable (FL), was determined on Lethbridge s o i l . The treatments were applied at 10 g h a ' using 125 L h a " volume with five replicate pans per treatment (EC, FL, and unsprayed c o n t r o l ) . The pans were set into the f i e l d i n a closely grouped, completely randomized design. Samples were taken at intervals over 0-16 weeks. 1

1

1988 Soil-Pan versus F i e l d - P l o t Experiment. A pan versus plot experiment was conducted to determine whether the soil-pan method gave representative results compared with a larger f i e l d - p l o t experiment. Prior to the pesticide applications, the s o i l in the pans and the f i e l d plots was prepared (cultivated, raked, watered and equilibrated) as i d e n t i c a l l y as possible. The f i e l d - p l o t experiment consisted of four r e p l i c a t e s , each 2 x 4 m. Lambdacyhalothrin (FL formulation) was applied at 15 g ha" in 125 L ha" volume using a bicycle sprayer with four nozzles on a 2-m boom. The plots were sampled by bulking two cores per s i t e from eight sites per replicate (sites chosen at random within a s t r a t i f i e d design). Pans were sprayed and sampled as before (four replicates) and set into the f i e l d - p l o t experimental s i t e . 1

1

1989 Soil-Pan versus F i e l d - P l o t Experiment. The 1988 pan versus plot experiment was repeated using an EC formulation of lambdacyhalothrin instead of the FL formulation. As a precaution against possible wind and water erosion, and loss of surface residue from the pans, the s o i l l e v e l was l e f t 2.5 cm below the l i p of the pans. Sample Handling and Residue Analysis Method. A l l s o i l samples were stored at -40°C u n t i l analysis. Samples from the soil-pan method were thawed and the whole four-core sample was analyzed d i r e c t l y . The composite samples (16 cores) from the f i e l d - p l o t experiments were thawed, a i r - d r i e d overnight, ground, mixed and 50 g subsamples taken. The subsamples were re-frozen and re-thawed before a n a l y s i s . The residue analysis method has been described previously i n d e t a i l (5). Briefly, samples were extracted by shaking with acetone/hexane, l i q u i d - l i q u i d partitioned into hexane, cleaned up on acid alumina microcolumns, and quantified by EC-GLC using a DB-1 c a p i l l a r y column.


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Soil-Pan Method for Studying Pesticide Dissipation

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Results and Discussion 1986 Soil-Pan Experiment. The d i s s i p a t i o n of deltamethrin was biphasic (Figure 1) with mean DT50=3.2 weeks and mean DT90-23 weeks (DT50 and DT90 are the times required for 50Z and 90Z of the i n i t i a l residue to disappear). The EC formulation degraded s i g n i f i c a n t l y (P

Soil-Pan Method for Studying Pesticide Dissipation on Soil - ACS

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