Remediation of Halogenated Fumigant Compounds in the Root Zone

Chapter 13

Remediation of Halogenated Fumigant Compounds in the Root Zone by Subsurface Application of Ammonium Thiosulfate Downloaded by STANFORD UNIV GREEN LIBR on July 17, 2012 | http://pubs.acs.org Publication Date: October 10, 2003 | doi: 10.1021/bk-2004-0863.ch013

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Sharon K . Papiernik , Frederick F. Ernst , Robert S. Dungan , Wei Zheng , Mingxin Guo , and Scott R. Yates 2

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George E . Brown Jr. Salinity Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 450 West Big Springs Road, Riverside, C A 92507-4617 Department of Environmental Science, University of California, Riverside, C A 92521 2

Fumigants are used for control of nematodes, fungi, weeds, and insects in high-cash-value crops. Because of their high volatil-ity, a large fraction of the applied mass may be volatilized from the soil surface following application. Emission reduction strategies are needed to prevent adverse human or environmental health impacts. Some emission-reduction strategies, such as tarping the soil surface with impermeable plastic, increase containment of fumigants in the soil. The resulting fumigant residues could cause atmospheric contamination once the tarp is disrupted, groundwater contamination if leaching is allowed, or phytotoxicity to the crop planted following fumigation. Previous research has shown that thiosulfate compounds, including ammonium thiosulfate (ATS), abiotically react with and detoxify halogenated fumigants in soil and water. In these experiments, we investigated subsurface application of ATS to reduce fumigant concentrations in the root zone, preventing off-site transport following fumigation. Results indicated that halogenated fumigants were dissipated more rapidly in soil receiving ATS application compared to those receiving water only. First-order dissipation half-lives were ≤1 day for the halogenated compounds 1,3-dichloropropene and propargyl bromide. For methyl isothiocyanate, a non-halogenated fumi-gant that does not undergo reaction with ATS, application of ATS had no impact on the rate of dissipation in soil. These results suggest that subsurface application of ATS may be useful for the root-zone remediation of halogenated compounds.

© 2004 American Chemical Society In Pesticide Decontamination and Detoxification; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Downloaded by STANFORD UNIV GREEN LIBR on July 17, 2012 | http://pubs.acs.org Publication Date: October 10, 2003 | doi: 10.1021/bk-2004-0863.ch013

170 Fumigants are used worldwide for the control of soil-borne pests, such as nematodes, fungi, weeds, and insects in high-cash-value crops. In the United States, fumigants are intensively used for strawberry and tomato production in California and Florida. The popular fumigant methyl bromide (MeBr) is being phased out in the United States and other signatories to the Montreal Protocol, and the phase-out is to be complete by 2005. Other chemical alternatives, particularly 1,3-dichloropropene ( 1,3-D), chloropicrin, and methyl isothiocyanate (MITC), are already available as partial replacements for MeBr. The potential of additional compounds, including iodomethane and propargyl bromide (PrBr), to serve as soil fumigants is being assessed. Fumigant compounds have high vapor pressures and are transported in soils largely in the gas phase. Without adequate containment, a significant proportion of the applied mass may be volatilized from the soil surface following soil application. For example, laboratory andfieldresearch has indicated that 30-60% of the applied 1,3-D is lost following subsurface application to uncovered soil (i, 2)

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Because fumigant compounds have broad toxicity and are highly mobile, reducing atmospheric emissions is vital to their continued use. The use of several fumigant compounds, including 1,3-D, has been restricted due to air quality concerns (3,4). Fumigation methods to increase containment and thus prevent air and water contamination by fumigant compounds are being developed. Containment also maximizes efficacy by increasing the time for which high fumigant concentrations are present in the soil. Management practices to reduce emissions include tarping the soil surface with impermeable plastic, increasing the soil water content to reduce gas-phase diffusion, and increasing the rate of fumigant transformation in the soil. Tarping the soil surface with a continuous sheet of impermeable plastic can drastically increase containment, resulting in near-zero emissions and reducing the fumigant application rate required to achieve adequate pest control (5). Another promising approach to reducing emissions involves enhancing fumigant degradation at the soil surface. Previous research has indicated that nucleophilic compounds such as ammonium thiosulfate (ATS) rapidly transform and detoxify halogenated fumigants such as MeBr, iodomethane, chloropicrin, PrBr, and 1,3-D in aqueous solution and in soil (6, 7). It is postulated that the reaction between ATS and these halogenated fumigants is via S 2 nucleophilic substitution, with thiosulfate replacing the halide (d). Similar reactions have been observed for reaction between ATS and other halogenated pesticides (8). Application of ATS at the soil surface can significantly reduce emissions of halogenated fumigants by forming a reactive barrier at the soil surface. Surface application of ATS reduced cumulative emissions of 1,3-D by >50% ( 7), and MeBr emissions by -90% (9). Addition of nucleophilic compounds has no impact on compounds which do not undergo nucleophilic substitution, such as the non-halogenated fumigant MITC. Thiosulfate compounds are commonly used as fertilizers, and integration of ATS application and fumigation with 1,3-D showed no phytotoxic symptoms in tomato planted following fumigation (7). N

In Pesticide Decontamination and Detoxification; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


171 Emissions-reduction strategies are required to minimize air contamination by fumigants and to increase efficacy. However, retention of significant concentrations of fumigants in soil following the fumigation period may result in air and groundwater contamination and crop phytotoxicity. For example, a large flux of fumigants to the atmosphere is observed following the removal of low-permeability tarps (5). Groundwater contamination by fumigants has been observed when heavy precipitation occurs when fumigant concentrations are relatively high, i.e., shortly after fumigation in conventional applications (70). Phytotoxicity to the crop planted following fumigation is possible where fumigant residues remain (77). In some current soil fumigation practices using 1,3-D, the fumigant is applied with water through subsurface drip irrigation lines. Similar application methods have been proposed for potential alternative fumigants such as PrBr. In these experiments, we investigated the potential for subsurface application of ATS to reduce soil concentrations of 1,3-D and PrBr following soil fumigation, thus reducing the threat of off-site transport and potential phytotoxic effects on crops.

Methods Four concrete mesocosms (3 m long by 1.5 m wide by 1.6 m deep) were filled with washedriversand to a bulk density of 1.4 Mg m". Beds were formed at the soil surface; beds were 50 cm across the top and 20 cm high, as indicated in Figure 1. As part of an emissions-reduction experiment (results not presented here), different management practices were represented in the mesocosms, including a bare soil surface, bed tarped with HDPE, or bed tarped with Hytibar® (a lowpermeability film). Subsurface drip lines (HDPE) were installed 15 cm below the bed surface. Fumigants were mixed with 24 L of water in HDPE carboys, which were then connected to the drip lines and pressurized to apply the fumigant mixture to the mesocosms. Application required 2-3 hours. Fumigant application rates were typical offieldapplication: 10 gal ac" of 1,3-D C-35 (In-Line®); 80 lb ac of PrBr; and 17 gal ac" of Vapam® (MITC precursor). These application rates corresponded to 0.2 to 0.3 mol of 1,3-D, PrBr, and MITC added to each mesocosm. After a 10-day fumigation period, ATS was drip-applied to half of the mesocosms in the same manner (380 mL of Thio-Sul® fertilizer in 24 L water, ~2 mol of ATS). The remaining mesocosms received 24 L of tap water (no ATS) to indicate fumigant concentrations remaining in the root zone with no ATS treatment. Soil gas samples were collected to determine fumigant concentrations remaining in the soil. Teflon tubing ( 1 -mm ID) was buried during bed construction and tubes terminated from 20 to 80 cm below the soil surface throughout the bed cross-sectional area (locations indicated in Figure 1) to provide information on the distribution of fumigant compounds in the root zone. Gas samples (50 mL) were collected on activated charcoal adsorbent tubes; syringes were used to apply vacuum and to measure the gas volume sampled. This approach has been successfully used in previous experiments to monitor soil gas concentrations (72). Fumigant compounds were extracted from charcoal using 3 mL of acetone. Fumigants in acetone extracts were identified and quantified by gas chromatography, using an electron capture detector for PrBr and 1,3-D and a nitrogenphosphorus detector for MITC. Soil gas concentrations of fumigants were 3

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Figure L (A) Dimensions of mesocosms and (B) location ofsoil gas samplers in xy plane


173 measured just prior to ATS application, and were monitored for 7 days following ATS application. Concentration data were kriged to construct contour maps of soil gas concentrations throughout the soil profile. The volume contained under the contours was determined to indicate the mass of fumigant remaining in the monitored zone of each mesocosm with time.


Results and Discussion Initial soil gas concentrations varied over an order of magnitude due to the management practices used in the experiments. Residual concentrations were greater in mesocosms which had better containment. For all fumigant compounds, tarping the bed with an impermeable film resulted in the greatest containment (Figure 2). Tarping with a more permeablefilm(HDPE) resulted in less effective containment, and leaving the soil surface bare resulted in the lowest soil concentrations of fumigants following the 10-day fumigation period (Figure 2). These soil concentrations indicated the potential for highly effective containment to result in higher post-fumigation concentrations in the root zone, which may result in more effective control with lower application rates, but may also result in air and groundwater contamination and crop phytoxicity. Destroying the residual fumigant in the soil decreases the threat of environmental contamination by these compounds and diminishes the potential for crop injury upon planting.

Depletion of Fumigants in the Root Zone by ATS Application Fumigant concentrations in soil gas samples were kriged to construct contours of the concentration of each compound at each sampling time (Figure 3). Because of the high variability in initial concentrations (Figure 2), the concentration at each sampling point was normalized to the concentration measured at that location just prior to ATS/water application in Figure 3. Results for each sampling time indicated that fumigant concentrations in the root zone were depleted to a greater extent in tanks treated with ATS than in those receiving water only. Integrating the area under the concentration contours provided a means of estimating the mass remaining in each mesocosm at each time. For the example given in Figure 3, the total PrBr in the ATS-treated tank at 52 hours after ATS application (Figure 3 A) was -20% of the PrBr present prior to ATS application. In the mesocosm receiving no ATS (Figure 3B), the PrBr mass remaining at 52 hours was ~60% of the initial PrBr. Subsurface application was effective at reducing the residual concentrations of halogenated fumigants (in this study, PrBr and 1,3-D), but had no impact on a non-halogenated fumigant (MITC). Each mesocosm was simultaneously treated with a solution containing 1,3-D, PrBr, and MITC. Within thefirstday following ATS application, the mass (normalized to initial conditions) of each halogenated



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In Pesticide Decontamination and Detoxification; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003. Distance (cm)

Figure 3. Normalized soil gas concentrations for propargyl bromide in (A) mesocosm receiving ATS solution and (B) mesocosm receiving water only (no ATS) at 52 hours after ATS/water application. Points indicate locations ofsoil gas samples. To aid in visualization, concentrations in the other half of the bed are indicated by the contours, assuming a symmetric distribution.

Distance (cm)


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Figure 4. Mass remaining in replicate mesocosms at 28 days after A TS/water application relative to the mass presentjust prior to application. (Mass is indicated by the volume under the concentration contours.) Values indicate the mean of two mesocosms and error bars indicate the standard error.

compound was significantly lower than that in tanks receiving no ATS (Figure 4). The normalized mass of MITC was not significantly different in ATS-treated and non-treated tanks (Figure 4). Fitting a first-order dissipation model (OQe"* ) indicated that the decay constant (k, h") for halogenated fumigants was approximately 2.5 to 5 times greater in ATS-treated tanks than in non-treated tanks (Figure 5, Table I). For PrBr, more than half of the residual post-fumigation PrBr was depleted within the first day following subsurface ATS application (Figure 4 and 5, Table I). In ATStreated mesocosms, 1,3-D isomers were dissipated more slowly than PrBr, and the first-order half-life for 1,3-D was approximately twice that of PrBr (Table I). These results are consistent with laboratory measurements of ATS reaction with fumigants in soil, in which degradation of PrBr by ATS in soil occurred at approximately twice the rate of 1,3-D degradation (6). The slower dissipation of 1,3-D may be due to the slower rate of reaction between ATS and 1,3-D in solution and because of compound-specific differences in the air-soil-water partitioning (6). Because MITC is not affected by ATS application (no chemical reaction occurs between MITC and ATS), MITC was dissipated in ATS-treated and non-treated tanks at approximately the same rate (Figure 5). 1

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177 Table L Rate of Fumigant Dissipation 1

First-order dissipation constant (hr ) ± standard error


Fumigant

No ATS

ATS

Propargyl bromide

0.046 ±0.009 (0.96)

0.009 ± 0.002 (0.88)

1,3-Dichloropropene (cisHrans)

0.023 ±0.004 (0.96)

0.009 ±0.001 (0.96)

MITC

0.0060 ±0.0005 (0.99)

0.0072 ±0.0005 (0.99)

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NOTE: Values in parentheses are r for nonlinear regression.

Initial fumigant concentrations in the mesocosms varied by an order of magnitude (Figure 2), and each treatment (ATS and no ATS) included one tank that had a bare soil surface during fumigation (low post-fumigation concentrations). Results were consistent between replicate tanks (Figures 4 and 5), indicating that with proper distribution, subsurface application of ATS can be expected to consistently reduce post-fumigation concentrations of halogenated fumigant compounds. These results indicate that for situations in which residual concentrations of halogenated chemicals subject to nucleophilic substitution are present in the soil, subsurface application of nucleophilic compounds such as ATS may be valuable for reducing the threat of leaching, runoff, volatilization, and phytotoxicity to crops. This approach may be particularly useful for remediation of chemicals applied via drip irrigation, including soil fumigants and other pesticides, since the application of ATS through existing drip lines would result in a minimal additional expense. Thiosulfate compounds, including ammonium, potassium, and calcium thiosulfate, are commonly used as fertilizers and pose little toxicity threat. Since ATS has been shown to degrade other halogenated agrochemicals, such as some chlorinated acetanilide herbicides (#), this approach has promise for root-zone remediation of many soil-applied chemicals.

Acknowledgments We acknowledge the assistance of Qiaoping Zhang and Christian Taylor in obtaining the experimental data. In Line® was provided by Dow AgroSciences. This research was funded in part by the Methyl Bromide Transitions Program, award number 00-51102-9551.



Figure 5. Decrease in (A) propargyl bromide and (B) MITC in the root zone with time. Values indicate the total volume contained under the concentration contours; the volume at each time was normalized to the volume observedjust prior to injection ofA TS/water. Values are the mean of two mesocosms and error bars represent the standard error. Lines indicate regression to a firstorder kinetic model. For MITC (B), regression omitted spurious 52-hour data.


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Wang, D.; Yates, S. R.; Ernst, F. F.; Knuteson, J. A. Volatilization of 1,3dichloropropene under different application methods. Water, Air, Soil Pollut. 2001, 127, 109-123. Gan, J.; Becker, J. O.; Ernst, F. F.; Hutchinson, C.; Knuteson, J. Α.; Yates, S. R. Surface application of ammonium thiosulfate fertilizer to reduce volatilization of 1,3-dichloropropene from soil. Pest Manag. Sci. 2000, 56, 264-270. California Department of Pesticide Regulation (CDPR). DPR approves limited use of soil fumigant. CDPR: Sacramento, CA, 1994; News Release 94-42. California Department of Pesticide Regulation (CDPR). California Man -agement Plan: 1,3-Dichloropropene. CDPR: Sacramento, CA, Jan. 30, 2002. Wang, D.; Yates, S. R.; Ernst, F. F.; Gan, J.; Jury, W. A. Reducing methyl bromide emission with a high barrier plastic film and reduced dosage. Environ. Sci. Technol. 1997, 31, 3686-3691. Wang, Q.; Gan, J.; Papiernik, S. K.; Yates, S. R. Transformation and detoxification of halogenated fumigants by ammonium thiosulfate. Environ. Sci. Technol. 2000, 34, 3717-3721. Gan, J.; Yates, S. R.; Knuteson, J. Α.; Becker, J. O. Transformation of ,13dichloropropene in soil by thiosulfate fertilizers. J. Environ. Qual. 2000, 29, 1476-1481. Gan, J.; Wang, Q.; Yates, S. R.; Koskinen, W. Α.; Jury, W. A. Dechlorination of chloroacetanilide herbicides by thiosulfate salts. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5189-5194. Gan, J.; Yates, S. R.; Becker, J. O.; Wang, D. Surface amendment of fertilizer ammonium thiosulfate to reduce methyl bromide emission from soil. Environ. Sci. Technol. 1998,32,2438-2441. Loria, R.; Eplee, R. E.; Baier, J. H.; Martin, T. M.; Moyer, D. D. Efficacy of sweep-shank fumigation with 1,3-dichloropropene against Pratylenchus penetrans and subsequent groundwater contamination. Plant Dis. 1986, 70, 42-45. Noling, J. W. Nematode Management in Carrots; Report ENY-021; Entomology and Nematology Department, Florida Cooperative Extension Service, University of Florida: Lake Alfred,, March 1999. Wang, D.; Yates, S. R.; Ernst, F. F.; Gan, J.; Gao, F.; Becker, J. O. Methyl bromide emission reduction withfieldmanagement practices. Environ. Sci. Technol. 1997, 31, 3017-3022.


Remediation of Halogenated Fumigant Compounds in the Root Zone

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