Jeq50331 1040..1048

High emissions from soil fumigants increase the risk of detrimental impact on workers, bystanders, and the environment, and jeopardize future availability of fumigants. Efficient and cost-effective approaches to minimize emissions are needed. This study evaluated the potential of surface water application (or water seal) to reduce 1,3-dichloropropene (1,3-D) emissions from soil (Hanford sandy loam) columns. Treatments included dry soil (control), initial water application (8 mm of water just before fumigant application), initial plus a second water application (2.6 mm) at 12 h, initial plus two water applications (2.6 mm each time) at 12 and 24 h, standard high density polyethylene (HDPE) tarp, initial water application plus HDPE tarp, and virtually impermeable film (VIF) tarp. Emissions from the soil surface and distribution of 1,3-D in the soil-gas phase were monitored for 2 wk. Each water application abruptly reduced 1,3-D emission flux, which rebounded over a few hours. Peak emission rates were substantially reduced, but total emission reduction was small. Total fumigant emission was 51% of applied for the control, 46% for initial water application only, and 41% for the three intermittent water applications with the remaining water treatment intermediate. The HDPE tarp alone resulted in 45% emission, while initial water application plus HDPE tarp resulted in 38% emission. The most effective soil surface treatment was VIF tarp (10% emission). Surface water application can be as effective, and less expensive than, standard HDPE tarp. Frequent water application is required to substantially reduce emissions. METHYL BROMIDE (MeBr) is being phased out internationally as a stratospheric ozone depleting compound. Other fumigants, such as 1,3-dichloropropene (1,3-D), chloropicrin, and metam sodium (methyl isothiocyanate [MITC] generator), are being used as alternatives to MeBr for soil fumigation. Uses of fumigants are regulated primarily based on their toxicology properties and air emissions. Minimizing emissions will be critical to protecting workers, bystanders, and the environment, and to maintaining practicable use of alternative fumigants to MeBr for agricultural production. Emissions from soil fumigation are affected by soil texture and water content, weather, and surface barriers as well as fumigant properties. Methods to reduce fumigant emissions include application methods such as chemigation (through drip irrigation), chemical remediation (e.g., use of thiosulfate to degrade fumigants), and surface barriers (plastic tarps and water seals) (Yates et al., 2002). Chemigation was shown to reduce fumigant emissions compared to shank injections in some cases (Gan et al., 1998b) but not in others (Sullivan et al., 2004). Chemical remediation at the surface (e.g., using thiosulfate) can reduce emissions substantially by reacting with and destroying the fumigants at the surface (Gan et al., 1998a). This method has not yet been tested in the field or evaluated economically. Surface plastic tarp (mulch) reduces fumigant emissions, depending on the chemical, and is generally expensive (about $2000 per ha for purchase, placement, removal, and disposal of high density polyethylene [HDPE] tarps over shank applications). High density polyethylene film is a poor barrier for 1,3-D, but is more effective with MeBr and chloropicrin (Wang et al., 1999; Papiernik and Yates, 2002). Plastic tarp was found to improve pest treatment of surface soil—especially weed control (Shem-Tov et al., 2005). Use of virtually impermeable film (VIF) is more effective in reducing 1,3-D as well as other fumigant emissions than standard HDPE tarp (e.g., Noling, 2002; Thomas et al., 2004, 2006; Wang et al., 1999), but this method is even higher cost and practical application methods that preserve the high barrier characteristics are still being developed. High surface soil water content was found to create a more effective barrier to 1,3-D movement than HDPE (Gan et al., 1998b; Thomas et al., 2003). Surface water seal (applying water uniformly to soil surface) is a technique that has shown potential to reduce fumigant emissions by forming a high water content layer at the surface that serves as a diffusion barrier. Fumigant diffusion rate in the liquid phase is much lower than through the gas phase. High soil water content and slight compaction at the soil surface reduces emissions also by reducing air-filled pore space. Water application to surface soil in column studies and small plots showed substantial reduction of MeBr emissions especially when combined with use of plastic tarp (Jin and Jury, 1995; Wang et al., 1997). More recent field trials studied use of surface water sprinkler irrigation (water seal) to reduce MITC emissions and found that intermittent water sealing was particularly effective to minimize off-gassing during night time periods when atmospheric dispersion conditions were relatively poor (Sullivan et al., 2004). This technique will usually cost less than using plastic tarps in areas where irrigation water is available, especially on farms where sprinkler systems are used. The objective of this study was to determine the potential of using surface water applications to reduce 1,3-D emissions in soil columns. Although soil columns S. Gao, USDA-ARS, Water Management Research Unit, San Joaquin Valley Agricultural Sciences Center, Parlier, CA 93648. T.J. Trout, USDA-ARS, Water Management Research Unit, Fort Collins, CO 80526-8119. Received 31 Aug. 2005. *Corresponding author (sgao@ fresno.ars.usda.gov). Published in J. Environ. Qual. 35:1040–1048 (2006). Technical Reports: Organic Compounds in the Environment doi:10.2134/jeq2005.0331 a ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: 1,3-D, 1,3-dichloropropene; HDPE, high density polyethylene; MeBr, methyl bromide; MITC, methyl isothiocyanate; VIF, virtually impermeable film. R e p ro d u c e d fr o m J o u rn a l o f E n v ir o n m e n ta l Q u a lit y . P u b lis h e d b y A S A , C S S A , a n d S S S A . A ll c o p y ri g h ts re s e rv e d . 1040 Published online May 31, 2006