Location: Contaminant Fate and Transport Research2013 Annual Report
1a. Objectives (from AD-416):
Soil fumigation is an important agricultural practice in California. Due to increasing regulatory oversight, new knowledge and improved methods for fumigating soil are needed to optimize the pest-control efficacy and minimize atmospheric emissions. The main objective of the proposed research project is to investigate the effect of co-formulations of 1,3-dichloropropene (1,3-D) and chloropicrin on emissions of each fumigant from soil. The research will provide periodic soil gas-phase concentrations, measurement of the cumulative emissions and measurements of the period-averaged (e.g., hourly) emission rates. A series of experiments, replicated three times, will allow comparison of 1,3-D and chloropicrin emissions when injected into soil as single products as well as comparison when injected as various co-formulations. Overall, this research project will provide information that can be used to determine if co-formulations of 1,3-D and chloropicrin alter the emission rate (period average and/or total emissions). This information will be useful in determining if a co-formulation effect occurs with these fumigants and, if found, will also be useful as a starting point in the development of a theoretical explanation of the co-formulation effect. Also, the results from this project may lead to a potential strategy to reduce emissions, if co-fumigant application results in lower emissions than either fumigant applied alone.
1b. Approach (from AD-416):
Experiments will be conducted in 12 cm (dia) x 150 cm (long) cylindrical soil columns and will be packed uniformly with a homogeneous mixture of soil with uniform moisture content and bulk density. The soil will be well mixed to ensure that soil properties and soil degradation rates are as uniform as possible. Soil properties will be reported and experiments will be conducted at 25 ± 1 oC. The tops of the columns will be fitted with stainless steel caps that allow for trapping of fumigants emitted from the soil surface. These columns are suitable for the 1-dimensional (vertical) study of fumigant behavior following a broadcast shank injection into ploughed soil. In these experiments, shank injection at about 46 cm depth will be tested by injecting a small volume of fumigant or fumigant formulation into the soil via an injection port. The volume of fumigant will be based on normal field experiment application rates. Immediately following fumigant application, air will be drawn across the soil surface (through the column headspace caps) and the emitted fumigant swept through a sampling system via Teflon tubing. The sampling system will use sorbent tubes (e.g. charcoal or XAD resin) to collect the fumigant vapor. Backup tubes will be installed in case of fumigant breakthrough. Solenoid valves will allow switching between sorbent tubes at regular time intervals and mitigate the need for human intervention. The sampling intervals will be the same for each column and will allow direct comparison between columns. The diffusion of the fumigant within the soil will be determined by periodic soil gas sampling; a small volume (e.g. 500 µL) of gas will be removed from the soil using a gas-tight syringe inserted through sampling ports along the side of the column. The duration of experiments will approximately 10–14 days. The experiments will involve 6 soil columns injected with one of the following fumigant/co-formulation levels. – 1,3-D alone – chloropicrin alone – Telone C17 (83% 1,3-D + 17% chloropicrin) – Telone C35 (65% 1,3-D + 35% chloropicrin) – Pic-Clor 60 (60% chloropicrin + 40% 1,3-D) – Pic-Clor 80 (80% chloropicrin + 20% 1,3-D). The experiment will be replicated 3 times and, for each replication, the columns will be randomly assigned to a treatment. The reported data will include the flux rate with time and will include the means and standard deviations for each treatment, cumulative flux with time (means and standard deviations) and soil gas-phase concentrations (means and standard deviations). The data will be subjected to a statistical analysis (Analysis of Variance) to test if the treatments are significantly different at the 95% probability level.
3. Progress Report:
This research is related to objective 1a of the parent project, "Conduct Laboratory Experiments and Modeling Studies to Test Accuracy of Simplified Approaches for Estimating Fumigant Emissions". The soil column experiments for the second and third replicates of each of the six treatments were performed. Soil was packed to a uniform bulk density of 1.5 g/mL and a uniform gravimetric moisture content of 5%. The experiment was carried out at a constant 25 °C. Emissions (4 h sampling periods) and soil gas were sampled until non-detectable levels of the fumigants were observed in the soil gas samples. The extraction of sorbent tubes and gas chromatography analysis of the solvent extracts has been performed and the data have been analyzed. Preliminary results indicate that 1,3-dichloropropene emissions, as a percentage of the total amount applied, were relatively constant across all treatments (33.4-35.5%). Therefore 1,3-dichloropropene emissions were not strongly affected by the percentage of 1,3-dichloropropene in the formulation, or the initial concentration in soil. For chloropicrin, however, emissions were highly correlated to the chloropicrin percentage in the formulation, with: 0.018%, 1.5%, 8.6%, 16.9%, and 14.5% total emissions for formulations containing 17%, 35%, 60%, 80% and 100% chloropicrin, respectively. Degradation studies, in which the same initial mass of total fumigant (1,3-dichloropropene + chloropicrin) were added to soil, found that the chloropicrin half-life for each formulation decreased with decreasing percentage of chloropicrin in the formulation (i.e., was dependent upon initial mass). A supplementary degradation study confirmed that these half-lives were similar in the absence of 1,3-dichloropropene. It is hypothesized that this degradation accounted for the large differences in emission losses (i.e., less chloropicrin was available for emission). A draft report of the column experiment has been completed and a peer-reviewed publication will be prepared during the first part of FY2014.