Location: Agricultural Water Efficiency and Salinity Research Unit
2019 Annual Report
Objectives
The overarching goal of this 5-year research plan is improving the understanding of the soil and environmental factors and agricultural practices that influence the fate, transport and emissions of pesticides in agricultural systems. This will be accomplished by developing new information related to soil mechanisms and their interactions, quantifying environmental factors that significantly affect fate and transport, and in developing a more accurate predictive model. Two objectives have been assigned to this project.
Objective 1: Quantify mechanisms and processes that affect exchange of agricultural contaminants between soil, water, and air.
Objective 1A. Obtain transport, transformation and partitioning coefficients for 1,3-dichloropropene (1,3-D) that can be used in predicting fate and transport.
Objective 1B. Obtain information on initial chloropicrin concentration and soil degradation rate and develop a mathematical relationship to describe this process.
Objective 1C. Test and verify the concentration-dependent soil degradation relationship using a radial-diffusion laboratory experiment.
Objective 2: Develop and test a comprehensive contaminant fate and transport model that focuses on improved prediction of off-site movement (with an emphasis on volatilization).
Approach
Research will be conducted to:
Objective 1A: Obtain basic information on vapor sorption, solubility, degradation, and Henry’s Law appropriate for the soil types and environmental conditions observed during the five 1,3-D field experiments. Develop relationships with soil type, soil water content, temperature and initial concentration that can be used for modeling activities described in Objective 2. Laboratory experiments will be conducted to measure the 1,3-D vapor density, degradation rate and adsorption in soil. These experiments will be conducted at 3 to 4 temperatures in the range 10 to 45 degrees Celsius. The soil vapor density measurements will be obtained at several water contents, including very dry soils. The effect of temperature on these transport parameters will be described using the Arrhenius equation.
Objective 1B: Conduct laboratory column experiments to reveal the effect of initial chloropicrin concentration on emissions and degradation in soil and develop a mathematical relationship describing this process. Soil degradation will be measured in laboratory incubation experiments (see Obj 1A) and in triplicated soil column experiments. The columns will be housed in a controlled temperature room at 25 degrees Celsius. Seven field application rates will be tested (50 to 350 lbs/acre) using native soil. Experiments will also be conducted using sterilized soil at two or three field application rates (e.g., 100, 200, and 300 lbs/acre). A model of the concentration-dependence of the degradation rate will be obtained using a logistic-response model, by fitting the initial concentrations and degradation rates.
Objective 1C: Test and verify the concentration-dependent soil degradation relationship by conducting a radial-diffusion laboratory experiment. Use soil degradation rates obtained in laboratory incubation experiments to parameterize concentration-dependent degradation in a 2-D numerical transport model and determine if the model more accurately predicts the radial diffusion within the 2-D soil monolith. CHAIN-2D, or similar, will be modified to enable simulation of a concentration-dependent degradation process and used to predict the soil concentrations and emissions in the 2-D soil monolith. Numerical predictions using constant degradation rates will be compared to predictions using concentration-dependent rates to determine if the concentration-dependent model performs better.
Objective 2: Develop and test a comprehensive contaminant fate and transport model that focuses on improved prediction of volatilization. Show that soil drying and vapor adsorption controls the timing of the peak fumigant emission rate during daily (i.e., 24 h) periods by comparing measured 1,3-D emission rates to predicted emission rates using a fully coupled heat, water, water vapor and chemical transport model coupled to atmospheric processes. Model accuracy will be determined by comparing field emission measurements and model predictions of soil temperature, soil water content, emissions, and timing of peak emission rates.
Progress Report
The only scientist assigned to this project retired in FY18. Besides an Agency-wide hiring backlog, filling this vacancy has been delayed by the fact that the position is identified as the Research Leader of the newly formed Agricultural Water Efficiency and Salinity Research Unit (created in FY19). Discussions are ongoing with Pacific West Area Leadership and National Program Staff as to how to best fill this position, as well as the future direction of this project and the new Research Unit.
During FY19, work on Sub-objective 1B, was conducted by a cooperating researcher from the University of California, Riverside, and ARS technical support staff in Riverside, California. Progress was made in better understanding the relationship between chloropicrin application rate and its degradation in soil. For this fumigant, it has been previously established that its application rate has a marked effect on degradation rate, with a potential further influence on chloropicrin emissions. This year batch degradation studies were conducted to better understand how this relationship is impacted by various soil/environmental conditions, i.e., gradients in soil temperature (10, 25, and 40°celsius), soil moisture content (one, eight, and 15 percent), and organic matter content (one, two, and three percent).
A general trend of degradation rate decreasing with increasing application rate was observed across almost all such gradients, which is likely attributable to decreased microbial numbers and activity (i.e., degradation) at high (toxic) application rates. The effects of these ranges in degradation rate on emissions from soil to air were then predicted using an analytical solution model, indicating that between the low and high application rates, total emissions percentage increased markedly (increases ranging from 69 to 99.8 percentage points, depending on prevalent conditions). The work strongly supports and elucidates our previous experimental data from soil columns, which showed that chloropicrin emissions rate is heavily dependent on application rate. We have now established that this relationship holds across a range of important soil conditions.
Accomplishments
1. Better parameterization of fumigant emissions. Micrometeorological methods are frequently used to observe emissions of fumigants and other pesticides. These observations are needed to evaluate the efficacy of methods to reduce emissions and to assess the potential impact of emissions on human and environmental health. However, the current methods to measure fumigant and pesticide emissions have significant differences in both peak emission rates and cumulative total emissions, which can create significant modeling and regulatory uncertainty. An ARS scientist in Riverside, California, and collaborators, worked to reduce this discrepancy by using a newer method to evaluate transport parameterizations for fumigant emissions. They found that using alternate transport equations reduced differences in observed emissions between measurement approaches and that these equations could be applied to previous studies, where they also reduced differences. Overall, they found that this correction reduced both the mean and uncertainty in flux fumigant emissions, which may be important for setting local application limits for protecting human and environmental health.
Review Publications
Ashworth, D.J., Yates, S.R. 2019. Effect of application rate on chloropicrin half-life and simulated emissions across a range of soil conditions. Science of the Total Environment. 682:457-463. https://doi.org/10.1016/j.scitotenv.2019.05.203.
Anderson, R.G., Yates, S.R., Ashworth, D.J., Jenkins, D.L., Zhang, Q. 2019. Reducing the discrepancies between the Aerodynamic Gradient Method and other micrometeorological approaches for measuring fumigant emissions. Science of the Total Environment. 687:392-400. https://doi.org/10.1016/j.scitotenv.2019.06.132.
