Location: Agricultural Water Efficiency and Salinity Research Unit
2020 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
Objective 1 involves research to reveal the important factors affecting the fate and transport of soil fumigants (i.e., pesticides) in the soil environment and obtain transport coefficients that can be used to characterize the effect of these factors on the accuracy of predictive models. In support of Objective 1B, progress has been made during Fiscal Year (FY) 2020 on incubation experiments using sterilized soil to better understand the role of microorganisms in the relationship between chloropicrin application rate and total emissions. Previously published work by researchers in Riverside, California, found that chloropicrin degradation rate is inversely related to its application rate, which has a marked impact on soil–air emissions of this important agricultural fumigant. It is believed that at high chloropicrin application rates, soil microorganisms are killed off, decreasing the rate of degradation and leading to disproportionally high emissions. Therefore, to study the effect of microorganisms in more detail, degradation experiments have been conducted at various application rates using sterilized soil (from the same location as used previously). The results show that (i) chloropicrin half-life remains relatively constant in the absence of microorganisms, with no statistically significant differences between the application rate treatments, which contrasts sharply with the behavior observed for non-sterilized soils; (ii) the measured half-lives are longer than those observed previously for non-sterilized soils, indicating that microorganisms do indeed play a significant role in chloropicrin degradation.
Under Sub-objective 1B, it will be important to consider this effect in emissions experiments; therefore, laboratory column studies using sterilized soils are planned for FY21. As a precursor to this, a simulation model was used to predict the chloropicrin emissions that might be obtained in such a study, using the half-lives determined in FY20 as parameter values in the model. For a range of application rates, compared with non-sterilized soil, sterilization leads to significantly greater predicted emissions owing to the absence of biological degradation within the soil. This work on chloropicrin degradation comports strongly with previous work conducted in the Riverside, California, laboratory, further indicating the very high importance of biological (microbial) degradation in terms of controlling chloropicrin emissions. Better understanding the role of microorganisms will allow for improved mitigation strategies to be developed to protect air quality and human health.
In addition, work continued on the use of customized biochar to increase degradation and reduce emissions of 1,3-dichloropropene (1,3-D). This study investigated the degradation of 1,3-D on iron (Fe)-impregnated biochar (FBC) amended with urea-hydrogen peroxide (UHP). The results indicated the degradation rate of trans-1,3-D on FBC-UHP was 54-fold higher than that on pristine biochar (PBC). The findings of this study provide a new approach for biochar application, especially for the emission reduction of hazardous volatile organic compounds from soil.
Collaboration also continued with a researcher from Dow Agrosciences. During FY20, the ratio between the cis and trans isomers of 1,3-D was investigated as a potential temporal indicator of when fumigant applications are made to a field. It appears that the ratio of cis/trans in ambient air is related to the time that has passed since the fumigant application was made. This result may be useful for developing a regulatory tool for dating 1,3-D applications and thus help to protect air quality/human health. The practicality of such an approach is being considered. Emissions modeling of the cis and trans isomers individually will be conducted to determine if these trends in cis/trans ratios can be reproduced.
Accomplishments
