Location: Agricultural Water Efficiency and Salinity Research Unit
2017 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
This is the first report for this project which began in January of 2017, please see the report for the previous project, 2036-12130-010-00D, "Reducing Contamination from Agricultural Chemicals” for further information.
Several experiments have been conducted in support of Objective 1a. Work has commenced on collecting vapor density measurements for 1,3-dichloropropene (1,3-D). This data revealed the relationship describing 1,3-D vapor density at prevailing soil water content and soil temperatures. The next step will be developing an equation that accurately describes the relationship over a range of water contents and temperatures. This equation will be used in a predictive model to simulate the effect of vapor adsorption on fumigant emissions. Experiments have also been completed to obtain Henry’s Law coefficients from 5 to 40 degrees celsius. These experiments were conducted at two starting concentrations, which gave similar results. Change in the Henry’s Law relationship with temperature was described using the Arrhenius equation, which yielded an activation energy coefficient (34 kilojoules/mole). During the next six months 1,3-D solubility and adsorption experiments will be completed.
For Objective 1b, experiments were conducted to determine the effect of application rate on soil degradation for chloropicrin (CP). Since lower soil concentrations occur for lower application rates, we hypothesize that a lower application rate leads to increased soil degradation due to lower soil concentrations. Previous research has shown that the mass of CP applied affects the soil degradation rate, which controls the level of emissions. In the current study, we further investigated this relationship across a wider range of CP application rates, i.e., 50–350 pounds/acre, and found that total emission percentages were strongly and positively related to application rate (i.e., initial mass). Total emissions varied from 4 to 34% over the 50–350 pounds/acre application rate range. Comparing the two datasets showed good agreement in terms of CP application rate versus emission percentage, and could be described by a second-order polynomial relationship with an R2 value of 0.93 (n = 12). Moreover, the degradation studies again showed that for lower application rates, the degradation half-life of CP was shorter. Since CP degradation was shown to be a predominantly microbial process, we hypothesize that this relationship is due to an increasing kill of CP-degrading microbes as the CP application rate increased. The influence of CP degradation on emissions was highlighted by a strong, positive linear relationship between half-life and emissions percentage (R2 = 0.90) across the application range. We consider that the shorter half-life (faster degradation) at lower application rates limited the amount of CP available for emission. At the higher application rates, plateaus in the curves of emission percentage (at around 32%) and degradation half-life (at around 75 hours) suggest that increasing application rates would not result in greater emission percentages.
For Objective 1c and Objective 2, progress has been made to incorporate a concentration-dependent fumigant degradation process into numerical models. The concentration-dependent degradation sub-model has been developed and accurately describes soil degradation in laboratory experiments with varying initial fumigant concentration. During the next 6 months, the computer coding will be completed and incorporated into both 1-Dimensional and 2-Dimensional simulation programs. Substantial progress has been made in preparing SOLUTE-1D for the addition of fully-coupled water-heat-vapor transport equations. The basic code for the non-coupled model has been verified by comparison to analytical solutions for appropriate soil and environmental conditions. Furthermore, SOLUTE-1D has been compared to a popular commercial simulation software program (i.e., HYDRUS) and shown to give equivalent results over a wide range of soil and environmental conditions and for various fumigant-application and emission-reduction scenarios.
Accomplishments
