Location: Agricultural Water Efficiency and Salinity Research Unit
2022 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 final report for project 2036-12130-011-000D, Predicting and Reducing Agricultural Contaminants in Soil, Water, and Air. The project has merged with expiring project 2036-32000-005-000D, Identifying, Quantifying and Tracking Microbial Contaminants, Antibiotics and Antibiotic Resistance Genes in Order to Protect Food and Water, and been replaced by new project 2036-12320-011-000D, Protection of Food and Water Supplies from Pathogens and Human Induced Chemicals of Emerging Concern. See the 2036-12320-011-000D annual report for more information.
Progress on this 1.0 scientist (SY) project was delayed due the SY position being vacant for most of the project. Nevertheless, notable progress was made on both objectives, which fall under NP212.
Experiments were conducted by researchers in Riverside, California, in support of Sub-objective 1A. Work was conducted to collect vapor density measurements for the soil fumigant 1,3-dichloropropene (1,3-D). This data revealed the relationship between 1,3-D vapor density and prevailing soil water content and soil temperature. Understanding this relationship across a range of soil water contents and temperatures is critical to the successful development of highly accurate predictive models for simulating the effect of vapor adsorption on fumigant emissions. Experiments were also completed to obtain Henry’s Law coefficients (air/water partitioning) for 1,3-D at temperatures ranging from 5 to 40 degrees C. These experiments were conducted at two initial 1,3-D concentrations, each of which gave similar results. Change in the Henry’s Law relationship with temperature was described using the Arrhenius equation, which yielded the activation energy coefficient of 34 kilojoules/mole. Experiments were also completed to obtain empirical data on 1,3-D solubility and soil adsorption. This information will provide useful parameter values for various model simulations of 1,3-D behavior in soil and its subsequent transport to the atmosphere; it is therefore of high importance to state and federal regulators who use such models to help protect air quality.
Under Sub-objective 1B, experiments were conducted to determine the effect of application rate on soil degradation for the soil fumigant chloropicrin (CP). Previous research at Riverside, California, has shown that the mass of CP applied affects the soil degradation rate, which controls the level of emissions. Since lower soil concentrations occur for lower application rates, it was hypothesized that lower application rates lead to higher soil degradation of CP due to greater microbial activity. This relationship was further investigated across a wide range of CP application rates, i.e., 50–350 pounds/acre, and it was 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. The relationship between CP application rate and emission percentage was well described by a second-order polynomial relationship with an R2 value of 0.93. Moreover, degradation studies 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, it was hypothesized that this relationship is due to an increasing kill of CP-degrading microbes as the CP application rate increases. 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. It was considered 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. A non-linear (polynomial) relationship identified between CP application rate and CP emissions, both as percent of that applied and as total mass, suggested that low application rates likely lead to disproportionally low emission losses compared with higher application rates; such a relationship could be considered when assessing/mitigating risk, e.g., in the setting of buffer zone distances.
Also under Sub-objective 1B, the relationship between CP application rate and its degradation in soil was studied. 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 CP emissions. Batch degradation studies were conducted to better understand how this relationship is impacted by various soil and environmental conditions, i.e., gradients in soil temperature (10, 25, and 40 degrees C), soil moisture content (1, 8, and 15%), and organic matter content (1, 2, and 3%). A general trend of degradation rate decreasing with increasing application rate was observed across almost all gradients, which is likely due 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, from 69–99.8% depending on prevalent conditions. The work strongly supports and elucidates previous experimental soil column data collected, which showed that CP emission rate is heavily dependent on application rate. Therefore, this relationship holds across a range of important soil conditions. These data are critically important for accurately quantifying CP emissions from soil to air to protect air quality.
Additionally, in support of Sub-objective 1B, incubation experiments were completed using sterilized soil to better understand the role of microorganisms in the relationship between CP application rate and total emissions. Previous work under this objective showed that CP 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 CP application rates, soil microorganisms are killed, which decreases the rate of degradation and leads to disproportionally high emissions. Therefore, to study the effect of microorganisms in more detail, degradation experiments were conducted at various application rates using sterilized soil (from the same location as used previously). The results showed that CP half-life remained relatively constant in the absence of microorganisms, with no statistically significant differences between the application rate treatments. This contrasts sharply with behavior observed for non-sterilized soils, and the measured CP half-lives were longer than those observed previously for non-sterilized soils, indicating that microorganisms do indeed play a significant role in CP degradation. These data are critically important for accurately quantifying CP emissions from soil to air to protect air quality.
Additional progress under Sub-objective 1B, included researchers at Riverside, California, further considering the impact of degradation on soil–air emissions under a range of soil conditions. To this end, a simulation model was used to predict CP emissions. For a range of application rates, sterilization led to significantly greater predicted emissions owing to the absence of biological degradation within the soil compared to non-sterilized soil. This work on CP degradation comports strongly with previous work conducted at Riverside, California, further indicating the very high importance of biological (microbial) degradation in terms of controlling CP emissions. This improved understanding of the role of microorganisms allows for the development of improved mitigation strategies to protect air quality and human health.
Under Sub-objective 1C and Objective 2, progress was made to incorporate a concentration-dependent fumigant degradation process into numerical models. A concentration-dependent degradation sub-model was developed that accurately describes soil degradation in laboratory experiments with varying initial fumigant concentrations. Computer coding was completed and incorporated into both One-Dimensional and Two-Dimensional Simulation programs. Substantial progress was made in preparing SOLUTE-1D for the addition of fully-coupled water–heat–vapor transport equations. The basic code for the non-coupled model was verified by comparison with analytical solutions for appropriate soil and environmental conditions. Furthermore, SOLUTE-1D was compared with 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. Numerical simulations of earlier field experiments were conducted in predictive mode (i.e., no calibration) to determine if simulation could be used as a substitute for field experiments to obtain information needed by regulators. The results show that the magnitude of the volatilization rate and total emissions could be adequately predicted for these experiments, except for a situation where the field was periodically irrigated after fumigation. In addition, the timing of the daily peak 1,3-D emissions was not accurately predicted for these experiments due to peak emission rates occurring during night or early-morning hours. This study revealed that more comprehensive mathematical models or adjustments to existing models are needed to fully describe emissions of soil fumigants from field soils under typical agronomic conditions.
Accomplishments
