Earth's near-surface environment sustains nearly all terrestrial life, yet this critical zone is threatened by the environmental migration of new and potentially harmful compounds produced to support a growing human population. Traditional transport equations often fail to capture the environmental behavior of these emerging contaminants due to issues such as flow heterogeneity. Thus, there is a need to better evaluate controls on pollutant partitioning in Earth's critical zone. Our first study investigated the transport and distribution of the neonicotinoid insecticide thiamethoxam (TMX) by growing TMX-coated corn seeds in coarse vs fine-textured soil columns maintained with versus without growing corn plants. Fine-textured soil transported TMX at concentrations that were two orders of magnitude higher than coarse-textured soil, due to preferential flow in the fine-textured soil columns and higher evapotranspiration (ET) concentrating more TMX in the coarse-textured soil. Living plants increased the concentration of TMX at depth, indicating that growing plants may drive preferential transport of neonicotinoids. For the second study we planted TMX-coated corn seeds and maintained field plots with and without viable crops (n = 3 plots per treatment), measuring TMX concentrations in three hydrological compartments (surface runoff, shallow lateral flow, and deep drainage) and soil. TMX was transported in the highest concentrations via surface runoff, while also showing continual migration within the subsurface throughout the growing season. Plants facilitated downward migration of TMX in soil yet restricted losses in drainage. For our final study, we used a simple isotope mixing method to evaluate how preferential flow alters the influence of compound chemical properties on solute transport. We applied deuterium-labeled rainfall to plots containing manure spiked with eight veterinary antibiotics with a range of mobility, and quantified transport to suction lysimeters (30 and 90 cm). We showed that low preferential flow (<20%) eliminates the influence of compound chemical properties and, contrary to conventional understanding, more preferential flow (~ >20%) amplifies these chemical controls, with more mobile compounds appearing in significantly higher concentrations than less mobiles ones. Altogether, we provide a refined understanding of solute partitioning in the critical zone necessary to improve process-based transport modeling.
Doctor of Philosophy
Earth’s near-surface environment sustains nearly all terrestrial life, yet this critical zone is threatened by the environmental migration of new and potentially harmful pollutants produced to support a growing human population. Additionally, traditional mathematical methods fail to accurately describe the behavior of these emerging pollutants in soils due to complex flow patterns. Thus, scientists need to better understand how these pollutants contaminate water bodies in the critical zone. We first conducted a greenhouse experiment to understand and measure the amount of the neonicotinoid insecticide thiamethoxam (TMX) that could move from coated corn seeds through the soil environment. Water draining from fine-textured soil had >100 times more TMX than water draining from course-textured soil, due to commonly occurring fractures/cracks in the finer-particle soil and more evaporation from soil and plant leaves sequestering TMX in the sandy soil. Growing plants amplified TMX movement through soil voids to lower depths. We then conducted a field study to determine how much TMX could move to the surrounding environment throughout the corn growing season. We found that plants aided in downward movement of TMX yet restricted total losses from the plot overall by removing soil water. Our third study investigated the degree to which chemical pollutant properties control movement of solutes when water flows preferentially through soil void space. Common dairy manure was spiked with eight pollutants ranging in chemical attraction to soil and was added to an agricultural field. After irrigation, we found that when total drainage water was less than 20% derived from preferential flow, chemical properties had a negligible effect on the amount of pollutant in draining soil water. Contrary to conventional understanding, when draining water contained more than 20% preferential flow, chemical properties had a strong influence on the amount of pollutant detected. Altogether, we provide new understanding of how solutes move though the critical zone. These findings are necessary to create mathematical tools that more accurately depict pollutant behavior below-ground.