Abstract
Preferential flow reduces water residence times and allows rapid transport of pollutants such as organic contaminants. Thus, preferential flow is considered to reduce the influence of soil matrix-solute interactions during solute transport. While this claim may be true when rainfall directly follows solute application, forcing rapid chemical and physical disequilibrium, it has been perpetuated as a general feature of solute transport—regardless of the magnitude preferential flow. A small number of studies have alternatively shown that preferential transport of strongly sorbing solutes is reduced when solutes have time to diffuse and equilibrate within the soil matrix. Here we expand this inference by allowing solute sorption equilibrium to occur and exploring how physiochemical properties affect solute transport across a vast range of preferential flow. We applied deuterium-labeled rainfall to field plots containing manure spiked with eight common antibiotics with a range of affinity for the soil after 7 days of equilibration with the soil matrix and quantified preferential flow and solute transport using 48 soil pore water samplers spread along a hillslope. Based on > 700 measurements, our data showed that solute transport to lysimeters was similar—regardless of antibiotic affinity for soil—when preferential flow represented less than 15% of the total water flow. When preferential flow exceeded 15%, however, concentrations were higher for compounds with relatively low affinity for soil. We provide evidence that (1) bypassing water flow can select for compounds that are more easily released from the soil matrix, and (2) this phenomenon becomes more evident as the magnitude of preferential flow increases. We argue that considering the natural spectrum preferential flow as an explanatory variable to gauge the influence of soil matrix-solute interactions may improve parsimonious transport models.
Highlights
Preferential flow reduces water residence times and allows rapid transport of pollutants such as organic contaminants
A growing and increasingly affluent human population is releasing ever greater loads of organic contaminants into Earth’s critical zone[1]. Many of these compounds are susceptible to rapid movement via preferential flow[2–4], which can often lead to orders of magnitude greater solute leaching than predicted by equations specific to a homogenous soil matrix[5,6]
Preferential flow occurs as a disequilibrium between water flowing through the low-permeability bulk soil and the highly conductive fraction of the total soil volume such as macropores with hydraulic conductivities > 0.01 cm h−113
Summary
Preferential flow reduces water residence times and allows rapid transport of pollutants such as organic contaminants. A growing and increasingly affluent human population is releasing ever greater loads of organic contaminants into Earth’s critical zone[1] Many of these compounds are susceptible to rapid movement via preferential flow[2–4], which can often lead to orders of magnitude greater solute leaching than predicted by equations specific to a homogenous soil matrix (e.g., the advection–dispersion equation)[5,6]. The primary objectives of this study were to (1) quantify transport of eight veterinary antibiotics under different preferential flow conditions and (2) determine if preferential flow can eliminate the influence of solute-soil affinity on transport of these solutes This analysis is necessary to provide a fundamental understanding of how preferential flow alters contaminant mobility and build process-based transport models needed to manage water quality and thwart water resource degradation
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