Preferential Flow in the Vadose Zone: Identifying Solute vs Media Controls on Contaminant Transport

Non-equilibrium flow often leads to orders of magnitude higher solute leaching through the vadose zone than predicted by equations (e.g., advection-dispersion) specific to a homogenous soil matrix. Widely occurring, preferential flow processes present a challenge to maintaining water quality standards. Further, a multitude of organic contaminants are transmitted through the critical zone, yet the degree to which their chemical properties dictate transport in the presence of preferential flow remains unknown. We used a simple isotope mixing model to determine the percent preferential flow required to eliminate chemical controls on solute transport. Eight common veterinary antibiotics were selected due to their environmental ubiquity and their range of mobility from low (tetracycline) to high (pirlimycin). Deuterium spiked rainfall was applied to 15 soil columns with varying magnitudes of soil structure (low-repacked and sieved, medium-semi-intact, and high-with artificial macropores) and to a series of no-till field plots. Antibiotic concentrations and deuterium signals were recorded in leachate (in greenhouse) and soil pore water (via suction lysimeters in the field) following a series of simulated rainfall events. Field results show that heavy rain (7 cm h^-1) produced as much as 60% preferential flow at a depth of 30 cm (B_t horizon) and up to 20% in a deeper C horizon (90 cm). We intend for these data, along with greenhouse results, to depict the physical conditions necessary to ignore chemical controls in vadose zone modeling.