Preferential flow of surface-applied solutes: Effect of lysimeter design and initial soil water content

Authors: Williams, Mark; CORONEL, OSCAR - Northeastern Illinois University; McAfee, Scott; SANDERS, LAURA - Northeastern Illinois University

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2020
Publication Date: 7/12/2020
Citation: Williams, M.R., Coronel, O., McAfee, S.J., Sanders, L.L. 2020. Preferential flow of surface-applied solutes: Effect of lysimeter design and initial soil water content. Vadose Zone Journal. 19:e20052. https://doi.org/10.1002/vzj2.20052.
DOI: https://doi.org/10.1002/vzj2.20052

Interpretive Summary: Preferential flow can result in substantial water and nutrient transport through the soil profile. Undisturbed blocks of soil extracted from agricultural fields (also referred to as soil columns or lysimeters) are often used to study water and nutrient movement in the laboratory. While lysimeters offer the advantage of a known soil volume and controlled laboratory conditions, unnatural preferential flow at the interface where the soil meets the lysimeter casing can produce unrealistic results. In this study, we use conservative solutes (bromide and chloride) and stable water isotopes to trace the sources and flow pathways of water through the soil under wet and dry antecedent moisture using lysimeters with and without edges sealed with petroleum jelly. Results showed that rainfall comprised 21-59% of the water leached through the soil profile regardless of lysimeter design or antecedent conditions. Sealing the edges of the lysimeters with petroleum jelly greatly reduced unnatural edge flow during rainfall simulations, but did not eliminate it. The greatest risk for edge flow occurred in unsealed lysimeters under wet antecedent conditions. Using multiple tracers, different initial antecedent conditions, and lysimeters with and without sealed edges not only allowed for the rigorous testing of laboratory methods, but also provided new insights into preferential flow processes and patterns that control subsurface nutrient transport.

Technical Abstract: Undisturbed soil lysimeters are widely used to study water and solute transport, where both natural (e.g., cracks, earthworm burrows) and un-natural (e.g., edge flow) preferential flow can greatly influence leaching rates. The objective of this study was to use chemical (Br- and Cl-) and isotopic (d18O) tracers to quantify the effect of initial soil water content and lysimeter design (unsealed edges vs. sealed edges) on preferential flows. Ten undisturbed lysimeters (900 cm2) were collected from an agricultural field, with the gap between the soil and lysimeter casing sealed with petroleum jelly for five of the lysimeters. Lysimeters were subjected to two rainfall simulations (3.3 cm h-1) under contrasting initial soil water contents, and leachate near the soil-casing interface was collected separately from leachate through the bulk soil. Three-component hydrograph separation revealed that event water (i.e., rainfall) comprised 21 to 59% of total leachate through the lysimeters irrespective of initial soil water content and lysimeter design. Sealing the edges of the lysimeters with petroleum jelly greatly reduced but did not eliminate edge flow during rainfall simulations. While water and solute transport were similar in both sealed and unsealed lysimeters under dry antecedent conditions due to the formation of shrinkage cracks on the soil surface, edge flow was substantially greater for the unsealed lysimeters under wet antecedent conditions. Unsealed edges under wet antecedent conditions facilitated the preferential transport of both event and pre-event water resulting in greater solute leaching. Using multiple tracers to contrast lysimeter designs under different initial soil water contents not only allowed for rigorous testing of a commonly used edge-flow suppression technique, but also provided new insights into preferential flow processes and patterns.