Electrical monitoring of saline tracers to assess subsurface hydrological connectivity in a flat ditch-drained field

Authors: ROBINSON, JUDITH - Pacific Northwest National Laboratory; Buda, Anthony; COLLICK, AMY - University Of Maryland Eastern Shore (UMES); SHOBER, AMY - University Of Delaware; NTARLAGIANNIS, DIMITRIS - Rutgers University; Bryant, Ray; Folmar, Gordon; ANDRES, SCOTT - University Of Delaware; SLATER, LEE - Rutgers University

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/16/2020
Publication Date: 3/17/2020
Citation: Robinson, J., Buda, A.R., Collick, A., Shober, A., Ntarlagiannis, D., Bryant, R.B., Folmar, G.J., Andres, S., Slater, L. 2020. Electrical monitoring of saline tracers to assess subsurface hydrological connectivity in a flat ditch-drained field. Hydrological Processes. 586:124862. https://doi.org/10.1016/j.jhydrol.2020.124862.
DOI: https://doi.org/10.1016/j.jhydrol.2020.124862

Interpretive Summary: Characterizing hydrologic processes that connect landscapes with surface waters is central to understanding the risk of phosphorus transfers from agriculture. This is particularly important in places like the lower Delmarva Peninsula, where intensive ditch drainage can lead to elevated phosphorus losses. In this study, we used innovative electrical methods to track the migration of a conductive salt tracer in shallow groundwater in the vicinity of an open drainage ditch. Findings revealed the importance of vertical leaching and lateral subsurface flow as key processes that linked the field with the ditch during storms. Even so, travel times in shallow groundwater were very slow, suggesting critical source areas of phosphorus loss were within 1 meter of the ditch during individual storms. Over the course of a year, these areas could extend to 10 meters from the ditch due to progressive phosphorus transfers in recurrent storms.

Technical Abstract: Time-lapse electrical resistivity imaging (ERI) combined with salt tracers provides a better understanding of how soil and antecedent wetness control hydrologic connectivity in ditch-drained agroecosystems. In a nearly level field adjacent to a drainage ditch, 192 electrodes were installed in a 72 m2 plot along with five hydrometric stations to study shallow subsurface transport. A salt tracer was applied within an upgradient trench and the migration was monitored during two storms typical of hydroclimatic conditions. Antecedent moisture conditions were markedly different, with storm SE1 having dry antecedent conditions and storm SE2 having wet antecedent conditions. In both storms, spatial delineation of tracer transport using time-lapse ERI showed rapid vertical percolation of the tracer through an argillic horizon followed by lateral movement within a lower, more permeable sandy horizon. Continuous electrical fluid conductance monitoring from the hydrometric stations confirmed these findings with elevated values below the argillic horizon after tracer application. Spatial moment analysis from time-lapse ERI supported by groundwater gradients revealed antecedent moisture conditions impacted 1) tracer flow direction and 2) seepage velocities. During dry antecedent soil moisture conditions observed in SE1, the flow direction was from the ditch to the field during a portion of the storm. Inferred as a hydrological disconnect of the ditch with groundwater, this could temporarily form a barrier to solute transfers from groundwater to the ditch during dry conditions. In SE2, the flow direction was consistently from the field to the ditch. Greater groundwater velocities during wet conditions suggest that solute transfers from groundwater to ditch are likely to be faster in storms during wet periods. Slow lateral transport times within the lower horizon indicate that critical sources areas of nutrient loss are likely within 1 m of the ditch.