Hydrogeophysical monitoring reveals primarily vertical movement of an applied tracer across a shallow, sloping low-permeability till interface: Implications for agricultural nitrate transport

Abstract

Intensive potato production is an important contributor to groundwater and surface water nitrate contamination in the province of Prince Edward Island (PEI), Canada. In PEI, the presence of a permeability contrast between soil and the underlying compact basal till at approximately 1 m depth is known to cause temporary perched water table conditions, but the implications for agricultural nitrate transport are not well understood. This study is the first detailed field investigation to examine the impact of the soil – compact basal till interface on vertical and lateral subsurface flow of water and solutes on a moderate hillslope in PEI. A surface-applied tracer test was conducted and a shallow drain-tile pipe system, with a tipping bucket gauge, was installed down-gradient of the tracer application to monitor lateral subsurface flow. In addition, three-dimensional electrical resistivity imaging (ERI) was employed to monitor the evolution of the applied tracer plume over a period of 1 year. A small scale, transient flow and transport model was used to simulate the tracer test and to investigate the sensitivity of tracer transport to selected parameters (primarily, the hydraulic conductivities of the soils and underlying compact basal till). Monitoring, conducted from October 2013 to June 2015, and numerical modeling showed a strong tendency for vertical flow of water and tracer through the soil – compact basal till interface as opposed to lateral down-slope subsurface flow. Water balance calculations estimated a total vertical flow of 94.5% of the infiltration for the October 2014 to January 2015 period. The preference for vertical over lateral flow and transport was consistent with electrical resistivity images for the same time period. These findings indicate that the fate of infiltrating water and dissolved nitrate is the deeper bedrock aquifer water table, and that intermittent perched water table conditions do not result in significant lateral flow toward nearby surface waters.