Intensive potato production in the Canadian province of Prince Edward Island (PEI) has contributed to the leaching of nitrate to groundwater and increased nutrient loadings to surface waters. The presence of a compact till layer at roughly one metre depth is known to cause temporary perched water table conditions, which may result in lateral subsurface flow that may effectively reduce the migration of nitrate to groundwater. The primary objective of this work was to determine the importance of this compact layer to vertical and lateral subsurface flow of water and nitrate. A surface applied tracer test was conducted at the Harrington Research Farm in central PEI. A shallow drain-tile pipe system, with a tipping bucket gauge, was installed down gradient of the tracer application to monitor lateral subsurface flow. A three-dimensional electrical resistivity imaging (ERI) array was employed on surface to monitor the spatial and temporal distribution of the applied tracer. A small scale, transient flow and transport model was constructed to simulate the tracer test. Results from the tracer test monitoring and numerical modelling show that the occurrence of lateral flow is dependent on the antecedent moisture conditions in the subsurface. The water balance yielded a total vertical flow of 94 % of the infiltration for the period beginning October 25, 2014 and ending January 1, 2015. The cumulative observed tracer mass in the tipping bucket as of May 12, 2015 (10.5 months after tracer application) was 0.125 % of the applied mass. The preference for vertical over lateral flow was further evidenced by the electrical resistivity images for the same time period. These results show a strong preference for vertical flow of water and nitrate through the compact layer as opposed to lateral subsurface flow.
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