Water and contaminant travel time through the unsaturated zone under an agricultural field in Prince Edward Island, Canada

English Abstract:

Knowledge of the travel time of water and contaminants through the unsaturated zone in intensive agricultural areas such as Prince Edward Island (PEI) is of crucial importance when assessing the impacts of Beneficial Management Practices (BMPs) on the quality of drinking water and of down-gradient aquatic ecosystems. This study was done at the Agriculture and Agri-Food Canada (AAFC) Research Farm in Harrington, PEI, in a field that has been under a potato production system since 2007. The unsaturated zone comprises a relatively thin soil layer (~1 m), underlain by a thick (~ 8 m) glacial till consisting of weathered local parent material (i.e., sandstone) and a 10-12 m thick layer of fractured sandstone. The estimated average downward velocities through the matrix of both till and bedrock has an upper limit of 12 m/year, however the estimated velocity of flow through the fractures can reach 280 m/year. In 2014, three boreholes disposed in an equilateral triangle (with 9 m long sides) were drilled and each equipped with an array of 11 Decagon 5TE sensors measuring temperature, moisture and electrical conductivity and 24 electrodes measuring resistivity to provide real-time vertical profiling of downward movement of water and solutes. A simplified flow and transport model has been developed in Finite Element Flow (FEFLOW) and the model will be refined using data resulting from a tracer test that is planned in the summer of 2015 as well as from the logging of the rock cores and overburden samples collected during the drilling of the three boreholes. The use of field data in combination with unsaturated flow and transport modelling as well as cross-hole three dimensional Electrical Resistivity Tomography (3D ERT) will provide insight into the travel times of water and solutes through the unsaturated zone as well as into the partitioning between fracture and matrix flow and the effects of heterogeneity in hydraulic conductivity of both till and bedrock materials.­­