Partitioning of solutes from agricultural fields within the hydrologic system at two sites in Southern Ontario and the subsequent impact on adjacent aquatic ecosystems
Measurements of the hydrologic water balance of two agricultural field sites in southern Ontario (a hillslope comprised of loam soils near Kintore and a clay plain near Woodslee) were undertaken to determine the major transport pathways of contaminants (including nitrate, chloride, and atrazine) below the root zone. As well, a numerical model of two-dimensional water flow which includes tile drains was developed and tested at the Woodslee site. At the Kintore field site only, we also investigated the potential impacts of the contaminants on the biological health of a drainage ditch located near the perimeter of the study field. The study fields were instrumented with meteorological stations and time domain reflectometry probes to measure potential and actual evapotranspiration, water flow metering systems at all tile drainage outlets and at upstream and downstream ditch stations (Kintore only), and groundwater monitoring wells to measure hydraulic heads below the water table. To quantify the contaminant flux soil samples were collected from the A horizon and water samples were collected at all tile and drainage ditch monitoring stations and from all monitoring wells. Water and soil samples were analysed mainly for nitrate, ammonium, and chloride content, however, a limited number of samples were analysed for a standard suite of anions and cations, isotopes (15N and 18O), and atrazine. Results indicate that at the Kintore site the tile drains are the primary transport path for nitrate to the drainage ditch. Concentration of nitrate in groundwater is highest in the upper slope area. There is good evidence that denitrification in the lower part of the field and beneath an uncultivated strip around the perimeter of the field substantially reduces nitrate concentrations. Groundwater contributes proportionately more chloride than nitrate to the drainage ditch in comparison to tile drains. Results at the Woodslee clay plain site suggest that up to 20% of infiltration may bypass the tile drains and recharge deep groundwater. Preliminary results from the numerical model agree quite well with the measured quantity of tile drain effleuent, however, the model over-predicts the quantity of runoff and underestimates the amount of deep groundwater flow. One explanation for the discrepancies between model results and measurements is that the point measurements of soil hydraulic conductivity used in the model do not include macro-features such as fractures and are probably too low. Based on an investigation of biological effects on the drainage ditch at Kintore, each of the biotic indices (EPT richness index, Hilsenhoff BI) showed that water quality declined downstream. The Hilsenhoff BI indicated some organic pollution at all of the study sites in Logan Drain, but water quality was still considered to be good.