GROUND WATER AGE VARIABILITY WITHIN A GLACIAL AQUIFER BENEATH A THICK UNSATURATED ZONE

The Waterloo Moraine Aquifer Complex supplies 56% of the drinking water to the Region of Waterloo. Within the last 12 years, a concerted effort has been made to better understand the flow system of this heterogeneous aquifer system. The aquifer complex consists of alternating aquifers and aquitards, reflecting the interlobate nature of the moraine. It has been recognized that the aquitards are laterally discontinuous; “windows” in the aquitards may allow hydraulic connectivity between the aquifers. In 1993-1994, a geochemical and isotopic (²H, ¹⁸O) survey of the aquifer was completed, and two transects were sampled for ³H, CFC and ³H/³He age dates. Several large nitrate plumes were discovered and mapped under the largely agricultural moraine surface. Regional flow models incorporating inverse particle tracking were also constructed to determine wellhead vulnerability. The flow models have not yet been calibrated to the age data.

Prior to using age data to calibrate the regional flow model, it is important to answer the question “How accurate are the age data in the heterogeneous aquifer?” A second round of geochemical, isotopic and age data were collected, this time utilizing ³H, ³H/³He and SF₆ environmental tracers. Nitrate concentrations have generally increased over portions of the aquifer that are upgradient from the production wells, although dissolved gas data suggests denitrification may be occurring. The geochemical data has been combined with dissolved gas and the age tracers to evaluate mixing and ground water flow patterns along the two transects.