DISTRIBUTION OF ARSENIC, FLUORIDE AND MANGANESE IN OVERBURDEN WATER WELLS, SOUTHERN ONTARIO, CANADA

Groundwater contamination has become a global issue regardless of different geological setting. Groundwater can be affected by anthropogenic as well as naturally occurring contaminants from surface or subsurface, but subsurface geology plays a critical role in the ultimate distribution and transfer of the contaminants into aquifers. In southern Ontario (S-ON), the current physiography and complex overburden geology was significantly influenced by Quaternary glacial events. As the glacial processes determine the nature and distribution of shallow subsurface sediments that overlie the water table, some of the physiographic settings may be more vulnerable in terms of groundwater contamination. This study focuses on water samples collected by the Ontario geological survey (OGS) from 515 bored/dug and drilled wells completed in overburden sediment across S-ON. Here we focus on As, F and Mn, 3 out of 84 major and minor constituents from the OGS Ambient Groundwater Geochemistry dataset to determine their spatial distribution pattern and probable sources. This investigation builds on the OGS groundwater geochemical data by integrating the OGS physiographic map and 3D surficial deposit mapping in the Brantford-Woodstock area of S-ON to help examine the possible geological influence. Geospatial analysis of constituent concentrations and physiography shows the distribution of impacted water wells and level of contamination at a regional scale.

All the physiographic settings have elevated Mn, except kame moraines. In general, drilled wells are more impacted by both As and F contamination; based on their average depth, an influence by bedrock is inferred, though the common As-F association suggests these may also come from phosphate fertilizers. Reducing condition in shallow clay could favor the dissolution of these constituents in groundwater of bevelled till plains. Moreover, clay minerals, common in till and clay plain deposits, could play significant role in controlling As and F contamination. The Brantford-Woodstock area in S-ON has As- and Mn- affected drilled wells on till plains, till moraines and spillways. Local geology and land use indicate that the reducing environment at depth can prompt the dissolution of As and Mn released either from underlying bedrock or fertilizers applied in farm lands.