Groundwater contained in Paleozoic bedrock formations and the overlying unconsolidated sediments (“drift”) in southern Ontario exhibits increasing salinity with depth (Carter et al, 2014). There is a downdip transition from fresh water at or near the surface in the drift and shallow bedrock, to brackish to saline sulphur water at intermediate depths, to dense brines in the deepest bedrock (Carter et al, 2015a, 2015b, Carter et al, 2014, Sharpe et al, 2014) (Fig.1). The base of the fresh water regime, i.e. the depth at which the water is no longer potable, has been previously mapped by Carter and Clark (2018) in southwestern Ontario. This study attempts to map regional variations in the depth of the interface between the sulphur water regime and the deep brine regime.