Groundwater recharge and contamination: Sensitivity analysis for carbonate aquifers in south-eastern Ontario. The Jock River Basin study.

Abstract

The Jock River Basin was selected as a case study representative of carbonate aquifers in South-Eastern Ontario, where development is increasing demands for groundwater. Monthly hydrochemical and isotope data were used to define aquifer recharge and response to recharge events. Groundwater discharge is important in maintaining the river base flow and wetlands. Deep bedrock groundwater recharge (between 10 to 25 m below bedrock surface) occurs in areas of shallow discontinuous overburden. Groundwaters show a high degree of mixing, but despite mixing impact by anthropogenic activity as road salting and nitrate pollution has been documented. In areas of thick overburden ($>$2m) two artesian confined interconnected aquifers have been identified, (i) a highly permeable basal overburden - upper bedrock aquifer and (ii) a deeper fractured bedrock aquifer. Circulation rates can be in a matter of days in the shallow aquifer, implying sensitivity to drought and development. While shallow groundwaters are highly sensitive to seasonal recharge, deeper bedrock groundwaters show essentially no temporal variations in chemistry and isotopes despite their shallow depth ( 25m). Their homogeneity and their low tritium content suggest a mean circulation time on the order of several decades. Susceptibility to mining and long term contaminant loading is a concern for long circulation time bedrock groundwaters. Isotope measurements suggest reversals of hydraulic gradients during periods of high groundwater consumption from both aquifers. The measurements show that these gradient reversals allow for rapid transport of surface waters into the aquifers. Conditions may allow for rapid introduction of contaminants in the long circulation time bedrock groundwaters.