Assessing the Hydrogeological Properties Influencing Nitrate Distribution in a Shallow Unconfined Dolostone Aquifer and Supply Well Vulnerability

This study investigated an aquifer system in Halton Hills, Ontario, Canada where elevated concentrations of nitrate (NO3-) were detected in two municipal water supply wells since 2007. An investigation core hole was drilled 20 m from the supply wells following the Discrete Fracture Network-Matrix (DFN-M) field approach to evaluate local hydrogeologic conditions and vertical extent of NO3- in the bedrock. Quarterly groundwater sampling allowed for assessing the spatio-temporal hydrochemistry variability and isotope analysis provided insight on rates of denitrification and flow path distinction. Hydraulic testing on the multilevel system (MLS) required a novel setup and mathematical development to address friction and acceleration losses for unsteady oscillatory annular flow conditions to obtain transmissivity (T) estimates. This study expands upon the understanding of the local hydrogeologic flow system in comparison to the existing equivalent porous media (EPM) based regional conceptual site model (CSM) and presents a methodology for obtaining improved T estimates.