Assessment of groundwater discharge pathways in a till-dominated coastal aquifer

Abstract

Study region: Mabou Harbour located in Cape Breton Island, Nova Scotia, Canada, is representative of the many natural harbours throughout the Maritime region of Canada as the surrounding landscape is overlain by glacial deposits, predominantly composed of glacial till. Study focus: Understanding the pathways facilitating groundwater flow and associated solute transport to the ocean is key for developing conceptual hydrogeologic models for coastal watersheds and for informing coastal zone management. Most local-scale field and modelling submarine groundwater discharge (SGD) studies have been conducted in high-permeability formations given the likely importance of SGD in these coastal settings. This study investigates direct (SGD) and indirect (seaward stream baseflow) groundwater discharge from a till dominated coastal aquifer using a combination of field measurements (river flow and baseflow separation, seepage meter measurements, and water temperature analysis) and a calibrated 3D numerical groundwater flow model. New hydrological insights for the region: Results show that seaward stream baseflow greatly exceeds direct SGD to the ocean on the scale of the full harbour watershed (96.1% vs. 3.9 of total groundwater discharge). Particle tracking shows that the vast majority of SGD originates in the subcatchment immediately surrounding the harbour with limited intermediate or regional flow. The shorter flow paths and residence times for direct SGD results in less opportunity for natural attenuation processes and may have implications for groundwater-borne contamination to this harbour.

Keywords

• Groundwater flow modelling
• Submarine groundwater discharge
• Groundwater-surface water interactions
• Coastal hydrogeology
• Groundwater pathways