Mapping submarine groundwater discharge at the Magdalen Islands

ABSTRACT: The Magdalen Islands in the Gulf of St. Lawrence are surrounded by salt water, causing groundwater to be the only source of drinking water for its inhabitants. Fresh groundwater in the archipelago is contained within a permeable red sandstone aquifer, and exists as a lens that overlies saline groundwater. Because of this configuration, exploitation of this aquifer requires complex management strategies due to potential saline intrusion in the pumping wells. Given the absence of any surface water bodies on the archipelago, the study of submarine fresh groundwater discharge (SFGD) is essential since it represents the only natural discharge for groundwater. SFGD is driven by differences in hydraulic head, and causes fresh groundwater to flow from the aquifer into the marine environment. Quantifying and mapping SFGD at the Magdalen Islands will allow for a refinement of the water budget and thus contribute to better management of local groundwater resources as impacted by climate change. Furthermore, mapping SFGD will supply additional data for the calibration of groundwater management models. The main objective of this study is to map the SFGD at the Magdalen Islands using thermal infrared remote (TIR) sensing. Unmanned aerial thermal infrared surveys allow for SFGD observation based on temperature contrasts at a given time in marine environments. However, several elements can affect the surface temperature in coastal waters, making the analysis of thermal infrared images challenging. In addition, there is little to no theoretical framework in the existing literature providing the appropriate methodology to assess the resulting images. In this study, surveys were carried out on multiple sites of the archipelago using a drone mounted with a TIR camera and a practical interpretation framework will be developed based on field observations and a numerical model to facilitate a standardized analysis of future TIR surveys.