Présentations Colloques

Oral Presentation
Session 8.04: Isotopic and residence time tracers
Janos Debora
Regional Flow and Groundwater Residence Time Simulations in Chaudire-Appalaches, Québec, Canada- Implications for interpreting Regional Geochemistry
A two-dimensional numerical model has been developed for simulating groundwater flow and mean groundwater age in the Chaudière-Appalaches region, Canada. As part of the regional basin-scale PACES projects in Québec, the study aims to advance our understanding of regional-scale groundwater flow dynamics and natural geochemical processes, and provides insights into the extent to which regional groundwater quality is shaped by flow dynamics. More specifically, the study aims to quantify the magnitude of regional flow, to estimate the maximum depth of active groundwater flow and the distribution of groundwater residence times, investigate the influence of normal faults on regional flow, and examine the contribution of formation brines and the Champlain Sea which covered part of the basin following the last deglaciation. Physical and chemical hydrogeological data were used to generate a regional conceptual and numerical flow model oriented in the vertical 2D plane roughly south-north towards the St. Lawrence River. The numerical model was first calibrated to the average recharge values+ then the role of faults was investigated by testing various fault configurations and hydraulic conductivities. Although some evidence for deeper regional flow exists, the area appears dominated by sub-regional flow systems on maximum scales of about 10-20 km, with significant flow through the shallow fractured sedimentary rock aquifer. This regional scale model is also used to support local groundwater geochemical interpretation, by providing insight into regional flow dynamics, including the extent to which deep basinal brines or marine waters might contribute to the regional flow. Finally, the simulated flow system in a transport model to simulate the distribution of groundwater ages, which are then compared to sampled 14C water ages. The use of residence times as the common parameter between flow dynamics and geochemical evolution helps verify the spacio-temporal coherence of the previously identified geochemical processes by bounding them to a time-scale. The insights obtained from the study will be used to better manage and protect the regional groundwater resources.