Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The present study presents a conceptual model of the chemical evolution of Canadian Shield groundwater that includes major, minor and trace elements. Four (4) stages of the natural chemical evolution of groundwater are examined: Stage 1- infiltration of rainwater; Stage 2- evolution of recharge water, Stage 3a-salinization by Water/Rock interaction, and Stage 3 b-salinization by Water/Clay interaction. The approach proposed is based on a standard of reference called the ‘Water Maturation Index’ (WMI) that establishes standardized concentration values for specific chemical elements occurring in groundwater. The WMI was developed to describe the natural process of multiparameter enrichment/depletion through ongoing salinization from a reference water (rainwater in this study). It is concluded from our study that to better understand the mixing dynamics and contribution of compositional hydrogeochemical poles (endmembers) in an area of the Canadian Shield that has experienced a post-glacial marine event, a groundwater sampling campaign should minimally include the major elements: chlorides, bicarbonates, sulphates, sodium, potassium, magnesium and calcium, to which should be added in all cases the minor and trace elements: iron, manganese, bromides, lithium, strontium, boron, fluorides, molybdenum, nitrite-nitrate, ammonium and silica. Analyzing all these elements together would allow a better understanding of the origin of the chemistry of sampled groundwater, without necessarily requiring isotopic analyses. This study also highlights the importance of using a passive or low-flow sampling protocol to limit anthropogenic disturbance of the natural environment during sampling. Finally, this study improves an existing conceptual model of the chemical evolution of groundwater for the study area.