Desalination of a sedimentary rock aquifer system invaded by Pleistocene Champlain Sea water and processes controlling groundwater geochemistry

Abstract

The objective of this study was to identify geochemical processes and Quaternary geological events responsible for the variations in groundwater geochemistry observed in a sedimentary rock aquifer system, including brackish to saline groundwater. Inorganic constituents and environmental isotopes were analyzed for 146 groundwater samples. Dissolution of carbonates dominates in recharge areas, resulting in Ca-, Mg-HCO 3 groundwater. Further along flow paths, under confined conditions, Ca 2? -Na ? ion exchange causes groundwater evolution to Na-HCO 3 type. Na-Cl groundwater is also found and it falls on a seawater mixing line. Using conservative tracers, Cl -and Br -, the original Champlain Sea water is shown to have been, in the region, a mixture of about 34% seawater and 66% freshwater, a composition still retained by some groundwater. Na-Cl groundwater thus results from mixing with former Champlain Sea water and also from solute diffusion from overlying marine clay. The system is thus found to be at different stages of desalinization, from the original Champlain Sea water still present in hydraulically stagnant areas of the aquifer to fully flushed conditions in parts, where more flow occurs, especially in recharge zones. The geochemical processes are integrated within the hydrogeological context to produce a conceptual geochemical evolution model for groundwater of the aquifer system.

Groundwater Quality, Geochemical Processes and Groundwater Evolution in the Chateauguay River Watershed, Quebec, Canada

“…Note that only the flow path A-A' is discussed in this paper. This approach supported the development of a conceptual model of the geochemical evolution of groundwater, which was compared to the conceptual model of the BassesLaurentides area developed previously by Cloutier et al (2010).…”

“…It can be concluded that the conditions present in the groundwater flow system in the Chateauguay River watershed allows more flushing of Champlain Sea water compared to the Basses-Laurentides, thus leading to the main differences observed in the groundwater geochemistry of these two areas. Areas of reduced flushing in BassesLaurentides are related to convergentdivergent groundwater flow patterns in two buried valleys of the Basses-Laurentides (Cloutier et al, 2010). Such flow patterns are not observed in the Chateauguay River, which has continuous flow paths leading to significant flushing of the entire aquifer system, thus reducing the sea water imprint on groundwater geochemistry.…”

“…Figure 5 shows the processes related to the salinity component #1. Cl -and Br -ions are generally used for identifying the origin of salinity and related processes (Richter et al, 1993;Cloutier et al, 2010). Two major trends are identified ( Figure 5).…”

“…The first shows a steady Cl -/Br -ratio with the increase of Cl -concentrations and is mainly correlated to water types of groups G2 and G3. Following the Champlain Sea retreat, about 9,500 years ago, saline water was left in the aquifer system (Cloutier et al, 2010). Considering this initial presence of saline water in the aquifer, the first trend can be associated with water mixing between fresh water and remnant Champlain Sea water.…”

“…Hierarchic Cluster Analysis (HCA) was applied to define geochemical zones. A conceptual model of the geochemical evolution of the groundwater was also defined (Cloutier et al, 2010).…”