|Résumé||(disponible en anglais seulement)|
The study of groundwater hydrogeochemistry of the Paleozoic Basses-Laurentides sedimentary rock aquifer system in Québec produced a large geochemical dataset.
Groundwater samples were collected at 153 sites over a 1500 km2 study area and analyzed for major and minor ions. The large number of data can lead to difficulties in the integration, interpretation and representation of the results. Two multivariate
statistical methods, hierarchical cluster analysis (HCA) and principal components analysis (PCA), were applied to a subgroup of the dataset to evaluate their usefulness to classify the groundwater samples, and to identify geochemical processes
controlling groundwater geochemistry. This subgroup consisted of 144 samples and 14 parameters (Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, SO42-, Fe2+, Mn2+, Br-, Sr2+, F-, Ba2+, HS-). Seven geochemically distinct clusters, C1-C7, resulted from the HCA.
Samples from clusters C3, C4, C6 and C7 are mostly located in preferential recharge areas. The majority of these samples have Ca-Mg-HCO3 recharge groundwater (C3, C6, C7) and Na-HCO3 evolved groundwater (C4). Samples from the other three clusters
(C1, C2, C5) are characteristic of an aquifer system under confined conditions. The majority of these samples have Na-HCO3 evolved groundwater (C1, C5) and Na-Cl ancient groundwater that exhibits elevated concentrations in Br- (C2). In addition to
recognizing the importance of hydrogeological conditions on groundwater geochemistry, the distribution of clusters also showed the importance of the geological formations on minor and trace elements, such as Fe2+, Mn2+, Sr2+, F- and Ba2+. The first
five components of the PCA account for 78.3% of the total variance in the dataset. Component 1 is defined by highly positive loadings in Na+, Cl- and Br- and is related to groundwater mixing with Champlain Sea water and solute diffusion from the
marine clay aquitard. The high positive loadings in Ca2+ and Mg2+ of component 2 suggest the importance of dissolution of carbonate rocks in this aquifer system. From their characteristic loadings, the first two components are defined as the
"salinity" and "hardness" components, respectively. Components 3-5 are related to more local and geological effects. The integration of the HCA and the PCA, with conventional classification of groundwater types, as well as with the hydrogeological
and geological contexts, allowed the division of the region into four main geochemical areas, providing an improved regional picture of the aquifer system dynamics and hydrogeochemical evolution of groundwater. The following factors were recognized
as influencing the evolution of groundwater identified in every geochemical area: (1) geological characteristics including sedimentary rock type and till mineralogy; (2) hydrogeological characteristics represented by the level of confinement and the
hydraulic gradient; and (3) the geological history including the latest glaciation and the Champlain Sea invasion. With its integrated approach, this hydrogeochemical study contributes to the characterization and understanding of complex groundwater
flow systems, and provides an example of the long-term geochemical evolution of hydrogeological systems after a major perturbation, in this case seawater invasion.