Hydrogeochemical characterization of groundwater in the Outaouais Region (Québec, Canada) - A regional scale study

Montcoudiol, N., Molson, J. W. and Lemieux, J.-M. (2013) Hydrogeochemical characterization of groundwater in the Outaouais Region (Québec, Canada) - A regional scale study. AGU Fall Meeting, San Francisco, CA USA, 09-13 Dec 2013.

Abstract

As part of the Québec regional groundwater characterization program (PACES), a detailed groundwater quality survey was undertaken in the Outaouais Region (Québec, Canada). During the summers of 2011 and 2012, 139 samples were taken from municipal and private wells which were analysed for major ions, nutrients, metals and sulphides. About 70% of the samples were obtained from bedrock wells, mainly in the Canadian Shield and the remainder from wells screened in Quaternary deposit aquifers. Hydrogeochemical facies were determined for 127 samples which had anion-cation charge balance errors within ×10 %. Ca-HCO3 is the dominant water type (65%) which was mainly found in unconfined aquifers, especially Quaternary deposits, and is typical of recently infiltrated rainwater. Other relevant water types are Na-HCO3 and Na-Cl (17 and 6% respectively), characteristic of confined aquifers. This classification by water type is supported by multivariate statistical analysis, namely Principal Component Analysis (PCA) and Hierarchical Clustering Analysis (HCA). PCA allows the identification of three factors controlling groundwater chemistry: salinity, silicate dissolution and F-bearing mineral dissolution. HCA results show that the samples can be grouped into seven clusters. Clusters 1 to 4 are mostly Ca-HCO3 water type and are representative of the enrichment in major ions due to carbonate and silicate dissolution, cluster 1 being closer to rainwater and cluster 4 the most evolved. Cluster 5, made of one sample with a particular chemistry, is not yet fully understood. Samples from cluster 6 present various degrees of Na-Ca exchange, a consequence of remnant Champlain Sea water (some samples from cluster 7 in confined zones) being replaced by infiltrating recharge water. Samples from cluster 7 are evolved waters with high Total Dissolved Solid (TDS) concentrations: they are remnants of the Champlain Sea in confined aquifers (bromide detected) or diluted/mixed by infiltrating rainwater in unconfined aquifers (bromide below detection limit). Secondary processes, related to the bedrock geology, are responsible for exceedances of Canadian drinking water standards. They include the dissolution of F-, U-, Fe- and Mn-bearing minerals known to be abundant in the region (deposits and mines throughout the study area). Chloride contamination was identified in some recharge areas, related to anthropogenic activities. The study has identified Champlain Sea invasion, cation exchange and freshwater recharge as the main geochemical processes affecting groundwater chemistry in this region. Secondary processes were also identified, based on exceedances of Canadian drinking water standards. These results will be applied at the local scale, along a flow path, to relate geochemical evolution to groundwater flow conditions. More data are available at this scale such as stable isotope content (δ18O and δ2H), groundwater age (tritium and 14C) and noble gas content. These additional data will help improve our understanding about groundwater chemical evolution and define a complete conceptual model.

• Item Type: Conference or Workshop Item
• Status: Published
• Refereed: No
• Glasgow Author(s) Enlighten ID: Montcoudiol, Dr Nelly
• Authors: Montcoudiol, N., Molson, J. W., and Lemieux, J.-M.
• College/School: College of Science and Engineering > School of Engineering > Systems Power and Energy