Groundwater geochemistry of the Outaouais Region (Quebec, Canada): a regional-scale study

As part of a province-wide groundwater characterization program, a detailed groundwater geochemistry survey was undertaken in the Outaouais Region (Qu,bec, Canada) in order to identify the primary processes responsible for groundwater quality and to develop a conceptual model for groundwater flow and geochemical evolution. During the summers of 2011 and 2012, 139 samples were collected from municipal and private wells which were analysed for major ions, nutrients, trace elements and sulphides. About 70 % of the samples were obtained from bedrock wells, mainly in the silicate rocks of the Canadian Shield and the remainder from wells screened in Quaternary deposit aquifers. Hydrogeochemical facies distributions were determined from 127 of these samples which had anion-cation charge balance errors within +/- 10 %. The classification by facies was also supported by a multivariate statistical analysis, namely factor analysis combined with hierarchical cluster analysis. The study identified Champlain Sea invasion, cation exchange and freshwater recharge as the main geochemical processes affecting groundwater chemistry in this region. Secondary processes, related to the bedrock geology, are responsible for exceedances of Canadian drinking-water standards, namely for fluoride, uranium, iron and manganese.