Characterization of a TCE-contaminated aquifer using tritium-helium dating and geochemical tracers, Valcartier, Quebec, Canada

The groundwater supply in the Valcartier area of Quebec is contaminated with trichloroethylene (TCE) from multiple sources, and hydrogeologic characterization is being carried out to understand its origin and migration patterns. A geochemical investigation was undertaken to independently verify groundwater flow and TCE transport in Valcartier groundwater to improve a numerical model that is currently being developed.;Geochemical parameters measured in groundwater along the flow paths as well as in Shannon and in the proglacial aquifer were examined for patterns. Multivariate statistics (Principal components analysis and cluster analyses) were applied to the data to distinguish different types of groundwater; the geochemistry of each group reflects recharge origin and lithology encountered. The groups verify flow paths delineated by the groundwater model, and help distinguish groundwater of different origins downgradient of source zones. One facies in particular is associated with high TCE concentrations and occurs where groundwater upflow from the proglacial aquifer in Shannon inferred from the 3H-3He data. This appears to be TCE coming from a previously unrecognized source in the south part of the study area, possibly an old bedrock dump.;This study demonstrates the value of integrating geochemical information in groundwater characterization programs with existing information to refine the understanding of groundwater flow, especially in the development of numerical models.;3H-3He ages were obtained along flow paths stemming from two main source zones and compared with advective model ages produced at corresponding locations. Ages obtained above and below a prodeltaic silty aquitard in the eastern part of the study area were used to calculate an average vertical hydraulic conductivity (Kv) of 1.3-3.1x10-8 m/s. 3H-3He ages were compared with modelled agesat different values of porosity ( n) and recharge; the bestcorrespondence was found to be at 0.35 and 300 mm/yr, respectively. Anomalously high concentrations of terrigenic He and unexpectedly old 3H-3He ages in some areas indicate areas where groundwater in the underlying proglacial aquifer may flow upward into the deltaic sand aquifer.