3H-3He groundwater dating and geochemical tracers to identify TCE origins and migration paths in a valley-fill aquifer

Samantha E.M. Murphy, René Lefebvre, Thomas Ouellon, Jean-Marc Ballard, Ian D. Clark

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Isotope Tracing I

ABSTRACT: A deltaic sand aquifer in a buried valley is contaminated with trichloroethylene (TCE) and its daughter products. A numerical groundwater model was developed to better define groundwater flow and dissolved TCE transport in order to guide the implementation of remediation and plume control strategies. In order to further constrain the model, independent data were required for model verification in addition to hydraulic head data. Tritium-helium (3H-3He) groundwater dating is a reliable method of determining groundwater residence times in shallow groundwater systems, which can be directly compared with simulated model ages. Groundwater ages and other geochemical tracers can therefore be used to relate different parts of the dissolved TCE plume to their origins from source zones and identify groundwater flow and TCE migration paths. The Valcartier deltaic aquifer is especially well suited for the application of groundwater dating as it does not contain silty layers, except in the eastern end of the system where a silty prodeltaic aquitard is found (Murphy et al., 2011). Figure 1 shows the study area where groundwater samples were obtained along flowlines originating from two previously identified TCE source zones, which converge to form a single plume downgradient in the aquifer. The geochemical characterization included physicochemical parameters, major ions, stable isotopes 18O and 2H, terrigenic 4He, perchlorate (ClO4) as well as TCE and its daughter products. Figure 1. Study area with sampling sites in wells located along two flow paths (north and south), indicated by particle tracking using a numerical model. The use of wells located along simulated flow paths facilitated the comparison of groundwater ages obtained from isotopic data with simulated ages from particle tracking. Geochemical water types were defined using multivariate statistical techniques. This included principal component analysis (PCA) and hierarchical cluster analysis (HCA), which defined 7 clusters that were used to define water types. Geochemical patterns from individual tracers and water types were contoured on the three vertical sections (Figure 2). These contours show distinct patterns that are coherent with groundwater flow conditions. The two flow paths are characterized by distinct geochemical HCA water types related to water origins. 3H-3He ages were compared to simulated ages using a range of values for porosity to determine the best match. In certain locations, anomalously old

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