Dating shallow groundwater using the 3H/3He method at the Bécancour River basin (Québec, Canada)

Marion Saby, Geneviève Vautour, Emilie Roulleau, Daniele L. Pinti, Marie Larocque, Maria Clara Castro

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: In the past few years the government of Québec has invested in characterizing available groundwater resources with the goal to protect them from future climatic and anthropogenic stresses. In this context, groundwater age is an important parameter that contributes to a better understanding of the global hydrological functioning of aquifers in terms of recharge and groundwater flow rates. The purpose of this study is to estimate the groundwater residence time in the Bécancour River catchment basin, located mid-way between Montréal and Québec (Canada). Groundwater is mainly recharged in the Appalachians and flows downward to the St. Lawrence River. A preliminary noble gas study performed in 2010-2011 showed that groundwater flowing in Holocene aquifers contains three helium components, radiogenic, atmospheric and tritiogenic. Radiogenic 4He-rich groundwater is mainly flowing in confined aquifers oriented NE-SW along a sinclinorum parallel to the St. Lawrence River. Groundwater flowing in granular unconfined aquifers in the plain shows exclusively atmospheric He, suggesting short residence times and indicating local recharge. On the basin eastern border, measured 3He/4He ratios (R) are higher than the atmospheric ratio (Ra = 1.386 x 10-6). Corrected for radiogenic and excess air contributions, all R/Ra ratios except one, show a linear increase with distance from the Appalachian recharge area (R/Ra = 1) to the plain discharge area (R/Ra = 1.70). The sample that does not display this linear correlation is located mid-way between the recharge and the discharge, displaying a R/Ra value of 2.50. These values clearly indicate the occurrence of 3He excesses produced from decay of 3H infiltrating aquifers after the thermonuclear bomb peak of the 60™s. The progressive 3He/4He ratio increase suggests a fipiston flowfl model for the aquifer, with an increasing input of 3H-decay 3He downstream, thus water recharged in older times when the 3H input was more important than today. Water has been resampled in the fall 2012 and tritium analyses are under way at the University of Waterloo to obtain precise 3H/3He ages.

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