Dating groundwater resources in central Québec using noble gases, 14C and water chemistry

Genevieve Vautour, Guillaume Meyzonnat, Daniele Luigi Pinti, Florent Barbecot, Marie Larocque, Maria Clara Castro, Chris M. Hall, Jean-François Hélie

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: Canada's water supply is endangered by climatic and anthropogenic stress. A multi-isotopic study involving noble gases, 14C and U, Th isotopes was initiated as part of a Quebec-funded program. The Bécancour River catchment area extends from the Appalachian Mts. down to the St. Lawrence River. Aquifers are mainly recharged in the mountains but also locally through the plain. Main aquifers are found in unconfined and confined Quaternary sediment. Noble gas results were obtained on two transects. The 3He/4He ratios versus 4He/20Ne clearly points to mixing between three water bodies. The first displays a near-atmospheric 3He/4He ratio of 0.79 Ra (Ra = 1.386 x 10-6) and a 4He/20Ne of 0.2, close to the atmospheric ratio in groundwater. The second water body shows the occurrence of tritiogenic 3He excesses up to 1.80x10-13 ccSTP/g. Using a local tritium decay curve and measured 3Hetri [1], ages between 2-34 years are suggested. The third water body contains large amounts of radiogenic 4He, up to 4.48x10-5 ccSTP/g. Paleotemperatures calculated using atmospheric noble gases suggest values varying between 0° to 7 °C. The isotopic composition values of oxygen and deuterium lie just below the Local Meteoric Water Line on a D vs 18O graph. This shift could correspond to ancient Holocene recharge or evaporation during snowpack melting. A simple in situ U-Th-4He age model [2] gives ages varying from 150 years to 2.25 Ma, which suggest that a 4He excess is ageing the water by either: (1) a diffusive crustal basal flux of 4He, or (2) upward advection of fossil brines, or (3) upward advection of 4He carrying methane from the Lorraine or Utica shales. [1] Murphy et al. (2011) Hydrogeology 19, 195-207. [2] Torgersen and Clarke (1985) Geochimica Cosmochimica Acta 49, 1211-1218.

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