Behavior of 234U, 238U and 226Ra in shallow aquifers of Quebec, Canada
Abstract
A study was initiated in order to quantify the groundwater resources available in two different hydrogeological systems. The first is the Bécancour basin located between Montréal and Québec. Groundwater flow in shallower sands intercalated with clays of the Holocene Champlain Sea and fractured carbonates of Ordovician age of the St. Lawrence Lowlands. The second is composed of unconfined and/or buried fluvioglacial deposits (eskers) produced during the Holocene deglaciation (ca. 8,000 yrs ago) in the region of Amos, Abitibi-Temiscamingue. Presence of uranium and radium is dependent of the redox conditions and the physical/lithological characteristic of aquifers. 234U/238U activity ratios can give precious information on the infiltration conditions at the recharge or the water-rock interactions. 226Ra can allow obtaining residence time in young groundwater thanks to its half-life of 2,622 yrs. In Bécancour basin, there is a clear decreasing trend of the 234U/238U activity ratio along the flow path, from a maximum of 3.12 measured close to the recharge (Appalachian Mts.) to a value of 1.14, on the plain close to the St. Lawrence River (discharge). This trend cannot be related to the radioactive decay of the 234U compared to that of 238U because this would mean a water residence time of 106 yrs timescale, while waters are young and they show the occurrence of tritiogenic 3He. There is a clear relation between the alkalinity of waters (and the HCO3- content) and the 234U/238U activity ratio, suggesting that the mobility of uranium might be related to HCO3- and CO32- ions complex, inducing higher mobility of 234U compared to 238U after its expulsion into water by induced α-recoil. In the region of Amos, Abitibi, preliminary results were obtained on freshwater samples collected in the semi-confined St-Mathieu de Berry, the buried one of Barraute and the unconfined moraine of Harricana. Three samples were collected in a confined saline aquifer in the fractured Archean bedrock beneath the clay plain separating the different fluvio-glacial aquifers. Total dissolved uranium abundance seems to be constant along the depth between 0.0612±0.0002 ppb and 0.4481±0.0016 ppb but an enrichment of 234U in all samples is observed as shown by an increase of the 234U/238U activity ratio with depth. The same behavior is observed for the 226Ra/238U activity ratios that show an increase with depth with a maximum concentration of radium of 0.271 dpm per liter. This gradient can be explained by accumulation of 226Ra excesses from older waters located in the fractured basement. More isotopic data should permit to quantify radium accumulation to obtain residence time of waters in these aquifers.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H11D1225M
- Keywords:
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- 1030 GEOCHEMISTRY / Geochemical cycles;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 1120 GEOCHRONOLOGY / Isotopic disequilibrium dating;
- 1832 HYDROLOGY / Groundwater transport