Linking groundwater quality to residence times and regional geology in the St. Lawrence Lowlands, southern Quebec, Canada

Abstract

The assessment of groundwater quality in shallow aquifers is of high societal relevance given that large populations depend directly on these water resources. The purpose of this study was to establish links between groundwater quality, groundwater residence times, and regional geology in the St. Lawrence Lowlands fractured bedrock aquifer. The study focuses on a 4500 km 2 watershed located in the St. Lawrence Lowlands of the province of Quebec in eastern Canada. A total of 150 wells were sampled for major, minor, and trace ions. Tritium (3 H) and its daughter element, 3 He, as well as radiocarbon activity (A 14 C) were measured in a subset of wells to estimate groundwater residence times. Results show that groundwater evolves from a Ca-HCO 3 water type in recharge zones (i.e., the Appalachian piedmont) to a Na-HCO 3 water type downgradient, toward the St. Lawrence River. Locally, barium (Ba), fluoride (F), iron (Fe), and manganese (Mn) concentrations reach 90, 2, 18, and 5.9 mg/L respectively, all exceeding their respective Canadian drinking water limits of 1, 1.5, 0.3, and 0.05 mg/L. Release of these elements into groundwater is mainly controlled by the groundwater redox state and pH conditions, as well as by the geology and the duration of rock-water interactions. This evolution is accompanied by increasing 3 H/ 3 He ages, from 4.78±0.44 years upgradient to more than 60 years downgradient. Discrepancies between calculated 3 H/ 3 He and 14 C water ages (the latter ranging from 280 ± 56 to 17,050 ± 3410 years) suggest mixing between modern water and paleogroundwater infiltrated through subglacial recharge when the Laurentide Ice Sheet covered the study area, and during the following deglaciation period. A linear relationship between 3 H activity and corrected 14 C versus Mg/Ca and Ba support a direct link between water residence time and the chemical evolution of these waters. The Ba, F, Fe, and Mn concentrations in

Fracturing‐induced release of radiogenic 4He and 234U into groundwater during the last deglaciation: An alternative source to crustal helium fluxes in periglacial aquifers

“…The maximum residence time has been taken as equal to 12.2 kyr, the age of deglaciation in the region. As noted by several studies in this region [ Cloutier et al ., ; Beaudry , ; Saby et al ., ], the maximum water residence time in the St. Lawrence Lowlands coincides with the Laurentide Ice Sheet retreat. This postglaciated groundwater contains large amounts of radiogenic 4 He, equivalent to that measured in BEC101, as also noted by Saby et al .…”

“…This postglaciated groundwater contains large amounts of radiogenic 4 He, equivalent to that measured in BEC101, as also noted by Saby et al . [].…”

“…Assuming an initial 4 He concentration in the rock of C 02 = 1.6 ± 0.9 × 10 −4 cm 3 STP , the total amount of 4 He accumulated in groundwater will range from 3.89 × 10 −6 cm 3 STP (t 0 = 6.7 kyr and D s2 = 1.0 × 10 −19 cm 2 s −1 ) to 1.22 × 10 −5 cm 3 STP (t 0 = 12.2 kyr and D s1 = 1.0 × 10 −18 cm 2 s −1 ). Except for the 4 He concentration calculated using the lower C 02 , a residence time of 6.7 kyr, and the slowest diffusion coefficient, D s2 , all other simulated helium concentrations are within the range of concentrations previously measured in St. Lawrence Lowlands groundwater, from 1.16 × 10 −5 cm 3 STP to 7.75 × 10 −5 cm 3 [ Pinti et al ., ; Vautour et al ., ; Saby et al ., ]. An important implication of the spherical diffusion model is that the fractured aquifer will release 4 He at a higher rate over thousands of years than will the steady state U‐Th 4 He production.…”

“…These values are in the range of those obtained by other authors in neighboring watersheds of the St. Lawrence Lowlands, ranging from 17–6 kyr (Nicolet‐St. François watershed) [ Saby et al ., ] to 14–4 kyr (Monteregie Est watershed) [ Beaudry , ].…”

“…More evolved waters have uncorrected 14 C up to 15 kyr, while NETHPATH 14 C-adjusted equivalent ages are up to 6.7 kyr [Vautour et al, 2015]. These values are in the range of those obtained by other authors in neighboring watersheds of the St. Lawrence Lowlands, ranging from 17-6 kyr (Nicolet-St. Franc¸ois watershed) [Saby et al, 2016] to 14-4 kyr (Monteregie Est watershed) [Beaudry, 2013].…”