TRACER POTENTIAL OF SR ISOTOPES IN GROUNDWATER FED PRAIRIE SLOUGHS

Strontium isotopes have tremendous potential as tracers of solute and water sources in surface hydrology, and water-rock interactions in groundwater hydrology, but the number of case studies lags behind H and O isotopes in the water molecule. To advance our understanding of the basic systematics, we measured ⁸⁷Sr/⁸⁶Sr ratios in dissolved strontium from a number of groundwater fed sloughs and springs near Biggar, Saskatchewan, including Lydden Lake, a closed basin Na₂SO₄ slough with seasonal salinities as high as 300,000 mg/L, and a permanent salt bed of up to 1.5 metres thickness. All the sloughs rest upon a thick glacial till sequence. Uniform ⁸⁷Sr/⁸⁶Sr ratios in these lakes sampled over multiple years indicates that they are well mixed. Of the 9 springs or seeps discharging groundwater to Lydden Lake, three have ⁸⁷Sr/⁸⁶Sr ratios lower than the lake from 0.7089 to 0.7092 (Group 1), and 6 are more radiogenic than the lake from 0.7103 to 0.7108 (Group 2). Strontium material balance calculations suggest that up to 70% of the solutes in Lydden Lake (⁸⁷Sr/⁸⁶Sr=0.7095) are derived from Group 1 springs. Group 1 springs also have more negative dD values of -160 permil (SMOW), typical of deeper, older groundwaters presumably recharged under a colder climate than at present, whereas many group 2 waters have dD values less than -145 permil suggestive of younger, shallower groundwaters, having been recharged locally.

The most interesting discovery is a decreasing trend of groundwater ⁸⁷Sr/⁸⁶Sr ratio with depth below the surface in the glacial till, with the lowest ratios (0.7065) occurring in groundwaters of the Cretaceous Judith River Formation, situated just below the till sequence at about 150m depth. The Judith River aquifer has sufficient hydraulic head to drive groundwater close to the surface. Thus we interpret the groundwater ⁸⁷Sr/⁸⁶Sr depth profile as a consequence of strontium exchange between upwardly advecting, relatively nonradiogenic, Judith River groundwater, and the more radiogenic Sr of the glacial till. Shallow groundwaters that are recharged locally acquire Sr only from the till, which explains why Group 2 groundwaters are more radiogenic than Group 1 groundwaters.