INFLUENCES OF A CHANGING CLIMATE ON THE SEASONAL VARIABILITY OF STABLE ISOTOPES AND RECHARGE: SPATIOTEMPORAL MONITORING OF MULTI-LEVEL BEDROCK WELLS

Droughts, floods and erratic snowmelt patterns are elements of climate change that have significant implications on groundwater recharge, especially in temperate climates where snowmelt can be the dominant source of recharge. Bedrock aquifers are highly prevalent in much of the world, including Canada and the United States and are particularly susceptible to extreme climate conditions due to the low storativity of the fracture network which governs groundwater flow. In 2016, severe drought conditions occurred throughout south-eastern Ontario, Canada, followed by extreme flooding in 2017. Snowmelt patterns consisted of one major and one minor event in 2017 and two to three major melt events in 2018. To explore the impacts of these extreme climate events on multiple bedrock aquifers, δ²H and δ¹⁸O were sampled at three geologically-distinct field sites in this region over the period of February 2017 – August 2018. Multi-level wells were constructed in Ordovician limestone, a mixed sandstone/Precambrian syenite setting and a gneissic formation, all overlain with thin (< 8 m) to no overburden. Results of constant head injection tests at 2-m intervals were used to identify and isolate high transmissivity zones (2–3) for well construction. Eight multi-level wells, totalling 22 piezometer intervals ranging in depths between 10 m and 55 m were sampled. Deuterium and oxygen-18 was measured in rain (n = 66), snow (n = 62), soil water (n = 117) and groundwater (n = 573) at biweekly to monthly intervals. The analysis was conducted using a ring-down mass spectrometer with an error of ±1‰ for δ²H and ±0.15‰ for δ¹⁸O. Initial results indicate pronounced isotopic enrichment of groundwater following the 2016 drought in unconfined crystalline formations, but less so in carbonate and sedimentary rock types where storage is higher. The enriched trend extends to deep crystalline intervals where connectivity exists, suggesting relatively rapid sensitivity to seasonal climatic extremes in low porosity environments. In some cases, depleted isotope values (i.e. recharge) in response to snowmelt were more pronounced when multiple melt events occurred. These results imply that erratic snowmelt patterns may aid in recharging bedrock aquifers, particularly where frozen overburden does not inhibit infiltration.