Investigation of Winter Recharge in Vulnerable Bedrock Aquifers Under Climate Change

Abstract

Many communities in cold regions depend on groundwater resources from shallow bedrock aquifers with thin overlying soil (0–5 m), yet these aquifers are vulnerable to surface contaminants and climate change impacts. The objective of this research was to investigate winter recharge in vulnerable bedrock aquifers for an improved understanding of how changing climate conditions may impact groundwater resources. In eastern Ontario, spatiotemporal sampling of stable water isotopes (δ2H and δ18O) from multi-level wells was conducted to improve the conceptual model of snowmelt recharge, groundwater flow/transport, and stable isotope attenuation in sedimentary and crystalline rock aquifers. This data was then combined with a regional southern Ontario dataset where statistical analyses aided in identifying the key drivers of groundwater recharge, and the limitations of isoscapes, in vulnerable bedrock aquifers. Lastly, surface-subsurface cryo-hydrogeologic conditions of a bedrock outcrop were monitored over an unseasonably warm and rainy winter to determine the mechanisms and conditions for mid-winter infiltration and recharge. Temporal tracing of the isotopic snowmelt signature with depth was critical to revealing how different bedrock aquifers with thin soil are vulnerable to changing snowmelt patterns, extreme drought and flooding, and surface contaminants. Winter rain and snowmelt were identified as dominant sources of recharge suggesting that changes to winter precipitation will have a greater impact on bedrock aquifer resources than changes to other seasonal precipitation. Despite frozen ground, mid-winter rainfall and snowmelt generated rapid recharge due to lateral infiltration along the unfrozen soil-rock interface. Results indicated that under warmer winter conditions winter recharge may increase on/near bedrock outcrops, but it can also be limited by the freezing of ponded surface water from intense mid-winter rainfall. The novel datasets and knowledge generated from this work have improved our understanding of vulnerable fractured rock systems in cold regions and can aid in managing and predicting impacts to these water resources under climate change.