The role of buried bedrock valleys on the development of karstic aquifers in flat-lying carbonate bedrock: insights from Guelph, Ontario, Canada

摘要

A spatial relationship between high capacity municipal production wells (>5,000 m3/day), completed in a deep bedrock aquifer, and a buried bedrock valley was recognized in the city of Guelph, southwestern Ontario, Canada. Most production wells are completed in a discrete zone, ～60 m below ground surface, within flat-lying dolostones of the Silurian Amabel Formation. Thick overburden and limited subsurface data make characterization of the karstic aquifer difficult. This study integrates hydrogeologic data with models of karst formation, deriving a conceptual model of porosity development as it relates to valley incision. Bedrock valley incision likely occurred prior to the early Wisconsinan age (>60–75 ka). Incision created steep hydraulic gradients within the flat-lying bedrock, and provided the driving force required to integrate regional groundwater flow into karst conduits that drained at the base of the valley. Dissolution in production zone dolostones was favoured over dissolution in shallower bedrock due to abundant bedding plane partings and fossiliferous facies with high intercrystalline porosity. Burial of the valley during subsequent ice advances reduced the valley’s hydraulic influence and the efficacy of the flow system to cause dissolution. The high capacity municipal wells near the buried bedrock valley tap into the now dormant karst aquifer system.