DISCOVERING A KARST CATCHMENT: RAPID, LONG-DISTANCE GROUNDWATER FLOW IN THE CANADIAN ROCKY MOUNTAINS

Extensive deposits of carbonate bedrock host alpine karst aquifers in the Canadian Rockies. Efficient groundwater infiltration and high flow velocities are the outcomes of sparse vegetation and high hydraulic gradients in mountainous terrain. Little research has focused on karstic groundwater flow in the Canadian Rockies owing to a lack of data from these rugged landscapes. The Watridge Karst Spring has highly variable discharge, ranging from 40 to nearly 3,000 L/s. The spring is issuing from a cave on a forested hillside, with no overhead water reservoir or drainage. Yet, water budget calculations suggest that the catchment area is on the order of 25 km². Prior to this study, surface connections and catchment extent were unknown.

Using continuous hydrochemical monitoring and dye tracing methods, the groundwater catchment has been delineated as an elongated area that extends the length of a regional syncline. The effects of rapid snowmelt and rainfall recharge from up to 14 km away are expressed at the Watridge Karst Spring by an increase in discharge followed by a lagged decrease in electrical conductivity (EC). The discharge and EC fluctuate over a diurnal scale as well as a hydrologic event scale (e.g., episodic snowmelt or heavy storm). Groundwater response times are defined here as the lag time between a hydrologic event and a resulting change in the spring hydrochemistry. This timing can be used to approximate celerity in the case of discharge, and velocity in the case of EC. Cross-correlation analysis between input and output signals has been used to constrain groundwater response times between 1 and 4 days. This is supported by dye tracing where groundwater velocities as high as 0.04 m/s have been recorded. An annually reoccurring and sudden change in outflow behaviour is characterized by a decreased recession rate, longer groundwater response times, and increased EC. This shift occurs below a flow rate of approximately 1000 L/s, where the water table drops to a threshold height. Thereupon, a shallow flow path carrying fresh snowmelt is drained and disconnected from a deeper flow network of longer-residing groundwater. This suggests a highly variable hydraulic gradient throughout the snowmelt season. A hypothesized dual flow regime is characterized by fast-flowing conduit flow, and slow-moving fracture flow.