A method and system using the measured adsorption of a tracer substance leached from a flexible liner sealing a subterranean borehole to indicate the flow of fluid, typically water, past the borehole. The liner is provided with contaminant collectors. The liner is impregnated in its fabrication with a tracer substance. The tracer is leached from the liner as water flows past the liner in/from fractures in the surrounding geologic formation. The water containing any leached tracer may also flow past the collectors. The collectors adsorb the tracer relative to the amount of tracer leached from the liner and into the passing water, and the concentration of tracer in the flowing water is proportional to the amount of toluene leached from the liner. The tracer level in a collector is tested as an indication of the flow past the liner.