Investigating Well Connectivity and Reactive Surface Area in a Sandstone Bedrock Using Ionic Tracers

Pump and treat remediation of bedrock groundwater systems is often impacted by an uneven sweep of the formation between injection and pumping wells.  Poor sweep efficiency may lead short circuiting of flow and treatment or, in the opposite extreme, poor water and reagent circulation in the impacted fractures.  We describe a field approach to measuring sweep efficiency using multiple ionic tracers.  Non-reactive anionic tracers are used to trace the water while reactive cationic tracers are used to probe the fracture surface.  Assuming the cation exchange capacity of the fracture surface is relatively constant, separation of cation breakthrough curves is a function primarily of available surface area.  We present results from a series of ionic tracer experiments conducted in a single bedding plane fracture in a low-matrix-porosity sandstone formation.  Tracer breakthrough was collected among five wells offset by 7 to 14 m.  Breakthrough was interpreted using numerical transport modeling that accounts for cation exchange.  We compare the tracer estimates swept surface area with estimates obtained from ground penetrating radar images of saline solution circulated between the same well pairs.  We find that, even though tests were performed in a single fracture, swept fracture surface area is highly variable among well pairs.