MEASURING HYDRAULIC CONNECTION IN FRACTURED BEDROCK WITH PERIODIC HYDRAULIC TESTS

Remediation of groundwater in bedrock often hinges on creating sufficient fluid circulation between treatment wells. Because of the close proximity of treatment wells, constant rate pumping tests are not well suited for measuring the hydraulic connection between two wells. Tracers are more effective for determining flow connections, but are expensive and time consuming. We explore the use of periodic (oscillatory) hydraulic testing for measuring hydraulic connectivity between wells in saturated fractured bedrock. In a two-well periodic test, head is varied periodically in one well and the head response observed in another. The head amplitude attenuation and phase shift between the induced and observed wells is indicative of transmissivity and storativity of the formation. In a radial configuration, the depth of penetration is a function of the square root of the period of oscillation, so the radius of influence can be “tuned” to the connection of interest. This allows the hydrogeologist to interrogate different portions of the fracture network using the same well pair. We will present a field example of this approach in a fractured crystalline rock groundwater system. One well underwent periodic injection and pumping while five observation wells 30 to 47 m away were monitored. Head response in the monitoring wells were highly variable and were consistent with the understanding of hydraulic connectivity based upon previous hydraulic and tracer tests. In particular, we will compare periodic responses to a previous constant rate pump test to highlight the additional information produced from the periodic tests.