Single borehole dilution tests using a permeable injection bag and a novel point-injection probe for the hydraulic characterization of karst aquifers

Single borehole dilution tests are a method for characterizing groundwater monitoring wells or boreholes and can either be conducted as uniform injection throughout the entire saturated length or as point injection at one specific depth. By injecting a tracer into a borehole and measuring concentration profiles, flow horizons and possible vertical flow can be identified and quantified. Compared to conventional methods, such as flowmeters, SBDTs are cheaper and require less equipment, but allow important conclusions about wells and aquifers.

Uniform injections deliver information about the entire saturated length and can be conducted using different techniques. The most common one uses a hosepipe filled with tracer solution to obtain a uniform concentration over the entire water column. We present a simplified method using a permeable injection bag to achieve close-to-uniform tracer distribution in the well.

However, uniform injections are not ideally suitable for the investigation of vertical flow. For this purpose, point injections are more appropriate. We introduce a newly developed probe which can be filled with saline solution, lowered into the intended depth and then be opened by a falling weight. Numerous tests have been carried out in the laboratory and several groundwater monitoring wells, to evaluate the simplified method for uniform injections and the new probe for point injections. Results show that with the simplified method, significant and reproducible results can be obtained. The functionality of the new injection probe was also demonstrated. Both techniques represent useful tools for efficient hydraulic characterization of boreholes in karst and other aquifers.