Active thermal tracer tests for improved hydrostratigraphic characterization.

Subsurface heterogeneity in hydraulic properties and processes is a fundamental challenge in hydrogeology. We have developed an improved method of borehole dilution testing for hydrostratigraphic characterization, in which distributed temperature sensing (DTS) is used to monitor advective heat movement. DTS offers many advantages over conventional technologies including response times in the order of seconds rather than minutes, the ability to profile temperature synoptically in a well without disturbing the fluid column, sensitivity to a wider range of flow rates than conventional spinner and heat pulse flow meters, and the ease of interpretation. Open-well thermal dilution tests in two multiaquifer wells near Madison, Wisconsin, provided detailed information on the borehole flow regimes, including flow rates and the locations of inflows from both fractures and porous media. The results led to an enhanced understanding of flow in a hydrostratigraphic unit previously conceptualized as homogenous and isotropic.