PREFERENTIAL GROUND-WATER FLOW: EVIDENCE FROM DECADES OF FLUORESCENT DYE-TRACING

Hydraulic conductivity depends on both the primary porosity of the aquifer matrix (particle size) and secondary porosity caused by structural features (joints, faults, solution cavities (karst), etc). Despite the paradigm found in most classical hydrogeology textbooks that ground-water flow is predominantly through porous media, travel times based on these assumptions often do not reflect those measured in the real world using ground-water tracers. For decades we have tested the hypothesis that fast-flowing, preferential ground-water flow paths through secondary porosity features are the dominant process for conducting water through aquifers, and that Darcy's law based on a granular porous media poorly predicts travel times with precision in the “real world”.

Numerous fluorescent dye-tracing (FDT) tests show that the ground-water seepage velocities in many different types of unconsolidated sediments and fractured rocks can be orders of magnitude faster than calculated standard measurements of hydraulic properties and hydraulic gradients. For example, the measured average linear velocities from fluorescent dye-tracing tests in a Triassic anhydrite/dolomite aquifer, a heavily-oil contaminated fluvial aquifer, a fractured bedrock aquifer, and a clayey unconfined aquifer were 20 to 110,000 times greater than the velocities calculated from routine hydraulic measurements.

FDTs also commonly show that groundwater flow directions deviate by refraction in the horizontal plane from the maximum direction of the hydraulic gradient. This refracted groundwater, if not recognized, may be the cause of why so many remediation strategies fail. Most FDTs confirm the presence of fast-flowing conduits and demonstrated the importance of preferential pathways at the project sites.

Our findings suggest that most conceptual hydrogeologic models need to include the possibility of fast-flowing, preferential groundwater flow. These can only be clearly defined using tracing techniques to determine the ground-water flow paths, effective seepage velocities, and accurate permeability estimates.