HYDROLOGICAL INVESTIGATIONS IN KARST AND FRACTURED ROCKS: SIGNS OF PROGRESS

What is “conventional hydrogeology”? It is whatever the site or setting presents, but in karst and fractured rocks there are challenges. What it is not is an ideal porous medium or equivalent porous medium. Those are conceptual models that can not exist in reality. It is also not simply the profile of a cave system between recharge areas with sinking streams and a discharge area with springs. However, using a combination of techniques progress can be made to show how to properly investigate such sites.

Quinlan et al., (1996) state that karst aquifers involve triple porosity: conduit (channel), fracture, and matrix. There are several definitions of a karst aquifer (Worthington et al., 2017). What makes it difficult is that karst is not isolated to one lithology and all the five major bedrock aquifers have similar characteristics and triple porosity (Worthington et al., 2016). Worthington (2009) in the Mammoth Cave Region modeled hydraulic conductivity in basins, delineated by tracing, and shows that when proceeding downgradient in a basin, hydraulic gradient flattens and hydraulic conductivity increases.

Davies (2008) and Worthington et al., (2000a, b) show that in any carbonate aquifer of any geological age > 94% of the groundwater flux is in conduits (channels). None is close to an ideal porous medium. Quinlan et al., (1997) defined conduit size (channels) by assuming a minimum traced conduit velocity (from >3,000 test in >40 countries) and a calculated a minimum size of channel or conduit that would permit turbulent flow (= a few mm). One doesn’t need caves to rapidly transport contaminants long distances.

Quinlan et al., (1995) show that in traced, delineated basins there is a relationship between basin area and discharge volume - normalized base flow. In addition discharge is via overflow and base-flow springs, and at lowest stage the entire basin can be discharging via a single base-flow spring.

In NW Florida Hazlett et al., (2004) show a numerical model can perform well: Cave divers physically traversed and mapped long conduit pathways. Quantitative tracing was done using fluorescent dyes with sampling at wells in conduits, open sinks and springs. Pathways were modeled. Natural uranium was also used as a tracer with similar results (Davies, 2008).