MONITORING BEDROCK SETTINGS AS A POROUS MEDIUM: THE EMPEROR HAS NO CLOTHES

Conceptual models are the fundamental basis for investigations of groundwater systems. The general simplified model - the porous medium was adopted for most hydrogeology despite abundance and obvious evidence that it might not apply in many cases ASTM (1996). Unfortunately it is obvious the model or derivations of it has characteristics unlike any bedrock setting. The EPM (equivalent porous medium) model is often used but is understandably difficult to set up. It’s use typically requires multiple tracers and large data sets to initially choose modeling parameters. This has rarely worked well.

We have a special case in karst, because there are obviously conduits and channels, some of which are extensively mapped. If there are springs, channels must also be present in all bedrock, and may be smaller, but the consequences identical. Most monitoring systems that rely on wells introduce complications. Funds are spent simply monitoring “plumes” outside channels, and not their true extent; this near the source becomes “monitoring natural attenuation.” Flow in conduits and channels is not attenuated. Most of the furthest migrated contaminants are rarely monitored.

In 1856 Henri Darcy wrote a report on obtaining a water supply for the City of Dijon. Every hydrogeologist should read it in its entirety; it is a marvelous piece of work. Most of the text is about springs, channels or conduits in the subsurface. It does not assume a porous medium. Darcy’s engineering feat for Dijon was hampered by the fact that clean spring water got fouled, which is normal in conduits and channels and his 15 km aqueduct system. He knew filtering such water would clean it up but he became concerned about the head loss. He derived an experiment and equation for that. We are sure he did not believe that this filtering work and that equation would form the basis of all flow, transport and modeling. However, this continues, but, we simply cannot persist with using an inappropriate conceptual model for monitoring groundwater pollution and attempting groundwater protection.

The act of monitoring groundwater in bedrock (and other settings) has to use methods such as multiple tracers and avoid inappropriate assumptions. It is time to be realistic.