HOW DO WE ESTIMATE HYDROGEOLOGIC PROCESSES GIVEN MAPS OF SUBSURFACE GEOLOGIC PROPERTIES?

In hydrogeology, we often have a limited view of the subsurface based on what has been characterized by well data. The physical template on which the movement of water occurs cannot be characterized by such limited data, so hydrogeologists frequently struggle to tell the story of complex processes in the subsurface. Geologic mapping may provide one of the best ways of interpolating the limited data we have, depending on the scale of the hydrologic processes of interest and the complexity of the underlying geology. While the understanding of hillslope-scale hydrology may not require detailed geologic mapping, larger-scale systems, from watersheds to continents, require data that we can’t easy get from standard hydrologic measurements, which tend to have support volumes at the point scale (like concentrations in a well) or at some aggregated scale (like stream discharge), neither of which may be the right scale for the needed hydrologic model. Airborne- or satellite-based platforms, with the exception of some geophysical methods, rarely see below the top few centimeters, which is almost useless to quantifying groundwater-related processes. Consequently, geologic mapping—as well as soil and plant mapping—can be important to providing information at a scale useful in large-scale hydrogeologic models, and perhaps in smaller-scale models of complex geologies such as fractures or karst, where “enough” data never exist.

Even with these data, the question from hydro(geo)logists often is: how do I use these geologic maps to estimate hydrogeologic processes in the subsurface? What relationships exist between geology and hydraulic conductivity, the great unknown property in hydrogeology?