11 June 2020

Ground Water Movement, Permeability, and Stratigraphy

One of the most common requests we're asked by environmental and engineering professionals is to decipher vague boring log relationships in order correlate subsurface units.  

People bring those questions to us because the topic of boring log correlations is the one of the original reasons for the formation of Midwest GeoSciences Group, over 15 years ago.

And a common thread between most questions is to understand ground water movement, typically asking to help place monitoring wells at the propert depth.  Most of the initial observations that are reported to us are:
  • Sometimes open borings are dry
  • Sometimes coarse-grained sediments are absent, but the boring is full of water
  • Sometimes buried sands are present, sometimes not.
  • Sometimes buried sands seem randomly present within and between boreholes.
  • Sometimes borings aren't sampled, but only their secondary properties are measured:
    • Cone Penetrometer Testing
    • Geophysical Logging
    • Hydraulic Profile Tool
    • HRSC tooling
    • Soil Classfications
    • Chemistry
    • Water Levels
  • Never does Alexa know the geology, LOL

We as hydrogeologists can relate to the some basic geologic explanations to the observations above:

  1. Water table is not always measured by the water level in open boreholes, especially shallow borings drilled into low-permeability formataions.
  2. Ground water can move horizontally in some low-permeability formations through oxidized zones dependent on the landscape and other subsurface conditions, sometimes flooding a boring where there's no coarse-grained sediments.
  3. Buried sands never occur randomly.
  4. Geologic content is the key to deciphering ground water movement, not permeability.
  5. The SOLE INDEPENDENT VARIABLE is what controls all secondary properties listed above.  

Taking the Mystery Out of the Subsurface (R) isn't easy. It requires both the willingness to get the training and apply it in the field and the ability to make keen observation skills and decipher the geology.  

Midwest GeoSciences Group was formed to help professionals basically improve their/our boring logs.   But preparing a boring log that is complete and accurate is like the pole vaulter going over the cross bar - it's the end result.    

There are steps that are required for success in both.  For the pole vault, there is a coach and practice not to mention the mechanics of the approach, the plant, the penetration, and getting into the position to fly over the cross bar.  For boring logs, there is a teacher and practice not to mention the mechanics of logging: sampling method, core management, core examination, understanding depositional processes and applying them to characterize the Sole Independent Variable.  

Neither good geologists and world-record pole vaulters happen by accident.  

Danny Korth, PG, GeoMan Extraordinaire

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