Saturday, April 9, 2011

PSSST..! It Could be the PST

PST stands for Practical Saturated Thickness - which differs from true saturated thickness.  It actually is pretty important in aquifers or aquifer sections that display a wide variability in storage coefficient and transmissivity.

According to the Kansas Geological Survey, who is doing a study on behalf of the Southwest Kansas GMD 3:
The PST considers only the net thickness of saturated sediments that significantly contribute to well yield from the water table down to the bedrock surface and differs from the saturated thickness (ST), which is the total thickness of saturated sediments between the water table and the bedrock surface. Thus, PST provides a more accurate picture of water availability and may also provide insight into future water-level trends at the scale of an individual well.
You may have 300 feet of saturated thickness in your well, but if half of that is comprised of tight sands, clays or other non transmitting materials, you're in for a surprise down the road.  SW GMD 3 is conducting a pilot study in the two areas shown in the attached graphic.

Drilling logs were used to estimate PST.  The logs used in this study are only from wells drilled into the bedrock surface. 207 logs were used for the Four Corners study area and 48 logs were used in the SE Stevens County area.  By evaluating the driller's logs and assigning aquifer parameters to the sections, it was determined that the pre-development PST fraction of ST was approximately 58% for both study areas.  The fact that only 58% of the saturated aquifer is contributing to well yields will likely impact future management decisions.

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