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skb505 (Civil/Environmental) (OP)
29 Mar 05 21:33
Does anyone have experience dealing with a perched water table?

Situtation:  Low lying area which is typically "dry".  After significant storm events(several inches of rain), the depression fills with runoff.  The runoff is retained for extensive periods of time (weeks to a couple of months).

With no positive outfall and a permitting nightmare to construct one, I'm looking for a way to increase percolation and decrease the time until the depression returns to a normal state.

I'm thinking their is a confining layer that significantly slows the percolation.  What would be the drawback to punching several "holes" through the confining layer to more quickly draining soils?
MEM1 (Civil/Environmental)
30 Mar 05 9:56
You may want to have NRCS or whatever state agency you are dealing with determine if this is a wetland.  You may not be able to do anything about it.
Helpful Member!  ctmtwilliams (Geotechnical)
30 Mar 05 11:51
First, please allow me to provide the following definition of a perched water table:

"An aquifer in which a ground water body is separated from the main ground water below it by an impermeable layer (which is relatively small laterally) and an unsaturated zone.  Water moving downward through the unsaturated zone will be intercepted and accumulate on top of the lens before it moves laterally to the edge of the lens and seeps downward to the regional water table."

Your surface water doesn’t fit the definition as perched as it isn’t an aquifer.  As mentioned, your problem may potentially be classified as a wetland and there are definite ramifications if the development of wetlands is regulated in your area.  Your local permitting agency should be able to help with this.  Additionally, let me add that perforating an aquitard is commonly frowned upon by most regulatory agencies as aquitards tend to protect drinking water supplies from surface contamination.  Your test holes (as you put it) could potentially be classified as injection wells and constructing/operating injection wells without permits could lead to major legal problems.

Now on a more positive side; your proposed solution of draining through the underlying aquitard is an interesting one.  Much of your proposed solution would depend upon the depth to the aquitard and its thickness, as well as the ability of the underlying strata to accept the water.  If the aquitard is shallow and thin enough, you may be able to build a drain system using trenches.  A test boring logged by a geologist would provide the information you are looking for.  
mitchell54 (Civil/Environmental)
31 Mar 05 15:38
ctmt is right on track, local agencies are your best resource.  One word of caution for you before you consider punching holes.  Sometimes in the most unlikely of places there are natual springs that exist.  I dealt with a land parcel that behaved like the one you have mentioned and made the mistake of digging a test pit that was only three feet deep.  Although I could have tiled off the parcel and diverted the water to an outlying area I found it easier to move the location of the home and incorporate a pond.
skb505 (Civil/Environmental) (OP)
1 Apr 05 23:41
Thanks for the input.  CTMT thanks for the correction, poor choice of terms on my part.  I don't think the area is a wetland by definition in that it is an existing "permitted" stormwater pond.  In researching the old files, it appears that the original facility was designed much larger, but not constructed to the permit requirements.

mitchell54, your past trouble is one thing that concerns me.  This area is significantly lower than the surrounding terrain and I think could potentially have the problem you suggest.

If I get a drill rig out to punch a deep hole, what's the likelihood that just in testing, I create a bigger problem than what already exists?

Again, thanks for everyone's input.
ctmtwilliams (Geotechnical)
2 Apr 05 19:30
A properly abandoned bore hole is typically backfilled with bentonite chips which seal the boring.  If for some reason the aquifer is under pressure (and flows to surface)the boring might need to be pressure grouted.  If you have a geologist on site logging the hole he/she is qualified (and would be legally responsible) for making a decision on the proper technique for abandonment of the boring should a problem arise. The local driller also should have experience with these localized problems.
greenone (Civil/Environmental)
7 Apr 05 13:26
Just one more caveat-you may need a permit from a local regulatory agency for your boring.  Here in CA, it varies by county.
queque (Civil/Environmental)
14 Apr 05 10:16
The detention basins approved in the Phoenix-Chandler area of Arizona allow runoff to be pumped into a canal and allowed to flow out of the  basin. It seems like such  a simple solution until you realize that you are moving a problem from point A to point B. This does not sit well with the receiving end folks or entities. No matter where water is temporarily stored, it normally has contaminants from the parking lots, cattle feed pens, etc. Any attempt to compound the problem by percolation to the groundwater system is frowned upon.

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