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Zero Pressure Drop vs. Time in Monitoring Well. What does it tell me?

Zero Pressure Drop vs. Time in Monitoring Well. What does it tell me?

Zero Pressure Drop vs. Time in Monitoring Well. What does it tell me?

I've always been an air guy so this is new to me.  I'm trying to determine negative pressure for a low flow (10 to 30 cfm) SVE system.  Pilot data show zero pressure drop vs. time in the monitoring well (25 ft from extraction point) except fo an instantaneous drop of 1 in. H2O at the initiation of the test.  The pressure remains 1 in. H20 for the remainder of the 4 hour test.  This seems to tell me that there is little resistance within the radius of influence (sand/gravel matrix under an impervious concrete surface).  Flow rate at the extraction point is 85 cfm.

Essentially, I want to use this data to approximate what the negative pressure will be at the extraction point in the 10 - 30 cfm range so I can properly size my equipment.  I'm considering a natural ventilator if I can convince myself and prove to my client that the negative pressure at the extraction point is negligible.  Thanks for the help.


RE: Zero Pressure Drop vs. Time in Monitoring Well. What does it tell me?

It sounds like the "pilot test" was somewhat limited.  Did you do any step tests?  Do your pilot test data allow to approximate the pneumatic characteristics of your vadose zone?  

The way your post is written, it sounds like you did one constant-rate test during the pilot.  Is that right?

Chapter 4 of the Army Corps' engineering manual for SVE systems is a good place to start for learning about the various tests that should be run in a pilot.


RE: Zero Pressure Drop vs. Time in Monitoring Well. What does it tell me?

Thanks for the help.  I came into this late and I don't have any of that info.  I will though investigate why this additional data was not collected.

RE: Zero Pressure Drop vs. Time in Monitoring Well. What does it tell me?


What are the characteristics of the extraction well (screen length in vadose zone, diameter, vacuum during test)? Pilot tests can help you determine air permeability and thus the radius of influence (which depends on how you define it). The Army Corps manual has provides input on the theoretical ways to determine this.

Doing step tests can tell you how the flow rate at the extraction well affects the vacuums in the extraction well and the monitoring well. Theoretically, you'll eventually reach a limit on your air flow as vacuums increase (if graphed it will look like a log curve).

I've done a lot of pilot tests and calcs before going in the field and every time there is a variable you don't think about until you do it (wet soil, bad screen installation). I've seen a pilot test in soil that theoretically should have had a high radius of influence but didn't and later they found a large void from buried rip rap about 10 feet from the extraction well. The monitoring well was about 30 feet away and didn't read any vacuum not matter how high they increased the flow rate.

RE: Zero Pressure Drop vs. Time in Monitoring Well. What does it tell me?

I'm joining this thread late, sorry. I have done a number of SVE pilot tests and several long-term SVE operations in California and Alberta.  The goal of obtaining data is two -fold:  first, to have it fit into a model such as Paul Johnson's mac-based model (1990) or, what I preferred, the model of Ronald Falta et al. (1996 Groundwater Monitoring and Review; was available from the EPA but I don't know) that takes your DYNAMIC observation well vacuum vs. time data and calculates permeabiities k horiz and k vertical (as long as your screen intervals vary in depth in the test zone) .  You don't need to have vacuums "stabilize," to get sufficient data for the Falta et al. model, which was the old practice of SVE tests.  Paul Johnson's model was the industry standard but requires steady-state vacuum readings.  I recommend getting both programs and comparing results.  Second:  Neither program calculates a "radius of influence."  That value is subjective.  Some write, without sufficient documentation, that it is the distance from the vac well in which vacuum is 1" H2O or greater.  Some have said 0.1" H2O or greater.  Take your pick.  I agree with Ekolog16 above that your screen interval is very, very important.  If it was screened in clay, don't expect anything.  If you screened it in the vadose zone and then found water 6 months later, expect water to come into your machine and have plenty of knock out pot volume (and it's also likely that GW level will be seasonal).   If it was screened in  clayey sand, or sandy clay, it may still not yield any flow.   For tests, I favor 2 foot screen intervals carefully placed in an asymmetric pattern around your vacuuming well.  And as ekolog mentioned, if there are any gravel back-filled areas you know of, they will ruin chances for sensible vacuum readings by providing short-circuit for air flow.  Best to stay more than 100 ft away from back-filled cavities.  Good luck, lad.   

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