Sizing an underground disposal field in silty sands

[SharkswithLasers]

A contractor is proposing to dispose of 250 GPM of pumped groundwater to an underground infiltration disposal gallery (piping to gravel bed) below frost line in the vadose zone (4 foot frost depth, 30 feet to water table, fairly uniform silty sands below 4 foot depth). The pumpout will occur over a long period of time (greater than five years).

The size of the disposal field/infiltration gallery recommended by the contractor is much less than I expected. Using an estimated long term soil acceptance rate, I estimated aproximately 4 acres needed (based on permeability 10E-4) and hydraulic loading (250 gal/min). The contractor (who has much experience with these matters) calculates 1/2 acre needed.

The main difference appears to be in the long-term soil acceptance rate or permeability and the way the soil will actually accept the water. We have no field tests yet, but we will be conducting some soon. We will use actual field test data to determine if proposed size is acceptable.

Has anyone designed/sized a similar underground disposal field? What are the equations and guidelines for such a system? Examples? I think I am missing something here because 4 acres is so much diffeent than 1/2 acre.


Any help you can offer is greatly appreciated.

 

[BigH]

I'd suggest that you set up a trial and do a bit of site testing - this is a very large difference. What do you get if you use 5x10-3cm/sec?? I take it your coef of perm quote was in cm/sec. I don't think that I would be cheap on this one - if you undersize, there may be consequences.

 

[Focht3]

I've done a similar system - but only for short-term storm water, not 250 gpm of steady flow for 5 years. My site was in southern California; over 70 feet of dry silty fine sand with a few gravel lenses - a dry well was the perfect answer for this site. But 250 gpm for 5 years - that's a lot of water! That's 657 million gallons - wow!

Where is the site? What's the natural groundwater level's seasonal range over a 100 year period, particularly the highest seasonal elevation based on the site soil conditions and/or historical records? Have you completed a hydrogeologic survey yet?

BigH is right - don't go cheap on this one. And remember that groundwater will mound around the injection point over time, which can drastically affect the efficiency of your design.

Have you done any three-dimensional modeling with MODFLOW or a similar program?



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[kmerl]

This is a drastic difference...

It sounds to me that the contractor may be using the gravel parameters instead of the soil parameters. Its worth a check...

Just some things to consider:

Is the groundwater being pumped in close proximity of the disposal field?

Not only is acceptance over long term effected by the permeability, but also the current groundwater potential under the site.

Another way to decrease footprint is to increase vertical area...

You indicate a gravel and pipe system, Focht3 eluded to using a dry well. These can increase infiltration surface area without increasing footprints.

Keithe J. Merl

 

[cdh61]

SharkswithLasers,

I suspect the contractor is using a 1D assessment, Darcy's. If you do a quick check, 1/2 acre, i=8, k= 10-4 cm/sec, and consider 15%(+/-) loss around perimeter to account for 3D, then 1/2 acre is what you get. 1/2 acre in plan could work, but not without completing a detailed field investigation and modeling as stated by others. There is to much risk, and the engineering must be completed to ensure contractors assumptions or proposed design will work.

Regards

 

[Focht3]

The real downfall with a simplified 1D analysis is the choice of i; while i = &[ignore]infin[/ignore]; may be approached when the project starts the value may be closer to i=1 (or less) after some period of time, or under certain weather conditions. Use of a 3D model - with realistic assumptions - can reduce the risk of missing the boat, so to speak.



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[cdh61]

I agree Focht3.

I am however interested in knowing how the contractor completed their assesment without the field work being completed, they must have data or completed some analysis, most likely a simplified quick assessment. Which is most likely not suitable for the application.

SharkswithLasers, could you or did you ask the contractor how they completed their assesssment to obtain 1/2 acre? It would be helpfull to assess the situation (1/2 acre verses 4 acres).

Regards

 

[SharkswithLasers]

Thank you, all, for the comments.

BigH - yes, cm/sec are the permeability units in my post.

Focht3/cdh61 - The site is in a northern climate (MN) with silty sands from 2 - 15 feet BGS. Below that is cleaner sands (based on previous soil borings conducted at the site).

A field investigation (e.g., saturated test hole perc pits) will be done to determine saturated infiltration rates at 4-foot BGS. I like the idea of dry well(s) in smaller area footprint to get rid of more water. FYI - the disposal field will not be in the extraction area. I have not done any MODFLOW or similar computer mdeling.

The contractor based his numbers on his familiarity with a similar site with similar conditions. That site pumped 500 gpm and had same dispsoal area (150 foot by 150 foot). Contractor thinks same disposal area for 1/2 the flow is being conservative (and maybe it is if we first verify with field tests).

A quick google search before I posted here provided me the MMR LF-1 site that has a 750 gpm extraction system (4 wells) with one disposal field 1/2 acre in size (I am not yet aware of the subsurface conditions at the LF-1 site). I'm in the process of obtaining the well field design report (which will hopefully shed some additional light for me on the subject of underground disposal).

Thanks again for all your comments. Even after I review the results of the field investigation, I still may consider the final design of the disposal field a candidate for a performance spec...

 

[BigH]

Thanks for confirming cm/s. I put that out - since traditionally geotechs used cm/s (about the only unit we did use in "metric" before we became metricfried. Now I find many using m/s instead. Ouch!

 

[Focht3]

I like performance specs - but can you keep the contractor on the hook for more than one year?



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