Slowing the rate of absorption of sandy soils

Question

Slowing the rate of absorption of sandy soils

dpa (Civil/Environmental)

(OP)

25 Aug 04 15:41

I have a problem posed by the local county officials.  They are concerned that the soils on a site that is being developed for recycling are too porous and that rain water may pass too quickly into the groundwater for any treatment of contaminants to occur.

1.  What is the best way to figure out if this is a problem?  Perk test? or other?

2.  If this is a problem, how do you slow the rate of absorption?

Thanks,
DPA

bltseattle (Civil/Environmental) · Answer 1 · 2004-08-25 17:17:06

Check if the agency will prescribe the desired range of infiltration rates and what testing methods they use.  If you are were in Washington State, the Department of Ecology has many pages in the stormwater manual addressing this issue, but each state and agency is different.

Two basic approaches to remedy over-rapid percolation:

1.  Provide a lined treatment system (e.g. pond or bioswale) upstream of the infiltration area
2.  Install engineered soil media on top of the native soils that will filter & treat the runoff.  The media material should be designed (gradation and organic content) by a geotech to meet the prescribed rates for water quality treatment.

Example:  ..recently had to specify an 18-inch layer of sandy loam filter material to filter runoff before discharging to underlying native soils that did not have treatment capability.  Another project we used 24-inches of sand, with the understanding that one day it would be a throw away layer to be replaced since it will clog over time.

dpa (Civil/Environmental) · Answer 2 · 2004-08-25 18:24:04

Thanks blt,

Your suggestion to hire a geotech is a good one. I'll contact the owners and let them know.

DPA

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Slowing the rate of absorption of sandy soils