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Methods to Reduce Infiltration Rates

Methods to Reduce Infiltration Rates

Methods to Reduce Infiltration Rates

Greetings. I'm interested in reducing the natural infiltration rate of sandy soils from several feet per day to say 2 inches per day. The only idea we have come up with is to amend the sandy soils with bentonite at a very low ratio...perhaps only 1% bentonite by dry weight. Our thinking is that with some combination of low bentonite addition and compaction, we can get rates reduced to the desired range.

We are dealing with existing stormwater dry retention areas and wastewater rapid infiltration basins. By design and siting, they currently have high infiltration rates. We would like to convert those basins to wetland treatment systems that discharge to groundwater (no surface outfall). In order to create an appropriate wetland hydroperiod with the available water (either stormwater or reclaimed water), we need to slow down, but not stop the infiltration. Without modification, the water will infiltrate too quickly and we will not be able support the desired wetland plant community. To be clear, we do not want to seal the bottoms of the basins. We've already demonstrated that these infiltrating wetland cells work in areas where soil modification was not needed. What we really want is the ability to engineer a natural sandy soil to give us a specific "approximate" infiltration rate.

Can anyone offer any recommendations or alternatives? Thanks.

RE: Methods to Reduce Infiltration Rates

More background info/specifics would be helpful.

RE: Methods to Reduce Infiltration Rates

Thanks for re-posting. It's not stated why the new design is necessary, but assuming it's regulatory-driven, if the permitting authorities are not on-board with it - ie doesn't meet their standards - our discussion here is moot. It could work for stormwater. I'd be more concerned about using it for wastewater and you'd want to check into compatibility (cation exchange as with a GCL). I'd also be concerned about preventing the roots from penetrating though the bentonite layer over time and degrading it.

RE: Methods to Reduce Infiltration Rates

Whether for stormwater or wastewater, the concept is being applied as a retrofit that provides increased water quality improvement through the various physical, chemical, and biological processes that occur in the wetlands. We just want to slow the water down to allow these processes to be most effective. It is only regulatory driven in the context that it could be implemented by utilities to meet nutrient TMDL requirements. Regulatory authorities are "conceptually" on board....each installation is evaluated on a case-by-case basis through the permitting process. Your concerns regarding cation exchange and root impacts are noted and helpful.

Most of the references we have found regarding bentonite addition are for pond sealing or liner applications where the goal is near-zero infiltration. We've been having difficulty finding references that give useful design guidance for slowing versus stopping downward flow.

Thanks again for your comments.

RE: Methods to Reduce Infiltration Rates

Compaction can help reduce the hydraulic conductivity (permeability) of the soils. Additionally, mixing and compacting the soil with silt or clays can also help. Depending upon the cost to transport and work the soil, will determine your best option (e.g., it might be cheaper to purchase the bentonite than to purchase & haul soil from offsite.)

RE: Methods to Reduce Infiltration Rates

irrespective of the infiltration rate, there has to be some consideration of the water table elevation. If the water table is at some near-surface depth, and if the pervious soils are of limited thickness, the rerouting of water into the newly-formed depression will result in some new flux to the water table recharge. This returns some mound. The development of a mound results in horizontal flow. To exaggerate the mounding effect, you can install a horizontal hydraulic barrier and model that affect.

You can also do blending as suggested. I don't think recompaction would return too great a value, but there is certainly some benefit. When considering the affect of recompaction, target compaction water contents at or greater than 90 percent saturation. I'm not saying, "Wet of optimum." As optimum water content for 95 percent compaction is not the same as optimum water content for 95 percent compaction. Refer to the Mitchell, Hooper and Campanerlla, ASCE 1965 for more information on permeability and compaction water content.


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