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permaculture - [permaculture] belated bentonite pond answer (7-26-04)

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  • From: "Albert Postema" <postema@nwlink.com>
  • To: <permaculture@lists.ibiblio.org>
  • Subject: [permaculture] belated bentonite pond answer (7-26-04)
  • Date: Mon, 20 Dec 2004 12:54:39 -0800

We are reclaiming an old pond which is full of alders and will need
something to plug up the bottom of the pond. Our neighbors' ponds slowly
drain throughout the summer so we probably need something to seal the bottom
and would like to avoid having to buy a huge piece of plastic (future
garbage). Does anyone have any experience with or information about sealing
large ponds with bentonite? If bentonite is a good way to go does anyone
have any leads on where I might find some in western Washington?

Thanks!

Anne Erlandson

Hello Anne,
In western washington there is typically a glacial till layer underlying most
soils in western washington. This layer is what we will dig down into and use
for lining the bottoms of ponds. The clay content (30-60%) will provide a
sealing layer. The thickness we use is 12 -24 inches dependant on the clay
content of the till onsite. Bentonite has 100% sealing but as you may know,
is expensive for use in large quantities. Let alone sourcing issues. Lighter
clay content glacial till does have some infiltration 2-5%. This information
is based on the assumption of equipment access and a >300sq ft pond size. For
small ponds and hand labor this method might be prohibitive. ie: galcial
till/hardpan is not easy to dig out in large quantities by hand and depth can
be an issue.

Typical procedure for pond/chanampas/wetlands construction. Dig test holes
(posthole) to determine till depth and clay content. Schedule equipment.
Remove topsoil, sub soil from pond area plus 2-3 feet and seperate/stockpile,
place in new location site dependant. Excavate till to required depth and
quantity, shape pond, place till material, compact. Build overflows, always
design for 100yr storm events so your work doesnt end up down the hill. Enjoy.

Albert Postema Director Earthwise Excavation Snohomish WA




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