It's been awhile since I read it, but Pub 590 has a section about collar location. We have a fairly large dam mass with a 300+ ft long 6" PVC drain pipe. We placed the first one just below the top toe of the dam (where the flat top breaks into the slope down to the pond) all the way down to the drain. We placed the second one 20' behind it, further downstream.
So far, so good.
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edit; welcome to PB forum, Paul O!

Last edited by Brettski; 10/11/07 05:56 PM. Reason: lousy manners