Just need surface soil tested. It would be a good idea to test the clay too, lest you run into a situation like Bob Lusk ran into when they found the perfect clay to line a pond with....

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=313538

Yeah, getting the surface soil tested will give you some idea what the water will be like that is running into your pond.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).