Best flooring for damp springhouse?

I have a springhouse sitting atop a mostly buried, 14' X 14' open-top concrete cistern. The plywood flooring is rotted and needs to be replaced. Though the cistern has some vents below the floor level, the underside of the floor is essentially a high-humidity environment. What is the best material or construction to use for this floor? Things I have considered:
1) pressure-treated plywood (rejected, since this sits just over our drinking water),
2) Marine-grade plywood (very expensive)
3) exterior-grade plywood, installed over a vapor barrier (6 mil poly sheeting?)
What will work best?
Thanks,
Kelly
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On Fri, 19 May 2006 20:17:08 +0000 (UTC), snipped-for-privacy@ptdcs2.intel.com (Kelly E Jones) wrote:

Concrete planks.
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Never heard of them before, but they sound promising. But, I imagine they weigh significantly more than plywood. WOuldn't I need to beef up the joists, as well?
Kelly
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They ARE the joists. I was talking about those 2' wide, 8" deep, hollow-core pre-stressed concrete planks that come in lengths up to 40' It would be a stupidly expensive way to solve the problem, but it would be permanantly solved.
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Kelly E Jones wrote:

2x deck boards? If you don't want the spaces then how about tongue and grove planks? They are available in Mahogany, fir and pine. You could maybe even route some PT pine decking to fit together T&G.
Mahogany T&G is not too expensive around here (relatively speaking) and is used on exterior porches. (Think Victorians, etc)
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