The answer is this:   # ...

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The answer is this:   # Zones where Mn/Fe are in the ground-water (or surface water if possible) x # of water treatment plants in each zone x unknown factor.  The unknown factor is what equipment was installed at each plant to mitigate Mn/Fe residuals, and its efficacy in removal.

Considering that this can remain a problem in certain zones, the unknown factor may be near unity.  Considering that the problem has already been encountered in these zones, and allowing for at least some effective plans on controlling the issue, the unknown factor may be nearer 0.1.