A correctly constructed dam of impermeable clay should not have water filtering through any of it (max 1 ft.) much less 50 ft. Iron is only one example as there can be many differences between rain water runoff and underground water. I would not expect pond water to pick up iron from a dam as the dam clay should not have much soluble iron in it.

Is the dam wet on the non-water (back side) ? If so does this appear to go through the dam to the front side? Is there a core trench in the dam ? Is there water running down the back side of the dam? I can't imagine how a dam could be built with a core directly on top of a spring.