I'm digging out in an existing depression that is naturally wet most of the year to create a small pond. This is, maybe, 600 feet inland from the ocean.

I've reached the water table and now have the beginnings of a small pond. The initial smell is that of rotten eggs, which I believe comes from bacteria that live in oxygen-deprived mud. But I expect that to change over time as the water becomes oxygenated.

Plants that I have tried -- even plants used to water such as bald cypress -- seem to suffer. I'm trying to come up with an explanation.

Would these plants be adversely affected by anaerobic water (which I expect will change over time)?

Could it be that the water is brackish? Would the water table that far inland still be salty?

I'd appreciate your thoughts in figuring this out.