Our allotment has no piped water, but plenty of water running in ditches on
the sloping site.
These never run dry as we are on a natural wetland with a clay subsoil.
However the sunken baths used to store water regularly silt up with a fine
almost black substance.
This is removed from the baths and piles at the sides.
After drying out it forms a fine (almost powder) soil.

Does anyone know what it might be.
Thanks
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