Here is something you could investigate as a possible cause. Check your
cooling water and see if it has been treated with a commercial algaecide.
Some of the products used to retard algae build up in cooling water contain
copper and zinc which could be the cause of elevated levels. Good luck.
>Currently, I am working with a silk screening plant which uses water
>based inks and no solvents during clean up, yet they are discharging
>excess Zinc and Copper into waste water. The Copper is most likely from
>the pigments, but it is a mystery as to the source of the Zinc. The
>majority of the water discharged is used in cooling. I am seeking any
>suggestions as to possible sources of Zinc in fabric printing operations.