A municipal wastewater plant (WWP) has biosolids with a zinc concentration above the biosolids criterion for zinc. The WWP found zinc concentrations in sediments contained in a fruit packing plant that discharges its wastewater for treatment. The plant's wastewater settling pit sediments measured at 1500mg/Kg. A water analysis yielded 162 ug/L. Another set of analysis of pit samples found 71.5 mg/L in a liquid/soild sample. The plant swears they do not use zinc containing chemicals at the packing house.
Are there any zinc containing compounds that are used in the orchards or packing house that the packing house would not be aware of. The high zinc levels in the biosolids are costly to dispose of at a landfill.
Zinc dithiocarbamate is sometimes used as a fungicide in tree fruit production. I suggest you examine production records for fruit supplied to the Packing House.
This is a very good suggestion, John!
In case, this cannot be confirmed, another possibility is the use of Zn-containing paints to prevent corrosion of metal constructions. I heard from high Zn concentrations in pork originating from stables where this type of paint was used.
Zinc oxide is sometimes sprayed in orchards at high concentrations to whitewash the fruit for sun protection. Foliar sprays of fertilizers containing zinc salts (such as zinc nitrate) are a common practice in apple orchards in some regions. The high zinc concentration in the wastewater suggests you should not be looking for the source of contamination in pesticides and fungicides, which are typically applied at rather low doses.
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