A zinc concentration in the range of 2-3 mg/L will cause toxicity in plants if used regularly. General hydroponics recommendation for continuous feeding is in the range of 0.05 - 0.1 mg Zn /L. Typical guidelines for zinc content in irrigation waters is a max of 1 mg/l to soils with pH less the 6.0 (acidity affects availability of zinc to plants), to 5mg/l with pH >7.0 ((ref:
http://www.env.gov.bc.ca/wat/wq/BCguide ... /zinc.html ) 1000ug/l = 1mg/l))
It is not uncommon for zinc levels in rain water tanks connected to zinc gal roofs to be in the range of 3-11 mg/l ( ref
My reference
Lead and other heavy metals: common contaminants of rainwater
tanks in Melbourne
M.I. Magyar1, V.G. Mitchell1, A.R. Ladson1, C. Diaper2
1Monash University, Institute for Sustainable Water Resources, Department of Civil
Engineering, Victoria 3800, Australia
2CSIRO Integrated Urban Water Systems, Graham Road, Highett, Victoria 3190, Australia
ISBN 0-858-25735-1 © 2008 publication: Water Down Under 2008
The general toxicity to humans in is in the order of hundreds of mg/L, though too much will make you sick.
It is quite possible for the zinc concentration from galvanized roofs, and reticulated water systems using galvanized iron to develop a level that is toxic to plants, yet have no effect to humans. Once again, "if you can drink the water, then it is OK for your plants" is a complete myth.
It is better to use colorbond iron ware.
The cans used for food storage are lined with tin
not zinc, this can corrode and can cause illness in people. Don't eat any food from a can that has rust in it or is bloated or swollen.
This is from wikipedia, so its not the best of references, but it will do. Type in "tin can" for a history of the tin can.
Dissolution of the tin coating into the food
Although tin is corrosion resistant, acidic food like fruits and vegetables can cause corrosion of the tin layer. Nausea, vomiting, and diarrhea have been reported after ingesting canned food containing 200 mg/kg of tin.[9] A 2002 study showed that 99.5% of 1200 tested cans contained below the UK regulatory limit of 200 mg/kg of tin, an improvement over most previous studies largely attributed to the increased use of fully lacquered cans for acidic foods, and concluded that the results do not raise any long term food safety concerns for consumers. The two non-compliant products were voluntarily recalled.[10]
Evidence of tin impurities can be indicated by color, as in the case of pears, but lack of color change does not guarantee that a food is not tainted with tin.
Paul
you tell that to my vet,after my dog drank some of my runoff fertiliser collected in a a galvanised bucket ,all I got was a $800.00 vet bill and a dead dog .so I would rather believe the vet and the other Paul. 
