The Question: 

Kate has 30-odd (very odd, she says) goats who get a zinc sulphate footbath (for scald and ...) every five to six weeks. After they’ve been treated, the contents are diluted and drained into the ground.

She wants to know is this environmentally safe and is there a better method of disposing of it.

“The footbath holds 40 litres but we’re about to get a larger bath that will hold more. Sometimes we use copper sulphate.” She adds that while re-using the solution is great in theory, the goats don’t understand theories so use the footbath as a lavatory while in there…

The Answer:

It depends on who you ask. A Dunedin-based soil scientist I spoke with says while it depends on how much is being disposed of, a lot of our soils are low in zinc anyway. “It could even have a small beneficial effect in some cases but it depends on what the concentration is. It’s best not to put it in the same spot every time but spread it around.”

Copper sulphate is quite similar, he says. “Again, this is fairly low in New Zealand soils and farmers use copper to improve fertility.”

It may have a minimal effect and could have a beneficial effect, but if the concentrations are high, this could be an issue. His advice - the best thing to do is do the sums and work out how much of the product is being used in the first place, and spread over how far an area.

Waikato agrichemical consultant Graham McBride is a founding member of the Waikato Pesticides Awareness Committee, which was formed in 1989. Two years ago looked at agrichemical legacy issues in the US and Australia as part of a Winston Churchill scholarship.

He believes there are some real deficiencies in New Zealand’s legislation and came up with 62 recommendations for the management of agrichemicals and contaminated land.

“These are based on the good, the bad and the ugly from overseas forums and from their experiences gained over some 30 years of managing contaminated land. We, however, are very late starters with this issue – and this creates some opportunities. I was trying to extract the take-home message for us.”

One of the recommendations dealt specifically with the disposal of toxic elements or more commonly ‘heavy metals’. “I’ve done a bit of work on zinc and believe that in this country we’re flat out setting ourselves up to recreate the same sort of legacy we’ve had with some of the old persistent agrichemicals. Only we’re doing it with modern chemicals, such as copper, zinc, cadmium, lead and some ectoparasiticides.  In a report on the issue by the New South Wales Environment Protection Agency (the equivalent of the Ministry of Agriculture), one of the authors said that in New South Wales zinc is probably becoming the biggest environmental problem and threat to aquatic ecosystems.’

Graham points out that in New Zealand the problem could be greater still – “we also use it for facial eczema control, which inevitably comes out the back end of the cow and on to pastures, and sheep farmers use it too for footrot and eczema prevention.”

Lincoln University studied aspects of footrot treatment in sheep, as part of a Ministry of Agriculture sustainable farming fund project.  They found that 304 sheep farmers used a total of 223 tonnes of zinc sulphate a year.

“I tracked the author down and asked how are people disposing of this, and there was a blank silence. And then he said ‘I suppose they’re just chucking it out on the ground, or into a nearby stream.”

Some calculations

According to Meat and Wool NZ, there are 13,700 sheep farmers in New Zealand. “Therefore as per the Churchill report that’s 13,700 x 734kg zinc sulphate per farmer per year. Which is 10,000,000kg, assuming the 304 farmers are representative of all sheep farmers. That’s 10,000 tonnes of zinc sulphate being ill-disposed every year, and that’s not taking into account residues from eczema drenches. Where is all that annual loading of zinc going to?”

While these are commercial farming operations, Graham, a lifestyle block owner himself (albeit a 200-acre one), says each year there are about 7000 additional lifestylers who also use various forms of zinc nationally. “As of the last count, I think there are 85,000 in total. Which will also add to the loading problems.” Additionally, the environmental accumulation from galvanising used on roofs and buildings which leaches off steel products is also becoming a serious concern in some overseas jurisdictions.

He referred to a photo taken of a zinc footbath that had sat unused for four years, the sort of bath that might be used on a farm and lifestyle block. “She’s just a biological desert. Not a green bit of grass or anything around it. And this is just the splash zone, not the disposal area.”

A gentleman’s proposal

But possibly the best solution is to analyse what the zinc concentrations are, and, the perfect gentleman that he is, he offers to do this for Kate.

“This will allow us to establish the concentration of the zinc to calculate the spray on pasture option (gram/zinc x area = gms/m2). This would tell us if it exceeds soil Maximum Residue Levels – were we to have a guideline as many other countries do.”

While New Zealand has no guideline, in Australia it is set at 200 mg/kg (or 200 ppm.)   In Holland, zinc is not considered a contaminant in soil when levels are below 140 mg/kg.  However, when zinc levels in one sample exceed 720 mg/kg, a further investigation is compulsory.

What further irks Graham is the total lack of advice for disposal of the products themselves.  “I checked out the agricultural labels on zinc products such as facial eczema drench and footrot treatments and there is not one word about safe disposal.

“So, we have a theoretical annual use of 10,000 tonnes of zinc sulphate which is being disposed of without any guidance or advice. New Zealand farmers were forced to dip sheep under the statute using another toxic element arsenic for nearly a hundred years until around 1970. They have unwittingly left an environmental legacy which will be around long after we are gone. Is zinc (one of) the next agrichemical legacy issues?”