Nitrate in water may cause cancer
Why is there so much fuss about a very small amount of nitrate in our water? Professor IAN SHAW explores where the nitrate in Canterbury's drinking water comes from and its toxic significance to consumers.
Without nitrate, plants can't grow. They use it as a source of nitrogen from which they build important proteins that drive their cells and hold them together. Without plants, our livestock couldn't survive and our ever-dependent primary industries would be unable to function.
Another take on this is, the more plants - particularly grass - we can grow, the more livestock we can support and the more meat and milk we can produce.
Clearly, this is of key importance to a primary industries-led export market like ours - and a very successful one too.
Extending this thinking still further; if grass needs nitrate to grow, if you give it more nitrate it will grow more, and greener and richer. This, of course, will enhance our primary productivity. Simple.
There is another angle to the use of nitrate fertilisers that is peculiarly New Zealand.
We have soils with lower levels of key plant nutrients, including phosphate and nitrate. To redress this, we topdress with nitrogen and phosphate-containing fertilisers.
Again, a simple solution to an agricultural problem. So far so good.
When nitrate (and phosphate for that matter, but I'll focus on nitrate here) is applied to fields, not all of it is taken up by the plants.
Some sits in the soil and is leached by rain - the nitrates (eg, ammonium nitrate) used in fertilisers are very water soluble.
This leaching takes the nitrate deeper into the soil and eventually it finds its way into groundwater.
Soil bacteria can use nitrate too and so they will remove some of the nitrate as it leaches through the soil. The deeper the nitrate gets, the fewer bacteria it comes into contact with and so its fate as a groundwater contaminant is sealed just a few tens of centimetres below the soil surface.
The time it takes the nitrate to get into groundwater depends on the depth of the groundwater; the shallower the quicker.
This is why some of the shallow bores that rural Cantabrians might use as their water sources are the first to be affected.
However, the nitrate is gently making its way to the deeper aquifers and has been detectable there for some years. Currently the levels in deep aquifers are not of concern, but they will rise as we load the nitrate on to soil.
So why is there so much fuss about nitrate in drinking water? The one-word answer is toxicity.
Nitrate is a powerful oxidising agent that can cause significant changes to some biological molecules.
For example, the red oxygen- carrying blood pigment haemoglobin is irreversibly oxidised to methemoglobin by nitrate. Methemoglobin cannot carry oxygen and so too much nitrate can have dire effects.
Babies are particularly prone to this because they can have bacteria in their gut which convert nitrate to nitrite; nitrite is an even more powerful oxidising agent than nitrate and rapidly converts haemoglobin to methemoglobin.
Interestingly, these bacteria only persist until the child's stomach fluids become more acid (at about 3 months old) - the bacteria that convert nitrate to nitrite can't tolerate acid.
Nitrate in water used to make up infant formula is the source of the problem for non-breast fed infants. If the nitrate dose the child receives is high enough, it can result in significant conversion of haemoglobin to methemoglobin and so impede the child's tissue oxygen supply and lead to blue- baby syndrome (haemoglobin without oxygen attached is a blue/ purple colour) also called methemoglobinaemia.
Some of the nitrate levels in rural Canterbury's shallow bores are high enough to lead to blue- baby syndrome.
This is serious and explains why midwives advise mothers in these areas to drink only bottled water.
In addition, there is a more sinister side to nitrate that is far less well understood and of uncertain impact on human populations.
High nitrate doses are associated with some cancers. This is thought to be because nitrate is reduced to nitrite in the gut and nitrite reacts with specific food breakdown products (amines) to form highly carcinogenic nitrosamines. This is a convoluted path to cancer, but is assuming greater importance as nitrate in food and drinking water slowly increases worldwide.
The next question is, can we remove nitrate from drinking water? Yes, but at significant cost.
Resins that exchange nitrate for another ion (eg, chloride) can be used.
The water is passed through a column of resin particles and for every nitrate removed, a chloride is added to the water.
Charcoal can also be used, but usually only to remove excessive nitrate in emergency situations. The cost of introducing water treatment to Canterbury would be huge and many pure-water loving Cantabrians would baulk at the mere thought of water treatment.
Water is not the only source of nitrate. Many foods contain high nitrate levels - particularly green leafy vegetables.
But perhaps more importantly, our use of nitrate fertilisers on food crops is leading to higher food nitrate levels.
Also, some meats, including bacon, are cured with sodium nitrate (chile saltpetre) which preserves the meat, makes it taste good and (interestingly) prevents botulinum toxin-producing bacteria growing.
The health risks of nitrates are dose-dependent and there are a multitude of nitrate sources in our food and drinking water. It is the sum of our nitrate intake that leads to the health risks.
Undoubtedly the nitrate issue is due to our desire to grow more food, more efficiently and in places it might not be best to grow it.
This is a gently ticking timebomb that has come to our attention because of the potential effects of drinking water on the children of Cantabrians. And we Cantabrians are very proud of the purity of our drinking water.
Ian Shaw is Professor of Toxicology at the University of Canterbury, Adjunct Professor of Food Safety at Lincoln University, co- director of the Canterbury/Lincoln Food Safety Centre and Author of Is it Safe to Eat? (Springer, 2005) and Food Safety - the Science of Keeping Food Safe (Wiley-Blackwell, 2013).