Press February 2002
Irrigation: too many unknowns
Irrigation has been promoted as the key to a dairying boom in Canterbury, but the costs of nitrate pollution and cryptosporidium infection may be high, warns WALTER C. CLARK.
Advocates of Central Plains and other irrigation schemes have loudly proclaimed the benefits but they have been silent on the down side, the enviromental costs and the costs to urban dwellers.
Outdoor recreationalists have drawn attention to the frantic grab for water that is an important cross-community amenity. No payment is made for the water taken.
Others have highlighted some of the aspects of enviromental degradation that attend and the changes in farming practices. The reduced, muddied, near lifeless, over-enriched trickles that wend their way between broken-down banks are well recognised.
Water to alleviate Canterbury's, summer drought, is cheap, but applying it may be costly.
To service bank loans farmers strive to maximise production and income. Water-soluble nitrogenous fertilizers make grass grow, and are applied liberally. When rain follows irrigation and fertilizer application, much nitrate moves below the root zone, and into the ground water
Up-country groundwater is down-country drinking water. Groundwater nitrate has been near World Health maxima in parts of Canterbury for some time.
Excessive nitrate in forage causes ill-thrift in stock. High nitrate levels in drinking water are incompatible with normal brain development in mammals.
Our most immediate concern should be for bottle-fed babies. They are small, they drink much, and are developing.
Nitrate is readily converted to nitrite in the normally acid stomach.
The nitrite combines with the haemoglobin in red blood cells to form methaemoglobin. Haemoglobin carries oxygen around the body, and releases it where needed.
Methaemoglobin does not give up oxygen. If the condition persists the baby will turn blue and may die.
Last year the Waimakariri District Council received a report that in four of its rural water supply systems the nitrate concentration had risen fourfold over 18 months ( the period of operation of the Waimakairiri irrigation scheme) and was well on the way to the maximum allowed by the New Zealand Drinking Water and World Health standards.
Since then, the report has apparently been withdrawn with murmurings about the quality of the science. The true state of these supplies is not known.
If irrigation-stimulated fertiliser makes water from wells and pipes non-potable, will the polluter pay? Who will pay for new schemes, and finding potable supplies?
Should it be those who had good water and now have none, or those who degraded the water? If the schemes goes ahead, and water is polluted by dung, urine and fertiliser, how long will it be before towns and cities must dig deep into the ground and their pockets for useable water?
Another problem is contamination of the enviroment by cattle with cryptosporidium, a protozoan, a small, single-celled animal. It thrives in the guts of many mammals and causes watery diarrhoea.
It is not a bacterium. It is like giardia and just as tough.
Calves are especially vulnerable. Just over half the calves studied in American research had active infections and released about one billion oocysts ( the infective stage) each day in their dung. Oocysts have tough coats which make them resistant to chlorination and ultraviolet light.
Flies transport oocysts. Some human infections have arisen from a single oocyst.
Water-borne infections are common. An outbreak in Milwaukee in 1993 infected 403,000 people, hospitalised 4400, killed at least 54. On average, infections lasted nine days and caused a 4.5kg weight loss in sufferers. Infections can be debilitating and they can be lethal in the young and immunologically compromised.
In New Zealand, little research has been done on cryptosporidium. At the hearing about discharging sewage effluent into the Heathcote Estuary Dr Mel Briesman produced the graph included. It plots cryptosporidium cases notified to Crown Public Health in Christchurch and North Canterbury from 1997 to 2000.
The low values in most Februaries probably indicate that by then the calves were immune and pasture run-off was reduced.
The later rise probably reflects calf births associated with winter milk supply. The continued increase to early summer probably mirrors oocyst accumulation and production by spring-born calves.
How much will cyrptosporidium cost if irrigation goes ahead? It will cost to visit the doctor and for tests and medication. Employers may pay much more for sick leave. Schools and hospitals will need relievers. Will the polluters pay?
Since 1997 northern hemisphere research has established that cryptosporidium occurs in sufficient numbers within shellfish to constitute a health hazard.
It enters the tissues of oysters, mussels and cockles. The blood cells are invaded and infections persist. One month after infection, oyster flesh fed to mice gave them all diarrhoea.
Will aspiring Pegasus Bay mussel farmers be able to demand that all effluent that might reach the sea by direct discharge or via rivers, be sterilised to remove cryptospordium and giardia?
It would cost millions, but the farmed mussels will still have to avoid the greatest source of infective material- that coming down the rivers from dairy farms.
We have been given a superficial assessment of the profits and losses associated with irrigation schemes but these may make us sick physically, environmentally and economically.
Too many facts are missing, but those we have, scare those who pause to think.
Walter C. Clark is an emeritus professor from the University of Canterbury, where he was head of the zoology department with a specialty in parasitology.