Moo-dunnit?

by Rebecca Macfie / 01 May, 2010
Is the mysterious contamination of a village well a sign of things to come for our drinking water?

Whodunnit? Who - or rather, whose ruminants - poisoned Dunsandel's town well?

Like other Cantabrians, the people of Dunsandel (population 450), just south of Christchurch, are accustomed to drinking untreated water that has filtered through layers of gravel and silts before being pumped up from deep underground.

Dunsandel's water comes from a well 70m deep. In 2001, when the local Selwyn District Council had the water tested to find out how old it was, it discovered it had been working its way through the cool dark aquifers under the Canterbury Plains for 50 years. It came out of the township's taps pure, sweet and unchlorinated.

But since last November, it has had to be treated with chlorine because of the presence of E coli - an indicator of faecal contamination. It's not the first time Canterbury well water has been contaminated, but it's thought to be the first time a well of this depth has been poisoned by animal waste.

So where's it coming from? And why should anybody beyond the quiet streets and lifestyle blocks of Dunsandel care?

Selwyn District Council utilities officer Lisa Duder says testing has shown the source of the contamination is ruminant animals, although scientists are unable to determine which species. Her colleague, water engineer Kris Kaser, provoked howls of derision from Green Party leader Russel Norman by suggesting late last year that the guilty ruminants could be sheep, goats or even llamas.

It's much more likely, said Norman, that the animal waste finding its way into the well is from dairy cows - the population of which has increased more than seven-fold in Canterbury in the past 20 years.

But - assuming Norman is right about the species - figuring out which herd of cows is responsible is to solve a major riddle. Norman and disgruntled Dunsandel local Peter Hill have been pointing the finger at the large dairy farm that borders the town well, an intensively irrigated 930-cow property owned by Lamasen Holdings, part of the Dairy Holdings empire that is one-third owned by Allan Hubbard's South Canterbury Finance.

Environment Canterbury records show that on January 14, the day before a particularly high E coli transgression in the well, one of Lamasen's pivot irrigators - which pump diluted cow effluent as well as water onto the soil - had created an area of saturated soil and ponded effluent measuring 1100sq m. The Environment Canterbury inspector deduced the irrigator had probably been allowed to "discharge effluent while stationary at the end of its run". The incident was described as "significant non-compliance" with the property's resource consent, and a formal warning was issued.

Bacterial counts in the well - which had been clear for the previous few weeks - subsequently topped the range of the council's testing equipment, suggesting a loading of at least 23 E coli per 100ml of water. "That's a big number," says Kaser. "It's not acceptable by any standards." In fact, any reading over 1 is judged unacceptable for consumption and the water must be treated.

But Environment Canterbury groundwater scientist Carl Hanson says it can't be assumed the effluent ponding on the Lamasen property is the source of the contamination. The mystery of Canterbury's groundwater system is such that the obvious answer is not always the correct one. "We don't know the direction that the groundwater flows precisely. But our best guess is that the area of ponding on that farm was not upgrading to this well. In other words, groundwater was not flowing from the area of ponding to the Dunsandel well. But we don't know for sure."

The problem is that despite Canterbury's enormous reliance on its aquifer system - even Christchurch's population of 370,000 drinks untreated ground­water - not much is known about it. And that's what elevates the contamination of Dunsandel's deep well from a minor issue affecting a small rural township to a matter of regional importance. If bacteria from animal effluent (and even Kaser admits dairy farms are "a likely source") can get down this far and cause such persistent contamination, what does that mean for the water supplies of other towns and cities around the region?

"The more we look at [Canterbury's groundwater system] the more complicated it gets," says Hanson. "The gravel below the Canterbury Plains is not a nice uniform pile of sand or gravel. There are old channels that seem to carry the water preferentially, and old silt bars where the water doesn't flow much at all, and these are all criss-crossing and cutting each other off."

Duder describes it as a "maze". "We don't know how quickly the water moves, whether it's running like a river or holding like a lake. We don't know how permeable the gravels are that everything is going through. The contamination could be coming from as far as 20 or 30km away."

Nor is it known whether the Dunsandel contamination is from a specific source, such as ponding on a farm, says Hanson, "or whether it is just generally that there are animals all over the plains and eventually the bacteria from these animals moves into the ground­water".

All this is further complicated by recent tests to "age" the water proving inconclusive - even though the Dunsandel water was put at 50 years old when tested in 2001 - and observations of the water level in the well showing it fluctuates quickly after rain or a "fresh" in the nearby Selwyn River. Kaser wonders whether this means the aquifer has created a "shortcut" that has changed the flow of water to the well. It all shows just how fragile the groundwater system and shingle plains are, says Hill, who last year paid $1000 for water supply to his lifestyle block.

As the revolution in Canterbury's land use continues, solving the questions raised by the Dunsandel contamination is likely to become increasingly urgent. Fonterra is assuming continued rampant growth in dairying in the region, and recently announced it will build a new milk powder plant at Darfield, 45km inland from Christchurch, to handle the tidal wave of milk. The plant will process up to 2.2 million litres of milk a day - equivalent to the output of 200 dairy farms.

And dairy farming in Canterbury - thanks to a massive increase in irrigation - is a high-intensity industry. The region's dairy farmers have the biggest herds in the country, and jam more cows into their paddocks than anywhere else - 3.5 cows per hectare; the New Zealand average is 2.8.

Environment Canterbury considers the Dunsandel problem significant enough to commission Environmental Science & Research to do an $18,000 study on bacterial contamination of deep groundwater sources in Canterbury, Southland and Hawke's Bay. Hanson says the organisation wants to figure out whether the Dunsandel contamination is a sign of things to come in the region. "It's always difficult to identify the first one or the first sign of something new. It's often easy to look back in the past and say, 'This is where it started', but it's difficult to recognise it at the time."

Meanwhile - despite an offer from Environment Canterbury to pay half the cost - Selwyn District Council has ruled out the option of drying out the Dun­sandel well to check whether the contamination is caused by a structural defect. Duder says spending $5500 to temporarily decommission and test the well didn't stack up, given that the investigation could prove inconclusive.

Instead, she says the townsfolk were offered the options of having a deeper well dug (at a cost of about $200,000, with no guarantee that the deeper water would be clean), continued chlorination of the existing well, or the installation of a UV water-treatment system. The locals opted for the latter.

Duder says the council has exhausted all its lines of investigation as to the source of the contamination. "We might never find out." She says it's now over to Environment Canterbury - shortly to be under the command of Government-appointed commissioners - to try to solve the mystery.

Hill is not satisfied. He claims Selwyn District Council has promoted intensive agriculture, including the proposed 60,000ha Central Plains Water scheme. Having backed "big dairy", he says, it has then put the cost of investigating the resulting damage to his community's water supply onto ratepayers. "It seems grossly unfair. It's not our fault that the water is contaminated."

Canterbury's medical officer of health, Ramon Pink, is monitoring the Dunsandel issue closely. "I think what we see and learn from Dunsandel will set the benchmark for how we respond in the future. It's very important to look at it and watch it."

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