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Effluent Issues Divide Experts

PUBLISHED: Farmers Weekly - 7 April 2016

Critics of Canterbury groundwater management are at an impasse with Federated Farmers and Irrigation New Zealand over the impact intensive livestock farming has in the region on human health.

Observations by Canterbury medical officer of health Dr Alistair Humphrey that the region's elevated gastro-intestinal diseases were attributable to intensive livestock farming on coarse, stony soils have been challenged by Irrigation NZ chief executive Andrew Curtis and Federated Farmers water spokesman Chris Allen.

Humphrey expressed his concern at Canterbury's level of gastro-intestinal infections being 50% greater than the national average (Farmers Weekly April 4).

He pointed to intensified livestock farming as a probable source of E.coli infection in groundwater supplies.

His concerns were echoed by veterinarian and ecologist Dr Alison Dewes who drew a direct link between irrigation, intensified cattle farming and E.coli infections.
She called for the industry to step back from further irrigation projects in light of the risk.

But Curtis said any "rumbly gut" issues in mid-Canterbury are attributable to point source discharges injecting E.coli into shallow groundwater supplies, rather than diffuse application of effluent from farmed cattle.

He acknowledged the quality of bore caps on farms was one that had contributed in the past.

But he said the problem had largely arisen from aged septic tanks in districts like Darfield not treating human waste properly and that waste becoming a point source for well contamination.

"We are disputing the whole pathogen pathway claimed to be causing these issues.

"Pathogens do not travel down through the soil, it acts as a filter to what animals deposit on top of it," he said.

INZ was distinguishing between human waste travelling through the soil and animal waste.

Data from Lincoln University dairy farm was quoted as evidence of no shallow bore contamination occurring from livestock farming.

However, critics challenged the validity of using Lincoln as an indicator for soil's ability to absorb stock effluent. They pointed to the Lincoln farm's heavier soils being less than representative of Canterbury's more prolific lighter, gravelly soils.

Two thirds of Canterbury's soils had been identified by satellite imagery and Landcare Research as being classed as stony soils.

Environmental models had also consistently predicted stony soils had a high risk of leaching and that inappropriate effluent treatment could result in contamination of water with nutrients and bacteria.

Landcare Research trial work showed E.coli bacteria did move through stony soils and stock effluent-affected shallow groundwater supplies with elevated nutrients and contaminants.

Researchers cautioned it was important that science and technology investment targeted the stony soils within irrigated areas for the risks such bacterial movement posed.

In 2014 Canterbury public health analyst Dr Jackson Green did a literature review detailing a "multi-dimensional" impact of irrigation on water quality, including leaching of contaminants into groundwater.

But Curtis said while the data did indicate an elevated level of gastro-intestinal diseases in mid Canterbury, it might be a reflection of more people in that region handling and working with livestock, the main transmitters of the diseases.

"It could well be a case that people are not washing their hands properly after handling them."

He also pointed to Environmental Science and Research work that showed with good management and no saturation of the surface and profile, there would be little bacterial movement down the soil.

It was done on heavier Lincoln University dairy soils.

However, the research using a RotoRainer irrigator on a thinner-soiled south Canterbury dairy farm found as soil became wetter, microbial removal became less.

The researchers found under heavy rain and irrigation conditions there were "lots of coliforms" and no campylobacter further down the soil profile.

But when fresh cow pats were added to the surface and heavy rain/irrigation conditions was applied, coliforms and low campylobacter levels were found 1.5 meters down.

The risks of the disputed E.coli movement and confirmed nitrate contamination were also elevated by the "flushing" effect of excessive water application, either natural or irrigated, could have.

Curtis confirmed only 20% of irrigators in Canterbury had soil moisture strips to aid in correct application rates to avoid over-watering soil.

Dewes was also concerned about the elevated nitrate levels being recorded in some Canterbury well supplies.

Part of pre-natal checks for new mothers in rural Canterbury in some districts included testing their water supply for nitrate readings to ensure babies could be safely fed milk made up with it.

There were also issues around how accurately nitrate losses on the plains were being measured and the accuracy of the losses being modelled.

Overseer general manager Caroline Read confirmed Overseer did not have trial data to base its assessments for many Canterbury soils on. Therefore, data from the Lincoln dairy unit was extrapolated out for much of the plains.

While challenging the bacteria issues in groundwater, Curtis acknowledged the elevated nitrate levels in some Canterbury water supplies. He was aware of Overseer's shortcomings in that respect.

"But what it comes down to is it is as good as we have got at the moment.

"You have to look at the numbers relative to each other, rather than the absolute numbers themselves."

He welcomed the proposed investment programme for Overseer that should result in more accurate nitrate loss predictions in the future.


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