Inevitable reduction in water quality

By Tony Orman and Peter Trolove

The NZFFA president Dr Peter Trolove says the massive increase in the amount of irrigated farmland across Canterbury in recent decades has resulted in a comparable reduction in the region’s water quantity and quality from over-allocation of water and nitrate pollution. 

The stimulus for investigation was in declining salmon and trout populations. “The region’s recreational fisheries at large braided river mouths and in smaller lowland spring fed streams and rivers have seen trout numbers decline to near extinction,” said Peter Trolove.

Initial researching of the limited data base at both Environment Canterbury (ECan) and NZFFA’s initial nitrate testing, suggested toxic levels of nitrate in groundwater was a prime suspect for the decline in lowland streams.

The loss of native and recreational fish from Canterbury’s large braided river mouth lagoons appeared related to lower residual flows and the loss of variable flows – a critical characteristic of braided rivers – due to excessive water draw-off for irrigation. So the Federation made the decision to investigate. The acquisition of a Trios optical sensor gave the NZFFA the ability to accurately test water for nitrate pollution.

The research process

Polluted river in Canterbury.

ECan’s maps showed nitrate pollution was a problem in wells across the Canterbury Plains. NZFFA’s initial testing showed very high levels of nitrate in the Hinds River in South Canterbury. and in most of the lowland springs and drains within the Hinds/Rangitata plain. The early signs strongly hinted at a nitrate problem. In 1986 a CRC report from Ministry of Works Christchurch warned that rural residents between Ashburton and Rakaia would need to source alternative drinking water due to increasing nitrate in groundwater.

“NZFFA decided to test surface water in the lower Selwyn District due to the recent massive Central Plains Water irrigation enhancement scheme which was completed between 2015 and October 2018.as there is no evidence ECan’s Land and Water Regional plan is (or will) manage this problem.”

Loss of fisheries

The consenting of the Central Plains Water (CPW) irrigation enhancement scheme has cost Canterbury anglers dearly said Peter Trolove. CPW has failed to meet its promise the scheme would mitigate pre‐existing over allocation of ground and surface water in the region.

CPW allowed large scale intensive irrigated dairy farming to be developed on 45,000 ha of porous soils especially vulnerable to nitrate leaching.

Diffuse nitrate pollution

Diffuse nitrate pollution occurs when synthetic or animal sourced nitrate leaches beyond the root zone into the underlying aquifers.

“The soils of Central Canterbury are porous and aerobic, making them especially vulnerable to nitrate leaching. Consequently these soils retain virtually no nitrate. Thus freshwater ecosystems sourced from Central Canterbury’s nitrate polluted aquifers are harmed by direct toxicity and indirectly at much lower levels from the effects of eutrophication,” said Peter Trolove.

Human Health Risk

Humans are at risk from drinking water from nitrate polluted wells tapping the polluted aquifers.

 A Danish study of 2.7 million people pointed to a direct link between bowel cancer and high nitrate levels in drinking water.

“Environment Canterbury (Canterbury Regional Council) knowingly chose GDP over pollution. Over the ecological health of the public’s rivers and the potential risk to human health from cancer, “blue baby” deaths, premature birth and foetal defects.” says Peter Trolove.

One of the ECan “sacked” councillors, was former Conservation Minister Eugenie Sage, whose submission tabled at the an ECan managed Hearing to amend the National Water Conservation (Rakaia River) Order 1988 to enable the Lake Coleridge Project (LCP), February 2012, stated that;

 “It would be reckless and irresponsible to proceed with CPW before the means and the regulations to manage nitrate leaching from the out of river use were in place.” Her warning was deflected to future hearings to consent the out of river use of this water.

Wholesale crisis as trout disappear

Peter Trolove said earlier reports indicated trout numbers were in decline.  One guide reported first the small fish then the larger fish were disappearing from the lower Hinds River.

In 2000, Wayne McCallum, North Canterbury Fish and Game’s Environment Officer, wrote about lowland trout rivers and said that “on careful study, there appears to be more than a problem. Rather the evidence points to a wholesale crisis.”

Wayne McCallum cited two examples, Canterbury’s Selwyn River and the Horokiwi Stream, north of Wellington, a stream that was the subject of scientist Radway Allen’s classic study of a “typical New Zealand trout stream.” In 2010 Aquatic Ecology Limited in a report for Fish and Game NZ “depicted significant shifts and declines in trout spawning activity.”

 A peer-reviewed US study (1979) found larval mortality of Chinook salmon, rainbow trout, and cut-throat trout occurred at concentrations as low as 2.3 to 7.6 mg/L NO 3 ‐N. Canterbury’s lower Hinds River nitrate levels tested at 7.0 mg/L NO‐3N.

Damming Results

Near the end of almost three years of the NZFFA’s monthly sampling of the Selwyn Water Zone’s lowland rivers, drains, and streams, the NZFFA results are damming said Peter Trolove. 

The August 2020 National Policy Statement for Freshwater Management NPS FM provides guidelines set by the Ministry for the Environment. The “bottom line for nitrate in rivers is 2.4 mg/L. “Hart’s Creek test results are consistently around 8.0 mg/L NO3-N,” he said.

The Selwyn River recorded nitrate levels over 9.0 mg/L in 2020/21 peaking at 9.98 mg/L NO3-N before the massive end of May 2021 flood flushed the lower river. As the surface flows recede and the Lower Selwyn becomes fed primarily from inflows of groundwater the nitrate levels have climbed back to over 7.7 mg/L. 

The Halswell River acts as a “control”. This large groundwater fed river is sourced from aquifers situated beneath Christchurch suburbs. The nitrate levels in the (urban) Halswell River are much less than in the rivers draining the (rural) Central Plains.

Toxic algae and pathogenic bacteria from cow faeces add to Selwyn River’s pollution woes.

Many of the smaller drains including the Irwell River suffer from over allocation, simply drying up completely during prolonged dry periods over summer.