Nitrate buildup in Canty water

An updated summary of information paints a bleak picture of the extent of nitrate pollution in Canterbury and sounds the alarm on the health of high country lakes.

The summary by Environment Canterbury’s surface water science team charting longer term surface water quality was presented to the natural environment committee last week, and follows the presentation in April of a summary on long term trends in groundwater.

The surface water summary found over half of monitored streams could be described as ‘‘moderately to severely impacted’’, while the proportion of streams in the low and mildly impacted category decreases.

The groundwater summary included longer term trend analyses showing nitrate levels are still increasing in three-quarters of the bores tested, and concluded there had been ‘‘no clear reversal of the trends’’ over the past 30 years.

The paper, written by surface water science manager Helen Shaw, relies on shorter data sets, despite her acknowledgement there are sites – set up by National Institute of Water and Atmospheric Research (Niwa) – for which they hold 30 years worth of data.

Over half (54%) of monitored streams show deteriorating quantitative macroinvertebrate community index (QMCI) trends over the past 20 years, an indicator of stream health.

‘‘The streams with poorer ecosystem health are typically in areas of more intensive urban and agricultural land use, but also on Banks Peninsula.’’

The ecological health of a further 22% of streams is considered ‘‘relatively stable’’, while just under a quarter (24%) are said to be improving.

The summary says 63% of all sites show an increasing trend in nitrate levels over the 22-year period, in line with groundwater quality trends, the ‘‘dominant source of nitrate-nitrate nitrogen in our rivers’’.

Shaw told the committee this is to be expected given the increases in nitrate in groundwater over the long term.

She pointed to improvements in phosphorus and turbidity, with phosphorous reducing in concentration in a number of sites, and turbidity which shows ‘‘some reductions’’ over a nineyear period.

However, Shaw noted the 22-year measure of turbidity was ‘‘a bit mixed, which is why we’re showing you the nine-year data’’.

Trend analysis of 24 high country lakes over the past 15 years showed chlorophyll-a increasing in more than 80% of

‘‘The streams with poorer ecosystem health are typically in areas of more intensive urban and agricultural land use . . ’’ ECan summary

‘‘It’s a classic example of corporate welfare in which the public pays for damage caused by the dairy industry.’’ Freshwater scientist Dr Mike Joy

the lakes, according to the summary, resulting in the lakes being downgraded from band A to band B within National Policy for Freshwater Management bands, which the report calls ‘‘a significant shift for these sensitive lakes’’.

Last month, ECan councillor and freshwater ecologist Lan Pham said she was lost for words to see ‘‘evidence this bleak’’, calling the results shocking and shameful.

Freshwater scientist Dr Mike Joy said there are glaring issues in the summary.

He said the use of short time periods is statistically problematic, as are the shifting baselines.

‘‘A few of the indices just sort of bottom or top out, so you don’t get a linear change – they start to plateau because they can’t go any lower or higher.

‘‘They’ve [ECan] been in there for 32 years. Water quality has been an issue the whole time – they can’t say they haven’t had time to collect that data.’’

Joy said he is also concerned about what information is and isn’t included.

‘‘There are huge gaps in the sampling sites, for example, around the Selwyn and the Rakaia [rivers]. If you’re the organisation that’s going to have to represent the results, there’s a tendency to drop the bad sites when self reporting.’’

He said the worsening health of high country lakes is certainly worrying, but queries the focus on the area.

‘‘The upland lakes are definitely being impacted, more than 80 per cent are getting worse for chlorophyll-a, which is nutrient driven, so it’s a bad sign, but it’s not clear why they are concentrating on high country lakes.

‘‘What we should be looking at is the lowland lakes. Where’s the data for Te Waihora/Lake Ellesmere, and the estuaries north and south of the peninsula?’’

Joy said the pollution of waterways affects all of Canterbury’s drinking water, and treating nitrate in drinking water is ‘‘incredibly expensive’’.

‘‘The public will end up having to pay for it, either through rates or through their own treatment systems in their houses. It’s a classic example of corporate welfare in which the public pays for damage caused by the dairy industry,’’ Joy said.

Nitrate, one of the most common drinking water contaminants in New Zealand, is largely driven by agricultural activity – nitrogen fertiliser application and livestock urine.

The safe nitrate level in New Zealand, 11mg/L of water, is mandated by the World Health Organisation in relation to blue baby syndrome.

However, overseas research points to other adverse birth outcomes at much lower levels, and cancer risks.