Nitrogen found in groundwater not associated with historic piggery plume

Jul 07, 2023

Nitrates are increasing in some Waimea groundwater aquifers, and "accumulating" and growing in size in one area, a council report says.

Tasman District Council science officer groundwater Melanie Westley presented the Waimea Groundwater Quality Survey 2021 report to the Tasman District Council strategy and policy committee on Thursday.

Westley, along with senior resource scientist water and special projects Joseph Thomas and other colleagues, sampled 149 sites, covering 137 groundwater bores and 12 river and spring sites in 2021.

Elevated nitrates in the groundwaters of the Waimea Plains were first identified in the 1970s. Age testing of that nitrate indicated that contamination could have dated back to the 1940s.

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An intensive piggery operation which operated near the Richmond foothills from that time until the mid 1980s was thought to be a "major contributor" to nitrate contamination in areas of the Waimea Plains near Hope, alongside nitrate from horticultural and agricultural activities.

Westley, speaking in the meeting, outlined a trend that was different to what they had observed in previous surveys.

While generally, over the past two decades, nitrates were decreasing and were making their way towards the coast, in the Upper Confined Aquifer there was an area which had elevated nitrates not associated with the historic piggery plume.

There was also a concentration of nitrates above 20 grams per metre cubed accumulating in the Bartlett Rd, Ranzau Rd, and Blackbyre Rd area which was higher than what they had seen in the past, Westley said.

That area, with higher nitrates, had also grown in size. Nitrates from the piggery plume were estimated as being between 11 and 16 g/m³, though these sites measured between 20 and 30.

"There's about 10 g/m³ that we’ve got running around coming from another source."

Westley said they were also noticing for the first time in 2021 an area on the western side of the Waimea River that was above 50% of the maximum acceptable value.

"This is the first time we’re seeing a wide area appearing in that area," she said.

Nitrate occurrence on the Waimea Plains came from a combination of "historic and present day inputs", and the areas with increasing nitrate trends had "overlying agricultural and horticultural activities".

Thomas said in the lower confined aquifer, along lower Queen St, which historically had elevated nitrate, it was now "coming down to just about the limit".

"But to meet the trigger value, we mix that water [for the TDC reticulated supply] with a much higher quality water from the gravel aquifer in the green area. So all our supplies will meet the drinking water standards," he said.

Elevated nitrate in drinking water is a contaminant associated with methemoglobinemia (blue baby syndrome), a serious condition in infants that leads to reduced oxygen availability and can cause death, Westley's report said.

In the Waimea Plains, groundwater is a "significant or principal source of water" for a range of uses, from urban supply for drinking, for industry, for irrigation, and also for private property use where property owners didn't have an articulated supply, Thomas said.

The use of the water was also a "key economic driver" for the Waimea Plains.

Council received the survey and green lighted the initiation of an action plan process for the Waimea Freshwater Management Unit to address nitrate in the Waimea Plains aquifers.