The Central App

High rainfall contributing to river water quality

The Central App

Aimee Wilson

03 July 2024, 5:30 PM

High rainfall contributing to river water qualitySwimming under the Shaky Bridge in Alexandra has been a popular summer pastime. PHOTO: File

Further testing of rivers across Otago over the 2023/2024 summer found that high bacteria levels were most often associated with moderate to heavy rainfall two days before sampling.

The Otago Regional Council's (ORC) Contact Recreation Monitoring Report 2024 was released last week, following the annual summer monitoring programme, assessing recreational water quality at primary sites.

Water quality scientist Helen Trotter prepared the report for the environmental science and policy committee, and six river sites across Otago have been graded ‘poor,’ including the Manuherekia River at Shaky Bridge, and the Arrow River at Cornwall St.

The ORC said increased run-off from urban and rural land carries contaminants to waterways and was associated with elevated bacteria concentrations.

In the 2023-2024 season, 560 routine samples for faecal indicator bacteria were taken at 33 primary contact sites, and 93 per cent of samples had water quality that was ‘suitable for swimming’ at the time of sampling. For 15 samples (3 per cent) across 10 sites, bacteria concentrations indicated the site was ‘unsuitable for swimming’ at the time of sampling and a health warning was issued.  

This included the Manuherekia River at Shaky Bridge.

A further 21 results (4 per cent) across seven sites, met the ‘caution advised’ guideline due to slightly elevated bacteria concentrations

The report noted that the Manuherekia River flows were low throughout the summer period.

While high bacteria levels were observed following rainfall, they were also persistent throughout the low flow period.  

Staff indicated that it was likely irrigation by-wash was a key source of bacterial contamination during low flow periods.

But Otago Regional councillor, farmer and irrigation company owner Gary Kelliher said the report was frustrating as it indicated irrigation bywash was the cause of the ‘caution’ ratings during the swimming season on the lower Manuherekia.

But he said from his questioning of staff, they had no evidence of that whatsoever, “it was just an assumption.”

He believed they were also mixing up bywash (where a scheme releases water back to a waterway), to runoff (where irrigation flow travels across land and re-enters a waterway).  

Gary said the one-in-40-year drought experienced last summer meant there couldn’t have been runoff either, “as for much of the season irrigation flows were very low.”

“The farmers in the Manuherikia are constantly wronged like this by council and others. The report could have mentioned the Omakau sewage outfall that would have been discharging its normal flow all season, but it didn’t. It also hinted avian may have been a contributor but didn’t specify to what extent.”

The 2024-2025 contact recreation monitoring period will commence in December 2024. 

Future work directions include exploring options for real-time monitoring to improve reporting efficiency, targeted sampling programmes for sites with frequent exceedances to investigate potential contaminant sources, and reviewing sites and procedures ahead of each season to ensure the programme maintains appropriate coverage across the region.