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The Otago Regional Council is seeking answers to the declining water quality of the Tokomairiro River near Milton.
The move follows the release of the Otago Regional Council's (ORC) State of the Environment study of water quality and ecosystem health in the Tokomairiro catchment.
The Tokomairiro River runs alongside Milton in South Otago and ORC director of environmental information and science, John Threlfall, said the water quality of the Tokomairiro catchment was steadily declining.
A seven-month study, which began in August last year, revealed that water quality has deteriorated significantly in recent years, with the entire catchment exceeding guidelines for bacteria, sediments, and nutrients.
The Moneymore drains were the most polluted sites in the catchment and contributed to increases in pollution downstream of their confluence with the west branch of the Tokomairiro River, Dr Threlfall said.
In the east branch, most of the water quality degradation came from the Gorge Creek and Salmonds Creek catchments.
Intensive farming and poor land use practices were the main causes of the degraded water quality and poor ecological health, Dr Threlfall said.
Run-off from State Highway 1 after heavy rain was also a contributing factor, although not to the same level, he said.
The report on the Tokomairiro area, which had a long history of agricultural use, said the climate and soil type meant that farming in the catchment relied on artificial drainage, specifically open and tile drains.
However, the regional council says changes in land use - especially in the lower catchment where dairy farm conversions are common - combined with inappropriate land management, are putting pressure on the water quality. This is also threatening the river's natural values, including native fish, five of which are considered to be endangered, and a trout fishery.
The monitoring carried out by the council between 2001 and 2006 led to the Tokomairiro River at the west branch bridge being classified as having good water quality. However, water quality declined sharply between 2006 and 2011, reducing its classification from good to fair, and making it one of the most polluted rivers in Otago.
The ORC has organised a public forum in Milton on Thursday to help farmers in the Tokomairiro address and resolve the water quality issues.
ORC chairman Stephen Woodhead said the forum was a chance for locals to hear about the water quality results and discuss solutions, as well as a chance for farmers to learn what they need to do to be compliant with the Otago Water Plan.
''Most locals understand that this sort of degradation of the waterways cannot continue. Unfortunately, it is a reality that some of the local farm management practices are contributing to the problem,'' Mr Woodhead said.
The council was keen to work with the local farming community to identify solutions to those problems, he said.
Tile-mole and surface drains are extensively used to enable farming on the catchment's heavy clay soils, and it was these drains which were often the conduit for most of the contaminants being discharged into the waterways.
The ORC accepted that these were not easy issues to solve but a start had to be made, and it was crucial that people got on board with improving water quality in the area, Mr Woodhead said.
''Changing some of the practices which have caused the water pollution doesn't require rocket science. With commitment from local landowners, this catchment can be rehabilitated.''
ORC would prefer not to go down an enforcement path, and hoped the Tokomairiro community would work with the council to identify practices which would ensure farm activities complied with ORC rules.
• Tokomairiro water quality forum, White Horse Inn, Milton, Thursday, December 13, 7pm to 9pm.