A survey of the Tukituki River has found that upgrades to the Waipawa and Waipukurau wastewater treatment plants have substantially reduced the amount of dissolved reactive phosphorus (DRP) they release into the river system.
The National Institute of Water and Atmospheric Research (Niwa) survey last year assessed the nutrients entering the Tukituki River, and their effect on toxic algae growth.
The study compared the river quality before the floating wetland treatment systems were commissioned in September 2014, and results from sampling done last year
The findings were presented to a freshwater conference in Invercargill last December.
The survey showed there had been a 97 per cent decrease of DRP from the Waipukurau plant and a 98 per cent decrease from the Waipawa plant since the upgrades.
In addition, it showed that before the upgrades the input upstream of the treatment plants contributed 15 per cent of the total phosphorus recorded, the plants' input was 34 per cent and downstream of the plants the contribution was 52 per cent.
Since the upgrades, the upstream contribution had remained the same at 15 per cent, the treatment plants were now contributing less than 1 per cent and the downstream input was 85 per cent.
CHB District Council technical services manager Steve Thrush said the results were pleasing, given that reducing the phosphorus input into the river was one of the main reasons for the upgrades.
The council's resource consent for the plants allowed for an upper limit of 0.5mg/l and a lower limit of 0.25mg/l of DRP discharged from the plants, with provision for some exceedances.
Mr Thrush said the target was to be under those limits, which was being achieved, typically recording 0.1mg/l of DRP or less.
The Papanui Stream had previously been identified as a significant contributor of DRP to the Tukituki sub-catchment, at any one time potentially contributing up to 40 per cent of the total in the lower Tukituki River.
The Hawke's Bay Regional Council had been working with landowners in the Papanui catchment and a draft catchment management strategy was formulated in 2015.
Hawke's Bay Regional Council land management adviser Warwick Hesketh said it was still a priority catchment and farmers were being encouraged to prepare their farm management plans earlier than required under Plan Change 6.
"It's important to note that there's no real smoking gun there," he said, in terms of the source of phosphorus entering the river.
"We had thought it might be feedlots, or the Otane wastewater treatment plant or intensive cropping, but it's looking like it's a lot more complex than that."
Any improvements made in the catchment would take a long time to show up in samples, but the message was getting through to landowners, he said.