The WWTP was upgraded in 2008 to remove phosphorus from the discharge in a bid to reduce the growth of algae, bacteria and microbes (periphyton) downstream thereby reducing the impact on the macroinvertebrate community - mayflies.
Last year concerns were raised after river sampling found a decline in the quality of the macroinvertebrate community downstream of the discharge.
PNCC and HRC agreed on a joint monitoring programme and engaged Opus International's principal environmental scientist Keith Hamil to help investigate the concerns and it's his peer reviewed report that is being publically released today.
The report has found periphyton grew three-times faster at a site 800m downstream of the WWTP discharge than upstream and that this has a significant adverse effect on the macroinvertebrate community during long dry periods.
Water and waste services manager Rob Green says work carried out in 2008 to improve the discharge has not been as effective as expected when the resource consent was granted.
Mr Green says, "the report has eliminated most of the causes of the growth in periphyton and council staff acknowledge it is clear nutrients emanating from the WWTP discharge are the likely cause".
However, he says further investigations are required into river sediments, ammonia levels and macronutrients to pinpoint the exact cause.
"We are committed to working proactively with HRC to ensure solutions are found. A work plan is currently being prepared and it will be work-shopped with Councillors before being formally reported back to Council."
The work plan is expected to include
• Operational changes - trial: increased alum dosing around long dry periods
• Further investigation - of river sediments
• Continued monitoring - of periphyton levels and the macroinvertabrate community
Both Councils have also agreed that a resource consent review should occur and work is expected to begin on that shortly.
PNCC acting chief executive Ray Swadel says PNCC is committed to improving the situation and ensuring the discharge meets the consent conditions in the timeframe provided.
Next week Councillors will receive the joint report and staff recommendations for consideration.
"At this stage it's too early to say how much the project will cost," says Mr Swadel. "Initially it will be carried out from within existing budgets however it is anticipated that at some stage staff will need to go back to Councillors to seek further funding to continue the project."
- Joint monitoring programme river report 2.9MB pdf