On some days the leachate could contribute up to 70 per cent of the ammoniacal nitrogen levels.
The council has an informal "handshake" agreement with Bonny Glen to accept the leachate.
But compliance reports from Horizons show persistent and "significant non-compliance" since at least 2005 - before leachate was being put into the treatment plant.
A report from March and April 2005 noted the discharge was not complying with the consent. That year the levels of ammonia and biological oxygen demand (BOD) in the discharge "significantly breached" consent conditions.
The council complied with its consent in March 2006 but failed later that year.
A year later, Horizons noted the impact ammonia levels in the discharge was having on the stream. "(It) will have had a significant detrimental effect on aquatic life," a report said.
In March 2008, it did not do composite sampling it was asked to do by Horizons following consent breaches.
The compliance report also shows RDC has not informed Horizons of consent breaches as it is required to do.
Between July 2008 and March 2009, RDC was given compliance based on Horizons' monitoring. RDC's own monitoring data, however showed breaches.
Later that year the MWTP was again deemed significantly non-compliant "due to the breaches of the downstream water quality limits".
RDC also failed to provide Horizons with data, including effluent flow and stream temperature requested in previous monitoring reports.
In 2011, water quality samples revealed ammonia levels downstream of the Marton plant were having adverse effects on the aquatic ecosystem and at times were 35 times the "acceptable" ammonia levels and seven times the "maximum allowable level". Significant non-compliance notices were issued.
In February this year the MWTP was again found to be "significantly non-compliant", breaching ammoniacal nitrogen limits and "conspicuous change" (water discolouration) criteria. RDC also did not provide data for dissolved oxygen levels despite the information being requested twice by Horizons.
Last month Horizons hinted at enforcement.
"Failure to provide this information, both in a tabular and graph form by 28 February may result in HRC initiating enforcement action," the report said.
Horizons has written to RDC seeking a formal explanation as to why it is consistently failing to comply with ammoniacal nitrogen standards and what steps it is taking to ensure this matter is addressed. A response was to be provided to HRC this week. A Horizons spokesperson said it was received.
Rangitikei mayor Andy Watson told the Chronicle the council was focusing on improving the MWTP so it was compliant.
He said, in the past few years, council's priorities had been on the Taihape and Hunterville plants and Horizons had helped set those priorities. But the Taihape and Hunterville plants are also currently non-compliant as is the Koitiata plant. Mr Watson was not sure why council had failed over a number of years to provide complete monitoring data.
"I can't give you an answer to that," he said. "Some it of course precedes me."
He said the consent breaches were not uncommon and "nearly every district in the country has plants that are non-compliant".
"I guess it's fair to say I'm disappointed that we have issue with regard to the Crofton site (but) to put this in perspective it's reasonably common."
Horizons consents monitoring team leader Greg Bevin said enforcement action had not been taken because RDC had other priorities.
"We acknowledge the fact that they had a series of plants that probably weren't performing as well as they should be," he said.
"At the end of the day ... the conditions are there for a reason. Ordinarily when people do non-comply we work with them to bring them into compliance."
Enforcement options include fines, prosecution, abatement notices or enforcement orders through the Environment Court.
Despite the breaches and enforcement RDC could face, Mr Watson was confident it would not come to that and RDC had been working with Horizons.
Inaccurate data:
In another case, biological oxygen demand levels in the Tutaenui Stream were supplied by RDC as "less than" three (grams per cubic metre).
Given the limit was two, it made consent conditions impossible to assess.
Horizons warned if low accuracy data was supplied again it would deem RDC non-compliant.
A year later the same thing happened but Horizons passed the consent as compliant noting the data "needs attention.
Reports not done:
Horizon's requests for information and data has been ignored for years.
A macroinvertebrate report (on aquatic life in the stream), due every three year was not done by RDC. One was received n 2002 but it appears not in 2005.
The council was asked for a report in 2008 but it was not received by Horizons. This happened again in 2010. The due date was pushed out to 2012.
Horizons noted then the report had been "commissioned" by RDC and it deemed council compliant on that basis despite the report being overdue.
Horizons again asked for the report in 2013 and with no report forthcoming, Horizons issued compliance failure and asked for it by the beginning of 2014.
This week Horizons consents monitoring team leader Greg Bevin said "some biota surveys had been completed at provided to Horizons. He was not sure when they were done.
The Chronicle has asked for copies of those surveys.
