The Wairarapa river often dubbed the "Sewer-mahanga" will be largely off-limits to swimmers as summer approaches.
The Ruamahanga is choked with agricultural runoff, treated sewerage and animal effluent, and is the dirtiest waterway in the Wellington region, the Environment Ministry's latest recreational water quality report shows.
Based on monitoring of the past five summers, four of six sites tested on the Ruamahanga had "very poor" water quality, exposing users to a high risk of illness from ingesting faecal matter. Possible ailments range from diarrhoea, stomach cramps and vomiting to serious diseases such as giardiasis, cryptosporidiosis, campylobacteriosis and salmonellosis.
Using a new "beach grade" ranging from "very good" to "very poor", the ministry study also shows some of the region's favourite swimming spots have made the grade, with Castlepoint and Riversdale beaches a mixture of good and very good.
Enclosed by native bush, the upper reaches of the Otaki and Waiohine rivers recorded some of the best freshwater results in the region.
Paekakariki Beach and Raumati Beach got universal "good" grades across multiple test sites.
Nationally, only 28 per cent of monitored sites were graded "fair" - signalling it was OK for swimming but that possible sources of faecal matter posed a risk of illness.
The Manawatu River, once regarded as one of the most polluted rivers in the country, earned a "fair" grade.
The report showed more than half of the freshwater recreational sites surveyed had poor or very poor water quality.
Massey University ecologist Mike Joy said the report underestimated the extent of the problem. "The reality is a whole lot worse than that."
The problem could be reduced or eliminated by increased planting alongside rivers and streams, and fencing off waterways to prevent contamination by cattle.
"The farming industry tries to get out of this by saying it's humans too, but it's not getting away from the fact the vast majority is coming from cows."
Labour environment spokesman Grant Robertson said the survey signalled a "fail grade" for the country and should be a wake-up call for the Government to do more to protect waterways.
Environment Minister Amy Adams urged against reading too much into the findings.
She told TV's Close Up last night that the report had looked only at swimming spots that councils were concerned about, and those that had returned bad results meant there was only a 1 per cent risk of getting sick from swimming there.
"It's quite wrong to give the impression that this is a systemic problem across New Zealand."
- The Dominion Post
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