A very wet summer downgraded water quality results in Marlborough last year, Marlborough District Council's environmental scientists say.

Water quality status had improved at four coastal sites and the Picton foreshore would improve as a result of work that the council was doing, Mayor Alistair Sowman said on Friday.

Marlborough's best swimming spots, based on the quality of the water, are Ngakuta Bay, Mistletoe Bay, Te Mahia, Bobs Bay, Marfells Beach and Whites Bay. All these beaches are graded very good, the highest level.

The grades are based on five years of weekly data collected during summer so water quality has to be consistently very good to make the top grade.

Swimming spots graded as good are Anakiwa, Portage, Shelly Beach, Tirimoana, Waikawa Bay and Pelorus Bridge.

A council report on water quality says Marlborough rivers and coastal waters were generally of good quality and safe for recreational activities.

"However, rivers that drain urban and intensive agriculture areas and coastal beaches which are located in urban areas and/or which have a significant river flow to them are more prone to poor water quality and are sometimes not safe for recreational activities, even during dry weather," the report says.

Wet weather often results in Environment Ministry's guidelines being exceeded so swimming after rainfall is not recommended, particularly in urban and intensive agricultural areas.

The swimming spots with the lowest grade, very poor, are Moenui, Oyster Bay, Picton Foreshore, the Rai River, the Pelorus at Totara Flat and the Taylor River.

The council report said excess readings at the Picton foreshore were mainly associated with heavy rainfall. About 10 per cent of the foreshore test results had "highly likely to be contaminated" readings, and another 5 per cent had levels indicated that the water was "potentially contaminated".

Microbial source testing showed cattle were the source of contamination at Moenui, the Rai River and the Pelorus at Totara Flat. The council has been working closely with dairy farmers to deal with this.

Momorangi's previous "poor" ranking improved to "fair" last summer.

Momorangi has had water quality problems and the source has been tracked to wildlife in the area, primarily birds.

Mr Sowman said it was a beach surrounded by bush so wildlife was part of the environment. "I don't think anyone would suggest that birds be eliminated. It's just a case of being sensible."

The council posts up-to-date information about water quality on its website and takes regular samples during summer at 18 coastal sites, as well as 13 river spots.

- The Marlborough Express

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