Understanding the harbour after heavy rain

Understanding the harbour after heavy rain

“Eastland Port water testing proves last month’s discoloured harbour water was unrelated to port activity,” says Eastland Port General Manager Andrew Gaddum.

The testing showed Kopuwhakapata Stream, upstream from Eastland Port’s discharge site, was experiencing increased suspended solids following heavy rain in February.

“And increased suspended solids in that stream means dark water in the harbour” says Mr Gaddum.

Mr Gaddum wants to allay people’s concerns and help the community better understand what happens in and around the harbour after a heavy rain.

He says Kopuwhakapata stream is one of Gisborne’s main stormwater runoffs and its water comes from about 300ha of Gisborne, from as far away as Titirangi Drive, Tyndall Road and the area of on-going sub-division works on Wainui Road.

When rain falls onto hard sealed surfaces such as footpaths, roads, and driveways, it can’t soak into the ground, and instead runs off the surface into a network of underwater pipes and eventually into the Kopuwhakapata Stream.

The stream itself winds its way through parts of Kaiti. You can see it most frequently along London St.  The water gets into the harbour through a pipe under the Tatapouri Fishing Club.

“Anyone who saw the stream after the rain in February will have seen it running black as a result of all the stormwater. As you would imagine stormwater includes dust off roads, soil, tyre rubber residue, petrol, oil, and other rubbish, making the water black”.

“This water then poured into the harbour. It will have stayed there until natural tidal flows washed it out to sea.”

Mr Gaddum said the port’s water testing measured suspended solids, the small solid particles which remain suspended in water, even after it’s been filtered. The testing was done in five locations including two within the port, two in the Kopuwhakapata Stream, and one where the stream flows into the harbour.

“By measuring the suspended solids we can work out which areas are worse than others.” Mr Gaddum says the day of testing the port’s contribution into the stream was 122 grams of suspended solids per cubic metre. Upstream from the port’s discharge point the water was dirtier, at 179 grams of suspended solids per cubic metre.

“The harbour water turned black because of the heavy stormwater pouring into it. Our upper log yard makes up only one percent of the land area contributing to the Kopuwhakapata stream catchment. And unlike the water coming from elsewhere, our water had been through a powerhouse of natural and mechanical filters first.”

Mr Gaddum says the testing did show elevated levels of nitrogen at all five testing sites including in the Kopuwhakapata Stream upstream from the port.

Nitrogen is found in fresh water systems but in excessive concentrations it may lead to excessive growth of algae and other plants.

He said the port’s consent parameters around nitrogen are very stringent and as a result “it doesn’t take much of a change in levels to put us over the trigger point.” Testing found nitrogen levels at 0.55 parts per million at the port’s compliance point. This is just over the port’s robust consent trigger level of 0.4 parts per million.

Mr Gaddum says the elevated levels of nitrogen at all five testing sites were difficult to explain and the port’s ecological experts are investigating further. He wants to work out where the nitrogen is coming from so it doesn’t happen again.The port is keen to learn the results of other water testing carried out around the region at the same time, added Mr Gaddum.