Contaminants pollute harbour

After the Mangawhai Community Wastewater Scheme expressed concern at a lack of quality water quality monitoring, members of the Mangawhai Harbour Water Quality Advisory Panel, Kaipara District Council and Northland Regional Council joined forces to find out how contaminants were entering the Mangawhai Harbour.

Water quality was studied in six sites. The testing included checks for salinity, water temperature, dissolved oxygen, turbidity and visual clarity, E. coli, ammoniacal nitrogen, nitrogen, and phosphorus.

The sampling was carried out at fortnightly intervals at low tide, with additional sampling taken after significant rainfall. The six sampling sites included Devich Road bridge, Insley Street bridge north and south, Molesworth causeway, Henry’s Bridge on Cove Road (Tara Creek), and the Forest Stream off King Road.

The work was completed by voluntary Advisory Panel members and skilled community members, and was supervised by two team leaders to ensure consistent sampling practice. Training and equipment were provided by Northland Regional Council and administrative support was provided by Kaipara District Council.

NIWA analysed the samples to see a) how water quality changed from the upper sub-catchments to where it enters the harbour, b) how water quality differed between pastoral and forested sub-catchments, c) how water quality was affected by rainfall, and d) the characteristics of each of the sampling sites.

The results found two extremes of water quality within the study: Forest Stream and Tara Creek at Henry's Bridge. Forest Stream had by far the lowest levels of phosphorus, nitrogen and ammonium and was within the ANZECC (2000) guidelines for lowland streams.

Of all the sites, Tara Creek at Henry’s Bridge showed the highest levels of phosphorus, nitrogen, ammonia, E. coli, and turbidity. At both Tara Creek and Devich Road Bridge nitrogen and ammonium levels exceeded ANZECC guidelines by three to six times.

Forest Stream showed high E. coli levels after heavy rain, however, the highest levels of E. coli in this stream were three times less than the maximum values recorded for any other site.

The high levels of E. coli found in Tara Creek during a period of no rainfall suggests faecal matter is directly entering the stream. A similar situation was found on the north side of Insley Road bridge.

As expected, high rainfall events during the testing period caused high turbidity and low clarity levels at all sampling sites, but Forest Stream was much lower and cleared a lot faster than at any other site. A very high correlation was found between turbidity and E. coli levels, suggesting overland flow is the most important contributor of contaminants.

The study shows that restoration planting and stream edge retirement will make the greatest contribution to reducing contamination of the streams and harbour during high rainfall events.

n The full NIWA report with detailed data from all sites is available from www.kaipara.govt.nz.

WATER QUALITY: Six sampling sites were tested.