Whangamata harbour water quality investigations, 1999-2000

Report: TR 2000/02

Author: Bill Vant

Surveys of the water quality of Whangamata Harbour and certain of its inflowing streams were conducted during June 1999 to February 2000. The investigations included:

1. surveys of the levels of faecal bacteria at three popular bathing beaches (two on the open coast, and one inside the harbour) at weekly intervals during the summer,
2. surveys of salinity, dissolved oxygen, water clarity, plant nutrients and faecal bacteria at 13 widely-distributed sites throughout the harbour on five occasions during June to February,
3. surveys of these constituents at approximately hourly intervals throughout the tidal cycle at a site in the Moanaanuanu Estuary on the same five sampling days,
4. surveys of nutrients and faecal bacteria in the lower Wentworth River—the main source of freshwater to the Moanaanuanu Estuary (and to the harbour as a whole),
5. surveys of these constituents at several sites further up the river, and in the lower part of the Waikiekie Stream (which drains the area of exotic forest where the treated Whangamata sewage effluent is spray-irrigated), and
6. occasional observations and analyses of surface foams and scums.

The bathing beaches were found to be suitable for swimming. In particular, the median level of enterococci at the harbour bathing beach site was <2 cfu/100 mL, or more than ten times lower than the national guideline level for safe bathing waters. This was despite the fact that substantially-higher levels of bacteria were found in the freshwaters which enter the harbour upstream of this site. Dilution with clean seawater was apparently sufficient to ensure that the water at this site was safe for bathing. However, the guidelines for safe shellfish-gathering waters require a lower level of contamination with faecal bacteria, and these and other results indicate that shellfish gathered from the southern half of the harbour should probably not be eaten.

At the time of sampling, the harbour waters were mostly (clean) seawater, and were generally in good condition: dissolved oxygen levels were generally high (>90% of saturation), and levels of nutrients and faecal bacteria were generally low. However, none of the surveys were undertaken during periods of high freshwater flow, and it is likely that levels of some contaminants may increase during and after flood events. Furthermore, although water quality was generally good over large areas of the harbour, it was found to be poorer in areas where moderately-contaminated river or stream water mixed with harbour water. As a result, contaminant levels were moderately-high at times in the Moanaanuanu Estuary, and near the mouth of the Waikiekie Stream.

In these areas, contaminant levels were generally highest when salinities were low, and vice versa. Most of the contaminants therefore entered the harbour from the land, rather than from the sea (although there may have been an exception to this at the time of the [very windy] December survey). The contaminants found in the Moanaanuanu Estuary appeared to have entered the Wentworth River from the catchment upstream of the Whangamata golf course. There was no evidence of any substantial leak of contaminants from the Whangamata wastewater treatment pond (which is located adjacent to a small stream which enters the Moanaanuanu Estuary downstream of the golf course).

Longitudinal surveys of the Wentworth River suggested that most of the contaminant load at the golf course entered the river from the largely pastoral area in the lower part of the catchment. Inspection of this area showed that livestock had unrestricted access to the river at places, and that contaminated runoff from the adjacent land was likely to enter the river. The moderate degree of contamination observed in the stream was broadly consistent with this type of land use.

Comparison of the levels of faecal bacteria in the Waikiekie Stream with those found in other small streams in the Waikato Region suggests that the bacterial load is partly due to the small amount of pastoral farming in the catchment, and partly to leakage from the spray irrigation area. The overall loads of faecal bacteria to the harbour from the Wentworth River and the Waikiekie Stream appear to be of similar magnitude.

Levels of nitrogen in the lower Waikiekie Stream were 30–100 times higher than in the Wentworth River. Together with the conclusions of a previous assessment, this fact suggests that leakage from the spray irrigation area is contributing a major load of nitrogen to the stream (and thus to the harbour). This load may have increased over the past decade. The potential for the nitrogen load to support nuisance plant growth in the harbour should be thoroughly assessed.

At times, small areas of foam or scum were seen floating on the water in sheltered parts of the Moanaanuanu Estuary. These appeared to be natural features of this (modified) estuarine area.