Whangamata Harbour: Contaminant loads and water quality

Report: TR 2001/04

Author: Bill Vant

In the Wentworth sub-catchment the area of native bush upstream of the upper-most sampling site contributed about half of the flow in the river. However, it generally contributed considerably smaller proportions of the faecal bacteria and total nitrogen, and only 10–14 per cant of the turbidity. The rest of the loads came from the largely-pastoral area downstream of this site. Two permanent drains through areas of farmland contributed relatively high loads of nutrients and faecal bacteria.

Field measurements in the two estuaries showed that the less dense river water tended to flow downstream above a layer of more dense seawater. At the more landward sites, concentrations of faecal bacteria and nitrogen were usually higher—occasionally much higher—in the less saline near-surface layer. At the seaward sites, however, the contaminants were generally more evenly-distributed. There was no evidence of any substantial input of contaminants into the Moanaanuanu estuary in the vicinity of the Whangamata wastewater treatment pond.

On each survey, the quality of the coastal waters entering and leaving the harbour on the inflowing and outgoing tides tended to be similar. Relatively-high concentrations of faecal bacteria were measured in these waters during the high freshwater flow event. On this occasion most of the bacterial load came from the Wentworth (28–39 per cent), Otuwheti (20–35 per cent) and Waikiekie (4–27 per cent) sub-catchments. Flushing of the catchment over the following fortnight, however, meant that contaminant loads at the end of the fortnight were much lower. As a result, bacterial concentrations in the harbour and coastal waters were also lower on the two surveys following the high flow event. While individual heavy rain events can reduce the suitability of the harbour and nearby coastal waters for bathing, an extended period of moderate-to-high flows appears to offset this to some extent by flushing contaminants from the catchment.