Wellington Regional Council environmental officers published in November
1997 a report of pollution studies in Porirua and Wellington Harbours.
At Porirua analysis of mashed cockle flesh showed that in some places
there are unacceptable levels of faecal coliform and enterococcal pollution.
These organisms are indicators of material from human or animal faeces
that the shellfish have filtered from the water. From five sampling
sites the worst was Ivey Bay in Pauatahanui Inlet, where the coliform
level was 2.4 times higher than the acceptable level for consumption.
Other sites marginal for consumption of shellfish were Horokiri Stream
mouth and Paremata Station in the Porirua arm. The survey, though limited
in extent, supports the view that most sites exceed water quality guidelines
for shellfish gathering. Since the report the Council has erected pollution
warning notices at strategic points in the harbour.
The same study also investigated chemical pollution. It showed raised
levels of Polycyclic Aromatic Hydrocarbons (mainly from incomplete combustion
of fuels) from the mouth of the Porirua Stream, and for one particular
chemical at the mouth of Duck Creek. These were not above the international
guide line levels, but did indicate that up-stream some businesses or
householders had been negligent. Analysis of heavy metals in sediment
muds showed raised levels for lead and zinc at the Porirua stream mouth,
and for zinc at the Mungavin site. Heavy metals in shellfish flesh were
not seriously harmful in Porirua Harbour, but samples showed higher
than expected levels of copper near Mungavin Point, and of cadmium at
Paremata Station. Lead has originated from petrol emissions in the past
- it is to be expected that levels will decline. Zinc comes mainly from
the coatings on roofing iron. It is not particularly toxic. Copper and
cadmium are more likely to have originated from industrial uses.
Oganochlorines, which are mainly from pesticides, are released on farms
and horticultural sites. They are fat soluble which helps them pass
up the food chains, but stops them being leached out in water. They
therefore tend to be long lasting. They accumulate progressively, and
can be toxic and carcinogenic. There were raised levels of organochlorines
at the Pauatahanui stream mouth - probably from DDT used more than twenty
years ago. Mungavin Point also showed raised levels of organochlorines.
Neil Bellingham
Click
here for Wellington Regional Council's Water Quality Guidelines
(4 Pages).
