The composite sample is done on a 24 hour flow proportional basis.
That means at the Northside Outfall, for every 100 tonnes of water discharged from the steel mill a sample of water is abstracted into the day's sample container. After 24 hours, these samples are taken to the laboratory for analysis. The results are electronically transmitted to operations and environmental staff who compare the results with the conditions set by the resource consent.
Discharge Water Quality Standards
On the northside and southside outfalls, the elements which are analysed in the laboratory and the frequency of sampling are listed below. These are specified in the resource consents held by New Zealand Steel.
- pH (acidity scale)
- suspended solids
- dissolved oxygen
In addition a continuous turbidity (clarity) meter operates on the separate ironsand dewatering plant discharge, where the only potential contaminants are clay minerals derived from the ironsand deposit.