A water quality index for contact recreation in New Zealand

We surveyed the opinions of 16 water quality experts in order to develop a water quality index for contact recreation in freshwaters in New Zealand. The index was developed by postal surveys using the Delphi method, involving feedback of information to the panel members at each iteration. Determinands selected for use in the index were as follows: faecal bacterial indicators (faecal coliforms or E. coli), pH, Munsell colour, visual clarity indicators (black disc visibility or turbidity), and nutrients promoting nuisance growths (filtered BOD5, and dissolved forms of phosphorus and nitrogen). "Sub-index" curves relating suitability-for-use to these water quality determinands have been developed. The mean ("consensus") sub-index curves can be used to interpret water quality data in terms of suitability-for-use scores. We advocate using the lowest suitability-for-use score for a water as its overall index value for contact recreation. Thus the water body's suitability-for-use is determined by its "poorest" characteristic. The index is now ready to be tested by water managers for its utility in state-of-environment reporting.