METHODS Samples were collected weekly or fortnightly throughout the period from the above plants.
Other treatment plants in the area were usually examined monthly while polluted rain water pools and streams were investigated only occasionally. Chemical analyses for works other than
Mangere were carried out on samples collected in 5 1 polythene containers while all samples for biological examination and chemical test at Mangere were collected in 250 ml glass-stoppered reagent bottles. Most chemical analyses were done according to A.P.H.A. 'Standard Methods' (Anon. 1965). Ammonia nitrogen was determined by Nesslerisation of steam-distilled samples. Algal numbers were calculated using a haemocytometer chamber by counting the cells in 10
* Present address: E. & W. S. Department, Bolivar Laboratories, Private Bag, P.O. Salis- bury, South Australia 5108.