Bacteriological studies on the Avon River, Christchurch, N.Z.

Abstract

The Avon River rises from springs and flows 26 km through the city of Christchurch to an estuary. The river has many tributaries, some arising from springs, others draining lowlying swamp land. Many stormwater drains discharge into the Avon. Variations in river flow are determined largely by rainfall. For most of its length the Avon River flows through residential areas, and is used for both recreation and stormwater drainage.

The danger from polluted water is mainly from living organisms it may contain. Recent faecal contamination is the greatest danger as pathogens may be present, which, if ingested, may cause epidemics.

Previous work has shown high levels of bacterial pollution to be present but no information is available on the sources of the organisms.

This investigation was designed to establish baseline levels of contamination. It extended over a period of very low rainfall during which the contribution of various sources of pollution were examined. The effect of a period of heavy rain on the numbers of bacteria was assessed. The survival time of the bacteria in the water was also examined.

Numbers of indicator bacteria occurring in water samples from 10 sites were investigated on 10 occasions during periods of low and high rainfall. The samples were examined for numbers of faecal coliform bacteria and faecal streptococci. Sediment samples were taken from 5 of the sites from which water samples were obtained and numbers of faecal coliform bacteria and faecal streptococci in them were estimated. At most sites during low flow numbers of faecal coliform bacteria exceeded the 200/100ml specified for Class C waters. Because numbers were high at sites closest to the source of the river samples from 6 sites in the upper reaches were examined on 4 occasions.

The inputs per metre during dry weather were found to vary over the length of the river although the sources of pollution were not determined. The relative contributions of humans and animals to the contamination has been estimated and it appears that in dry weather the ducks are minor contributors to the pollution.