|12 July 1999|
PRELIMINARY STUDY INDICATES LIKELY
ENVIRONMENTAL IMPACT OF OCEAN OUTFALL
Preliminary findings from scientists considering the options for discharging Christchurchs treated wastewater from a pipeline off South Brighton confirm that higher environmental standards would be possible if such a pipe were built.
An extensive computer model of the Estuary and 500 km2 of Pegasus Bay shows that tide and weather patterns would ensure far lower concentrations of the material would wash back onto nearby beaches than is the case at present from water discharged into the western end of the Estuary.
Christchurch City Council wastewater engineer Walter Lewthwaite is managing the study, which is being undertaken by Woodward-Clyde Limited of Christchurch and Sydney-based environmental science company Unisearch. He says that dilution and dispersal of the treated wastewater pumped out of a long pipe are two of the critical factors to be considered.
"From a long ocean pipeline, discharging into 12 to 15 metres of water, the initial dilution before the treated discharge would reach the surface is at least 65 times, and perhaps as much as 100 times. This compares to the present Estuary outfall, discharging into shallow waters on an outgoing tide, where there is perhaps a five-fold initial dilution before the discharged water starts its journey out of the Estuary. A pipeline would therefore represent a considerable improvement on this factor alone.
"The length of the pipe would also offer significant advantages. Discharging up to two km beyond the low tide mark, the discharged water would move back and forward with the tides in calm conditions. An appreciable wind however, would carry the top two or three metres of water independently of the tidal, estuary, or Waimakariri flows. As the treated water is effectively fresh water, and therefore floats to the top of the ocean, it is these surface layers that would contain the highest levels of diluted and treated discharge. In the most adverse weather conditions - a brisk, sustained easterly - the treated discharge could start reaching the surfzone at South Brighton, which is the closest beach, after about seven hours.
"In comparison, at present the treated wastewater discharge flushes from the Estuary on the outgoing tide. In a southerly wind it moves northwards along the Brighton beaches over a few tide cycles. In a northerly or westerly wind it circulates east towards Sumner and Scarborough before re-entering the Estuary on the next incoming tide and combining with the next discharge," he said.
Constructing an ocean outfall, plus improved treatment, is estimated at $48 million. Alternative methods of treating Christchurchs wastewater are also being considered as part of the Councils wastewater review. Further analysis will give more detail to compare the water quality that will be achieved with different treatment standards and discharge locations.
Christchurch City Council will present viable options for the disposal of Christchurch wastewater for public consultation in September, prior to a final Council decision, which is expected towards the end of 1999. The assessment of environmental effects is expected to be completed in August, in time for these options to be developed in more detail. Christchurch residents will receive a background brochure in their mailboxes this week outlining the wastewater disposal issue.
For further information contact: Walter Lewthwaite
Christchurch City Council
Tel: 941 8367