Bacteria levels in the Taieri River at Waipiata, a popular bathing spot, continue to be up to three times higher than the national health guidelines, making it unsafe for swimming.
Bacteria levels have exceeded the red action guideline of more than 550 E. coli per 100ml - which indicates the water poses an unacceptable health risk to swimmers - in every Otago Regional Council test conducted this summer.
On December 23 1986 E. coli per 100ml was recorded after significant rainfall two days earlier, but on December 30 it recorded 770 E. coli per 100ml without any rainfall in the days prior.
Last year the council launched an investigation after unexplained levels of bacteria were recorded three times during last summer's recreational monitoring programme.
The regional council monitors the water quality at popular marine and freshwater bathing sites once a week between December and March.
It tested for the concentration of E. coli in fresh water and Enterococci in salty water, which indicated the presence of faecal material and disease-causing organisms.
Council environmental services manager Martin King said yesterday two staff last summer investigated the nearby small tributaries of the river seeking sources of the contamination, but no obvious source was found.
''It seems it could be a mix of a number of things.''
Many water fowl were seen in the backwaters and there was the potential for irrigation run-off.
''There's a raft of things really, but we came to no major conclusions.''
Because of recent rain enough mixing was occurring to ensure the contamination was not at such significant levels downstream at the river's popular Outram Glen swimming spot, Mr King said.
The site at Outram had breached the red alert guideline on December 23, recording 2420 E. coli per 100ml, and amber alert levels on December 16 and 30 after some rainfall on those days or the days before.
Staff would meet in coming weeks to analyse the recent data from Waipiata to see what else could be done, Mr King said.
''There is a bit of work to be done, but to be honest there is no quick and easy fix.''
The Kakanui River site at Clifton Falls had also continued to record high levels of bacteria since December 9.
It was found last summer the contamination at this site was probably because of droppings from nesting gull colonies upstream of the site.
Other sites to record red action levels of bacteria were the Clutha River at the Balclutha Lagoon and the Waikouaiti River at Bucklands Crossing, both on December 9 and 23.
