A recent investigation into the Papakaio aquifer, which sits under North Otago and South Canterbury, found the water was of ''great age'' and, as a result of that and a lack of oxygen, was acidic, sulphuric and contained dissolved metals, such as iron and manganese.
Council groundwater resource scientist Jens Rekker said as a consequence the groundwater was ''chemically aggressive'' and corroded mild steel used in the casings of wells drilled into the aquifer.
This, combined with aquifer pressure above the surface in some places, meant at least six bores in the Enfield basin had uncontrolled seepage greater than five litres a second, ''which has become a significant water resource problem within the aquifer''.
The council had policies that sought to prevent uncontrolled bore seepage, he said.
''Observation of the significant number of bores in North Otago that continue to discharge without adequate control, due to corrosion, suggests that these polices have not had the desired effect.''
The report suggested limiting the consenting of groundwater from the aquifer to 50% of the replenishment that tops up the aquifer at its exposed margins.
At that rate, the Maerewhenua and Southern allocation zones would be over-allocated.
The council's environmental services manager Martin King said the council was aware of attempts to cap bores but there was no easy answer.
Resource management director Dr Selva Selvarajah has said in the past when bores were drilled the corrosive nature of the water was not taken into consideration when choosing the materials used.
Requiring the use of materials such as stainless steel might need to be considered when permits were issued in the future.
It would cost ''hundreds of thousands of dollars'' to close down the leaky bores and the water quality also led to clogged pipes and irrigation systems.
Some farmers had put in ponds to capture the run-off from the bores.
Cr Doug Brown said leaky bores were a controversial issue in North Otago. Some farmers were unhappy seeing water continually flowing down gullies and going to waste.
The report would now go to the council's policy and submissions committee for consideration during consent and allocation processes.
