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Tougher rules governing septic tanks in at-risk areas of Otago could be looming after a discovery leaking tanks could be contaminating the region's underground water supplies.
The risk of groundwater being contaminated by leaking tanks has been investigated by the Otago Regional Council and a report will be considered by the council's regulatory committee today.
It recommends the council adopt a plan change to deal with the risk septic tanks pose and move to a different management regime.
With no exact records of how many septic tanks there are in Otago, or where they are, the council used information from local authorities and modelling to estimate 14,600 properties have tanks in the region, with 2200 to 7300 in some stage of failure and 2500 exceeding the 6A water plan's nitrogen discharge limits.
''Approximately 70% of the aquifers within Otago may be at medium or high risk of contamination from surface sources,'' the report says.
Hotspots identified as at a high risk of contamination and of high priority for action were parts of the lower Taieri aquifer, the Pomahaka aquifer, the Wakatipu basin and parts of the Wanaka-Cardrona and Hawea Flats aquifers where groundwater is close to the surface.
The greatest proportion of the tanks were in the Dunedin district and the highest concentration of tanks was in Central Otago, the report said.
Under council rules, small-scale discharges from septic tanks and long drop toilets are allowed provided certain conditions are met.
Enforcement of the rules was ''reactive'', with the council acting on complaints and taking action only when necessary, the report said.
''The current reaction approach, however, may be inadequate.''
However, this only captured a small number of septic tank failures, mostly those inconveniencing a neighbour or affecting public land.
''Septic tanks that fail silently - for example, if the bottom of the tank is no longer intact or discharges are relatively contained - tend to pass unnoticed.''
The silent failures breached council rules and were ''likely to be occurring in significant numbers''.
''As a result, they pose a significant risk to the groundwater resources of Otago.''
The report suggested using nitrogen loading limits in plan change 6A as a tool to control septic tank densities and prioritise communities without a reticulated water supply to control the risk of groundwater contamination by septic tanks.
While it would reduce the number of tanks requiring action, it would not substantially change the ''hotspots'' identified for some form of targeted action, it said.
Another approach suggested, but not recommended in the report, was for a programme of targeted inspections to ensure septic tanks meet set criteria.
''Doing so may prove onerous on ratepayers, and would require a workload comparable to dairy farm inspections.''