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
It recommends the council adopt a plan change to deal with
the risk septic tanks pose and move to a different management
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
''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
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
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
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.''