Contaminated lake’s decline alarms

A Golden Bay lake has become an unswimmable, contaminated mess and residents say stormwater runoff from a dairy farm and housing is to blame.

For the past two summers significant algal bloom, or cyanobacteria, has been increasing in Lake Killarney, nestled in a reserve behind Commercial St near the town’s centre.

Since 2004 stormwater from neighbouring farmland and residential properties has been diverted into the lake.

Resident Dave Kennedy said the lake was ‘‘clear blue’’ when he first shifted there 30 years ago, but these days it was often a ‘‘murkybrown’’.

‘‘I think it’s only since they directed the stormwater into lake that it’s started causing the harm,’’ he said. ‘‘Our kids used to swim in it, but you wouldn’t like to now.’’

While sediment analysis is due in October, Tasman District Council results to date reveal a ‘‘significant’’ nutrient and organic contamination.

Neighbouring Takaka residents are increasingly concerned about the discolouration and potential effects of the algal bloom on adjoining properties.

John Lewis said the lake was still very blue when he moved there five years ago, but in the last few years he had noticed the discolouration increase during the summer months and after rain events. ‘‘You wouldn’t like to stick your toes in there now,’’ he said.

Lewis is taking regular water samples and photographs for the council, whose engineers are trying to solve the problem.

He said the contamination was coming from the council-owned pipe, which was draining from a sump on the adjacent dairy farm previously owned by Fonterra, and into the lake.

Algal blooms are typically the result of a buildup of nutrients and need warmth to grow. Another potential source was from the leaves of deciduous trees, he said.

During heavy rain several weeks ago, his wife Anne Lewis said she noticed water coming out of the drain was ‘‘the colour of mud’’.

‘‘You could see the discolouration spreading through the whole lake. The area of discolouration got bigger and bigger until it covered the whole lake.’’

The lake is home to the rare dabchick duck. Anne Lewis said there was a ‘‘marked reduction’’ in the number of ducklings in the lake last spring, but wasn’t sure if this was because of the contamination, or a snapping turtle that had taken up residence.

A report by TDC environment and planning manager Dennis Bush-King said the pipe to the lake was originally installed in 1970.

However, only since 2004 had floodwaters from farmland and residential properties on Meihana Street been accepted into the system. ‘‘It appears that these overflow events to the lake are common even in relatively small rainfall events when soils are saturated,’’ he wrote.

‘‘Results from samples of this stormwater are pending, and discussions about possible diversion of this stormwater have begun internally.’’

Dissolved oxygen profiles and sediment samples were collected in July. Oxygen was fully depleted in the lower few meters of the lake.

TDC spokesman Chris Choat said it had applied for Envirolink funding to get advice on developing a more comprehensive plan ‘‘to inform us as to the situation and remedial options. ‘‘It’s not toxic at this stage, but we would advise people to avoid contact with the water,’’ he said.