Water storage lake to be drained

Greater Wellington Regional Council has this week begun draining one of its water storage lakes, in an effort to remove an organic compound, recently found at higher than normal levels in the lakes.

The naturally occurring compound, known as Geosmin, can cause water to taste and smell ‘earthy’. Geosmin poses no health risk and the water remains safe to drink.

Lake water with normal levels of Geosmin is routinely treated at the Te Marua Water Treatment Plant using powdered activated carbon (PAC). However due to the exceptionally high levels in Lake 2 in recent weeks, not all of this taste and odour causing compound can be removed from its water. The level of Geosmin in Lake 1 is presently normal, which means that the PAC treatment is able to effectively remove the Geosmin.

Because storage lake water is only used as a back-up water source when river water is unavailable (for example, after heavy rainfall when river water is too dirty to treat) and water from Lake 1 is able to be used, regional council officers have made the decision to drain Lake 2 and refill it with fresh water.

Greater Wellington Regional Council Water Supply General Manager Chris Laidlow says that while all water being supplied to the public remains completely safe to drink, it made sense to drain the lake.

“When we’re using water from the storage lakes – which isn’t often – organic compounds such as Geosmin are removed using PAC. While that process sometimes doesn’t remove all of the compounds from our water, it is completely safe and complies with the NZ Drinking-water Standards.

“We decided this week that it is better to drain Lake 2 rather than continue to try and treat the high levels of Geosmin. Now is a reasonable time to empty the lake as there is a very low risk of a water shortage at this time of year and we have plenty of time to refill the lake before next summer.

“We don’t yet know the cause of the high levels of Geosmin but we’re looking into this and also methods for dealing with high levels of Geosmin in the future.”

Lake 2 is the southern of the two lakes and holds 2,067 million litres of water. It will be empty by 20 May and is expected to take 6-8 weeks to refill.

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