Land and Water Link

April 2003

Boost for Blue Lake's Future

Residents of the City of Mount Gambier, South Australia, have always relied on the famous Blue Lake for their everyday water. Thanks to a new study conducted by CSIRO Land and Water, they now have a better means of safeguarding this vital resource into the future.

Concerns about how much longer the lake's water would be suitable for drinking began to emerge when it was detected that nitrate concentrations had been gradually rising between 1970 and 1997.

CSIRO scientist Dr Sébastien Lamontagne, an expert on the interaction between groundwater and surface water and nutrient cycles in aquatic ecosystems, has been working with the South Australian Environment Protection Authority to investigate the situation.

'With clean water becoming increasingly precious to Australian communities, it is vital to find out how the water of Blue Lake can best be managed to ensure Mount Gambier residents have a lasting supply', says Dr Lamontagne.

Blue Lake, a deep volcanic crater lake, is famous for the striking blue colour it takes on every year between November and March. This colour change is caused by the natural formation of calcite crystals.

Following European settlement in the Mount Gambier region there have been dramatic changes to the landscape, which in turn have affected the groundwater supply. Agriculture, urbanisation and disposal of various wastes through local bores and sinkholes have all contributed to a widespread increase in nitrate concentration in the aquifer that feeds the lake.

According to the National Health and Medical Research Council, acceptable levels of nitrate in drinking water are less than 11.3 mg N/L. The level of nitrate (NO₃^–) in the lake is currently around 3.5 mg N/L. Although this is well below the recommended limit, the concern has been that nitrate levels still appear to be increasing.

Dr Lamontagne says, 'Whilst action was being taken on other fronts to secure the future of this vital water resource, for example by implementing measures for controlling waste disposal, it wasn't clear how the nitrate concentration would evolve and impact the lake in the long term'.

Lamontagne and his team reviewed the data available on the Blue Lake to determine how groundwater entering the lake had affected nitrate levels in the past. This gave them the basis for forecasting future trends. Their study found that the slow increase in lake water nitrate levels reflected a similar increase in the concentration of nitrate in the groundwater entering the lake.

'We estimated the past groundwater input rates of nitrate to the lake by reviewing long-term data and then combining it with a simple nitrate mass-balance model of the lake', explains Dr Lamontagne. 'Based on our models, we were then able to forecast trends in nitrate concentration for 1998 to 2028, using best and worst-case scenarios.'

'We identified one aspect of the Blue Lake's management that could have a significant impact on concentrations of nitrate – the extraction of lake water by pumping.' Dr Lamontagne elaborates, 'The hydrology of the lake has been significantly changed since the middle of the last century because of pumping withdrawals. On average a drop of water now stays in the lake for around nine years instead of perhaps thirty years a century ago'.

In their forecast, the research team considered three different pumping scenarios: a low rate (3,000 ML/y), the 1997 pumping rate (3,340 ML/y), and a high pumping rate (4,000 ML/y). The best-case scenario was based on a combination of low pumping rates and a low rate of increase in nitrate concentration in groundwater entering the lake. High pumping rates and a high rate of increase in groundwater nitrate concentration represented a reasonable worst-case scenario based on present conditions.

For all scenarios, the research team found that lake nitrate levels would increase by 2028. But as Lamontagne explains, 'This isn't necessarily all bad news. In all the scenarios we considered, nitrate concentration should remain well below the drinking water guidelines for the next few decades.'

In the short term, pumping management schemes that increase the lake water residence time will slow down the increase in nitrate concentration. 'What was not realised in the past, is that the lake probably consumes a significant proportion of the nitrate input from groundwater. So by allowing water to remain in the lake for a longer period of time before being pumped out, we can capitalise on the lake's self-cleansing process', points out Dr Lamontagne.

Considering that over a scale of centuries the concentration of nitrate might exceed guidelines, Lamontagne adds that future studies will be needed to help monitor and manage the lake for the long term. 'We certainly need an accurate understanding of the hydrogeology of Blue Lake – the current focus of research by local and state authorities such as the Department of Water, Land and Biodiversity Conservation, South Australia.

'However, it will also be important to better understand what happens once nitrate enters the lake', says Dr Lamontagne. 'While some aspects of Blue Lake, such as the annual colour change, are relatively well known, there is still much to learn about its unique ecology.'

The results of this research have recently been published in a report entitled Predicted trends for NO₃^– concentration in the Blue Lake, South Australia (PDF 552 kB).

For further information

Contact
Dr Sébastien Lamontagne
Ph: 08 8303 8713