Brown Lake Update and someBackground Research
September 19th 2002
Lake still dropping: Traditional owners report that Brown lake is still dropping as of 10th September 2002. This reinforces the trend we observed at Brown lake on 20th June, the water level was lower than May even with rain in between visits. Brown Lake catchment inspections have revealed that Swallow lake is also low and this points to the whole perched aquifer being down. Traditional owners assert that while they have seen Swallow lake dry they have never seen Brown Lake so dry and still believe that drought is not the only cause.
What is the expected fluctuation in brown lakes water level
J.W. Laycock Hydrogeology Of North Stradbroke Island 1975
Indicates that the lake fluctuated upwards from its nominal level by over 3.5 m in 1973-74. There is no record that we could find showing the level dropping from its nominal level, as indicated by established vegetation, by over 1.5m. Inspecting coastal rainfall records for the last 5 months shows there should be have been a loss due to net evaporation of some 400mm which is less than the observed drops over the same period. Measurements from 31st may-2 June showed a drop of about 25mm over 3 days !.
Possible Causes of Water level drops
Dry Weather projection of some of the adjacent coastal rainfall and evaporation records reveals that there could of been a nett loss of around 700mm due to evaporation in the last12 months. This still leaves over 700mm more of drop to explain.
Water Extraction While in general the lake is perched above the water table , however within the lakes catchment the lower water table rises on the higher dunes and can contribute water to streams and wetlands that flow into Brown Lake.
Water extraction for water supply purposes along the other areas of the island can lower the water table for some distance away. This possible link between water extraction and decreased Brown Lake recharge from higher dunes could account for some of the observed drop.
It has been noted in some reports that Brown lake could lose more water if the gradient between the perched aquifer and the underlying one increases ie more water leaks over the edge in to the underlying aquifer.
Damage to underlying impervious layers. At present given the rapid drops observed and past experiences of perched lakes on the Island this is a highly probable cause. The soil type around Brown Lake is the Giant Podzol which has an impervious humic layer at 12-20m in depth. This layer is responsible for holding in Brown Lakes water .On other Island lakes when this layer has been breached there is a rapid and permanent drop in water levels.
The underlying layer can also leak more if the water table under it is lower. If it was significantly drier underneath the peat can even crack.
Other Metrology Data the BOM has indicated in only has partial data for Stradbroke island for 2000-2002 and little evaporation data BREC is obtaining this information for analysis.
1. That there is evidence that suggests the lake is drying out faster than can be accounted for by drier conditions.
2. That RSC seems to have no drought policy with regard to water extractions on the island
3. That the current information such as climate and water table information doesn't seem to be publicly available.
4. That the regulators such as EPA & DNRM seem disinterested in water problems on the Island. Given that the Island has similar values to World Heritage Areas such as Fraser island the EPA has duty of care to ensure these ecosystems are maintained in healthy state.
1. That a working group involving EPA, DNRM, RSC ,Traditional owners and other stakeholders urgently examine known information on the current status of Brown Lake.
2. This working group should make its findings and all information publicly available.
3. That any water extractions from the Brown lake aquifer be halted.