Live Oral Presentation delivered remotely as part of National Virtual Conference AFSS Conference 2020

Aquifer source determination for Brown Lake, Minjerribah, suggests that licensed groundwater extraction is not a contributing cause of falling lake water levels (#44)

Harald Hofmann 1 , Jonathan Marshall 2 3 , Cameron Schulz 2 , Allen Gontz 4
  1. School of Earth and Environmental Sciences, The University of Queensland , St Lucia, Queensland, Australia
  2. Queensland Department of Environment and Science, Dutton Park , Queensland, Australia
  3. Australian Rivers Institute, Griffith University, Nathan , Queensland, Australia
  4. Department of Geological Sciences & Environmental Science Program, San Diego State University, San Diego , California, USA

Brown Lake is one of the iconic wetlands on North Stradbroke Island (Minjerribah). Water levels in Brown Lake have been continuously dropping for the last decade, raising community concerns that licensed groundwater abstraction from the Minjerribah regional groundwater system may be a contributing cause. This study aimed to identify aquifer sources associated with the lake and the licensed extraction to help clarify their potential impacts on Brown Lake water balance.

Water was analysed from groundwater bores around Brown Lake and surface water from Brown Lake, Blue Lake and Swallow Lagoon to establish relationships between the isotope and ionic chemistry of groundwater sources and the surface water systems. Relative elevations were used to determine potential groundwater flow paths to further assess potential groundwater linkages to surface water features.

Mean residence times differentiated groundwaters, with regional groundwater generally exceeding 80 years, while perched groundwater was decades younger. High hydraulic head differences (>20 m) indicated that the regional aquifer and perched systems were disconnected. Differences in water levels between Brown Lake, golf course aquifers and Swallow Lagoon indicated at least three perched aquifer systems in the area. Brown Lake is generally gaining groundwater and surface water levels are supported by at least one perched groundwater system. Residence time estimates in combination with hydraulic heads also indicated the perched aquifer under the golf club is disconnected from both Brown Lake and Swallow Lagoon and the perched aquifers associated with them.

Overall results indicate that licenced groundwater extraction is unlikely to have any influence on Brown Lake water level.  Rather, declining water level is likely associated with the long-term local rainfall deficient experienced by Minjerribah.  Further research is recommended to better characterise the hydrology of this and other complex perched aquifers and wetlands, which are poorly understood yet common and significant in coastal south-east Queensland.