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Sustainability of fresh water lenses under major rivers

This project has provided new knowledge to understand and predict the hydrogeology of the lenses of freshwater that contribute to the maintenance of Murray River baseflow.

The physical hydrogeology and environmental isotopes (stable isotopes O, H, C, 14C and 3H analyses 222Rn concentrations) were used to constrain the age, origins, and stability of a low-salinity groundwater lens formed by periodic river recharge in the Nyah-Hattah fresh groundwater lens of the Murray Basin.

The Murray Basin is the major groundwater basin in southeast Australia. However, its potential to be used as water supply is limited by the high salinity (Total Dissolved Solids, TDS, commonly >10,000 mg/L) of much of the groundwater. Fresh (TDS < 5,000 mg/L) groundwater lenses underlying some reaches of the major rivers represent the only major fresh groundwater resources over large parts of the basin. These lenses are up to 500 m wide and up to 40 m deep, and extend for up to tens of kilometres beneath the river channels. While these are potentially important sources of water, a complete understanding of their hydrogeology is required to ascertain whether they represent a sustainable source of water.

Recent large-scale land clearing in the Murray Basin has caused the regional water table to rise. In addition, regulation of the Murray River has substantially reduced the frequency of flood events. These changes have increased the hydraulic gradients between the regional groundwater and the river while reducing recharge to the freshwater lens. The low salinity groundwater in the lenses originated by recharge from the river and through the floodplain during flood events. The margins of the lens are mixing zones between water derived from the river and the regional groundwater. The lenses are dynamic systems – their formation is controlled by the relative rates of recharge from the river and mixing with the regional groundwater. This will inevitably result in their degradation. Ultimately, if the lens does completely degrade and the regional water table intercepts the base of the Murray River, saline groundwater will feed the river as baseflow leading to a reduction in water quality and impacting riverine ecosystems.

Project Objectives

Key objectives were to determine how the Hattah-Kulkyne fresh groundwater lens responds to changes in hydrological conditions. This response was used to develop a conceptual model that clarifies how fresh water lenses beneath rivers in dry environments respond to hydrologic change. Specifically, to:

  • determine the time frames over which the lens was formed and the age distribution of groundwater in the lens.
  • identify the controls on the boundary between the lens and the regional groundwater system.
  • determine the rate and conditions under which the lens is replenished or degraded.
  • Identify the contribution of the lens to baseflow in the river under current hydrologic conditions and the impact of degradation of the lens on future river water quality.

The outputs of the project have been submitted to a peer-reviewed journal and have framed successful ARC and other funded research. The results have been shared with the Mallee CMA, CSIRO, MDBC, BRS & Monash University researchers.

Outcomes

  • The low salinity groundwater in the lenses originated by recharge from the river. The observation that 14C concentrations of groundwater within the lens decrease with depth implies that recharge through the floodplain, not just directly from the river is important.
  • The lens is probably recharged during flood events. That the longitudinal and lateral margins of the lens are transitional in their major ion, stable isotope, and 14C geochemistry implies that they are mixing zones between water in the lenses and the regional groundwater.
  • The lenses are dynamic systems – their formation is controlled by the relative rates of recharge from the river and mixing with the regional groundwater. Minor variations in hydrogeology; for example, small changes in lateral or vertical groundwater gradients, floodplain width or infiltration rates, or water table depths probably controlled the initial location of the lenses. The same controls also govern their future stability.
  • Recent changes to the hydrogeology of the Murray Basin include large-scale land clearing that has increased basin-wide recharge and caused the regional water table to rise. In addition, regulation of the Murray River and the use of the river for water supply have substantially reduced the frequency of flood events. These changes will increase the hydraulic gradients between the regional groundwater and the river while reducing recharge to the freshwater lens, and will inevitably result in their degradation.
  • Any attempt to use groundwater from the lenses for water supply is probably unsustainable, and will accelerate the rate at which the lens degrades. Ultimately, if the lens does completely degrade and the regional water table intercepts the base of the Murray River, saline groundwater will feed the river as baseflow leading to a reduction in water quality and impacting riverine ecosystems.

Publications and Resources



None listed


Citation

Land & Water Australia. 2009. Sustainability of fresh water lenses under major rivers. [Online] (Updated June 25th, 2009)
Available at: http://lwa.gov.au/node/2998 [Accessed Wednesday 23rd of October 2013 01:35:41 PM ].

Metadata

Program

  • Innovation
  • Theme: Innovation Call

    Project Code:

    002364

    State & NRM Region(s)

    Related Topics

    id: 2998 / created: 11 March, 2009 / last updated: 25 June, 2009