Oral Presentation NCGRT/IAH Australasian Groundwater Conference 2019

Hydrogeological, climatic and anthropogenic drivers of acidification within an inland acid sulfate soil wetland – investigations to inform remediation  (272)

Nicolaas P. Unland 1 , Louise Lennon 1 , Greg Hoxley 1
  1. Jacobs, Melbourne, VIC, Australia

The Barwon Downs borefield has been a crucial back-up source of water supply for the Geelong region during drought, and has been operated since the 1980’s. This has caused drawdown throughout the Barwon Downs Graben and some impact to groundwater receptors. The most notable of these is Boundary Creek, a tributary to the Barwon River in its upper catchment, in which reaches have been converted from predominantly gaining reaches to predominantly losing reaches. As a result, the flow regime in the creek has changed from a perennial system to one that flows during winter months or following high rainfall events.

Furthermore, flows through Boundary Creek have historically supported a wetland known as the Yeodene or “Big” swamp, an inland peat swamp which contains acid sulfate soils. In response to reduced flows and drying throughout the Boundary Creek catchment, sulfidic soils within the swamp have oxidised. This has resulted in the release of acidic and metalliferous leachate to the creek when it flows. This has been realised in the form of fish kill events in the Barwon River.  

While the cause-effect paradigm here would seem simple at first glance, it has been complicated by a number of additional factors. These include climate change, the construction of an on-stream dam, the development of agriculture and farming throughout the catchment and the occurrence of peat fires in the wetland. The study presented here assesses these potential drivers by considering the timing of such changes with shifts in flow and water quality in the creek.

The study builds on this understanding and presents the findings of subsequent investigations including soil sampling to ~6 m depth at 17 locations across the wetland for geochemical analysis, groundwater-surface water level and quality monitoring.

The findings of the above investigations have been compiled to build an understanding of the hydraulic and geochemical nature of the creek and wetland system. In doing so, the viability of remediation strategies such as the release of supplementary flows to the creak or the treatment of soils or water with neutralising agents are better interrogated.