What are Sulfidic Sediments?

Introduction

Sulfidic sediments form naturally when soils are inundated for extended periods by sulfate-rich water.

  • Sulfate is reduced to sulfide by anoxic bacteria in the presence of organic carbon
  • Sulfides react with metals in the soil to form sulfidic minerals such as iron pyrite
  • Sulfidic sediments cause no harm if left undisturbed and submerged
  • If exposed to air (as in a natural or manipulated drying event), sulfidic sediments oxidise to produce sulfuric acid and other toxins
  • When the sediments are re-wetted, excess acid may be flushed into the water and cause harm to fish, water bugs and vegetation.

Diagrammatic representation of chemical processes that occur to form sulfidic sediments

Identifying Sulfidic Sediments — What to look for

Red discolouration from bank seepage which indicates groundwater intrusions high in iron content

Coppery coloured scum on banks and debris

Iron bacteria residue which has an 'oil slick' look and rust coloured deposits

Just under the surface of the scum, the soil resembles black 'ooze'

Waters look 'unhealthy', are murky with an orange-brown tinge and a distinct 'salty' odour

Wetlands which are most at risk are those that have

  • Been inundated for extended periods of time.
  • Elevated groundwater levels and/or disposal basins in the surrounding area.
  • Been identified as definitely containing sulfidic sediments or most probably contain them.
  • Water with an electrical conductivity of >1750 mScm-1 and/or
  • Sediment salinities of > 400 mScm-1.

What To Do?

If you suspect a wetland has sulfidic sediments, you will need to undertake a full assessment for the presence of sulfidic sediments before initiating a drying cycle. It may be necessary to keep water in the wetland in the short-term.

The Murray-Darling Freshwater Research Centre (MDFRC) has developed a Rapid Screening Tool to assist in identifying wetlands that are at risk. More information can be sourced via http://www.mdfrc.org.au/resources/biogeochemistry/ActionSupportTool.htm