Dear All
The orange stain (Ochre) you are obseving is as a result of the rapid neutralisation of acidified groundwater associated with acid sulfate soils.
See http://www.der.wa.gov.au/your-environment/acid-sulfate-soils/69-ass-guidelines?highlight=WyJhc3MiXQ==
Acid sulfate soils (ASS) are naturally occurring soils, sediments and peats that contain iron sulfides, predominantly in the form of pyrite materials. These soils are most commonly found in low-lying land bordering the coast or estuarine and saline wetlands and freshwater groundwaterdependent wetlands. In an anoxic state, these materials remain benign and do not pose a significant risk to human health or the environment. However, disturbing ASS, and exposing it to oxygen, has the potential to cause significant environmental and economic impacts including:
fish kills and loss of biodiversity in wetlands and waterways,
contamination of groundwater resources by acid, arsenic, heavy metals and other contaminants
corrosion of concrete and steel infrastructure by acidic soil and water.
Characteristics of ASS to look out for include,
- waterlogged soils
- water pH usually neutral but may be acidic
- oily looking iron bacterial surface scum (the similar appearances of iron bacterial scum and a hydrocarbon slick can be differentiated by disturbing the surface with a stick - bacterial scum will separate if agitated whereas a hydrocarbon slick will adhere to the stick upon removal)
- waterlogged soils – unripe muds (soft, sticky and can be squeezed between fingers, blue grey or dark greenish grey mud with a high water content), silty sands or sands (mid to dark grey) or bottom sediments (dark grey to black e.g. iron monosulfide ―black oozes‖) possibly exposed at sides and bottom of drains, cuttings or in boreholes peat or peaty soils
- a sulfurous smell e.g. hydrogen sulfide or ‗rotten egg‘ gas
- soil pHF >4 and commonly neutral and soil pHFOX <3, with large unit change from pHF to pHFOX, together with volcanic reaction to peroxide.
Cheers
David E Jackson
Sometime Freelancer
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