A Naturally Toxic Watercourse

Hydrogen sulfide (H2S) or rotten egg gas is often the smell of mangroves.  This poisonous gas is produced  by decomposing organic matter in the below the surface of the mangrove muds.  Normally the gas is only present in trace amounts and is wafted away in the wind or currents.   Very occasionally in prolonged still conditions, it builds up to levels that become toxic to life resulting in fish kills.  At very high concentrations, I suspect it kills patches of mangrove trees and it could be the main cause of the mass mangrove tree death caused by Cyclones.  Obviously understanding what hydrogen sulfide can do to ecosystems is important.  Yesterday, I found a strange example of an ecosystem stressed by this gas - the Blind Barron.  

In about 1942, the Barron River suddenly broke through the beach and created a new mouth about 1 km to the north.  The channel to the river old mouth filled with sediment and a flat beach ridge formed in what was the mouth.  A 1952 view of the Barron River mouth is presented below.  A pair of orange points have been placed on the image to mark two features that can be identified in both photos.

However mixed in with the river sand and mud that filled the old channel was a lot of mangrove leaf detritus, which creates hydrogen sulfide as it decomposes.  And this is where it gets strange.  The sand flats at the mouth of the Blind Barron (name of the old mouth) are higher than the bed of the Blind Barron so the water flows from the sand flats into the Blind Barron when the tide goes out – it looks like the watercourse is flowing backwards.   When the tide has gone out, the flow keeps going!  It is sustained by groundwater coming out of the beach ridge that has built up in the mouth the Blind Barron.  As the groundwater has been in a low oxygen environment that is rich in organic matter, it contains hydrogen sulfide.  When the tide comes in, it refills the beach ridge ensuring that on each low tide there will be plenty of groundwater.  The net result is a watercourse that fills with clean seawater at high tide and runs with contaminated water at low tide.  

Freshly deposited mangrove detritus at mouth of Blind Barron which will generate H2S when buried.  The white material is shell grit.  Normally the presence of shell grit neutralises acid generated by H2S so I am assuming that the water is not acidic, only contaminated with H2S.

This 50 m long pond is filled with contaminated seepage from the adjacent beach and sand flats

Pond water flows back into the Blind Barron at a high rate- shown here still flowing strongly many hours after high tide

Strange water colour - clearer than estuary water with a hint of colloidal cloudiness and blue tint.

View up the Blind Barron - it has high banks and a sandy bed

The extent of the toxic backflow is shown in this 2008 imagery.

The impact of the hydrogen sulfide is only on animals that live in the waters of the Blind Barron.  Life on the banks is not affected as the hydrogen sulfide is a low tide phenomenon.  The water was clear and inviting but smells like a volcanic spring.  It was noticeable but not powerful.  Toad fish were swimming around randomly in the water column and a mud crab was sitting in a hollow with its tail hanging down (It didn't want to be photographed).  One fish, a rarely seen hairtail or cutlass fish was lying on the bottom and died as I photographed it with my underwater camera.