Wetlands – a sleeping giant
Variously known in the urban environment as detention basins, stormwater management systems or sediment basins, they are generally better known as wetlands. While their primary function may be to mitigate flooding and reduce pollutants entering major waterways, they also fulfil important ecological and amenity functions.
Wetlands are typically constructed as a chain-of-ponds system which harks back to a core function of the Australian landscape before European settlement. Because of Australia’s wet / dry seasonal fluctuations, chains-of-ponds developed in the deeper waterholes after the river / creek stopped flowing in summer. Also known as billabongs, these waterholes were critically important for landscape function, not to mention the animals and birds who also frequented them. And of course such resources would have been critically important to Aboriginal people.
Unlike Andean countries with their vast ice and snow fields, and northern European countries with very large peat deposits, the ancient landscape of south-eastern Australia had little in the way of water storages to keep rivers running through the dry season. For this reason, the chains-of-ponds and billabongs were critically important in delaying the movement of water through the landscape.
Along came European settlement and in a bid to increase the use of the land, our predecessors straightened creek lines and drained the chains-of-ponds, thereby inadvertently accelerating the dehydration of the landscape.
So in the modern day we understand the error of our ways and almost every new development in urban areas has such structures to slow water, trap pollutants, and mitigate surges in urban water flow.
The success of these structures may be seen in the effectiveness with which they trap sediment. The sleeping giant referred to above concerns the need for such structures to be regularly maintained. By design, they will fill up, and need to be routinely cleared out so that they can continue to function as designed.
Depending on location, some wetlands will accumulate unacceptable levels of contaminants washed off roads or from industrial premises in the catchment. Given that a high proportion of entrapped sediment will be very fine material such as clay particles and organic matter, they can be very good at binding pollutants such as some heavy metals and organic pollutants.
SESL Australia has tested many of these sediments. Those that exceed EPA thresholds for beneficial re-use must be disposed of to licenced premises. For many sediments that are not contaminated, the major issue preventing re-use is weed seeds which are found in super-abundance in sediments.
We recently carried out some laboratory testing to evaluate optimal temperatures required to achieve weed kills in this material. We then proceeded to trial blends of sediment and compost to see if we could achieve sufficient temperatures to effect good control of weed seeds and other propagules.
The trial was a success as determined by a massive reduction in the weed burden of the material. Further work is required to demonstrate that full control of weeds in sediment is possible using compost technologies.
This work has shown that beneficial re-use of dredged sediments is possible and material that previously was consigned to landfill can now be successfully recycled into landscape soils.