North Cronulla has one. West Sutherland has one. Engadine has two.
These are "constructed wetlands". Council-built, they strive to do what several thousand natural wetlands are doing all around the (mainly) coastal areas of Australia.
The 'kidneys' of a catchment.
The essential function of a wetland is to FILTER water, making murky water clean. It's a kidney-type role - or like the liver, which filters toxics out of the blood stream. They also act as giant sponges, gradually releasing water into catchments, thus buffering them against extreme wet and dry spells.
Essential as they are, wetlands have been treated appallingly by Australians. We've called them 'swamps', 'bogs', 'marshes' or 'wastelands', not fit for agriculture, only fit for breeding mosquitoes.
We've drained them, filled them, dumped rubbish on them. Then we've built parks, playing fields, even houses on them.
Only in recent years, after destroying half the nation's natural wetlands, have we woken to the fact that wetlands are vital to a healthy environment.
So today the National Parks and Wildlife Service says it will guard remaining natural wetlands.
And in suburban areas where destruction has been rampant, Councils are expected to help.
Our Shire Council has responded by saving an endangered natural wetland just in time: at Australia Road in Menai. Council knows that much more is needed.
Shire's big stormwater problem.
Rampaging stormwater periodically rushes off Shire streets and gardens, carrying vast amounts of dirt, debris, droppings, detergents, oils, chemicals and sometimes even sewage-overflow into drains and thence into the Georges, Woronora and Hacking Rivers. Sinful pollution.
Our worried Council has learnt a lot from Mother Nature. In just the past five years it has built the four "constructed" or "artificial" wetlands as a valuable start to cleaning up stormwater at some of the Shire's more needful points. Cost is high, between $200,000 and $300,000. Another, at Gymea, is on the drawing board.
They deserve a visit by families and field study by schools.
How the wetlands work.
A wetland is exactly that - wet land , typically saturated and sometimes inundated by heavy rain or storms. A constructed wetland can readily be observed to have four sectors.
First, the water, flushing off scores of streets and gutters, converges into pipes which deliver it to the initial stage of the constructed wetland, a Gross Pollutant Trap (GPT), a cage for straining out the large litter: tins, bottles, plastics, sticks, twigs and coarse sediment from sand to pebbles. The stormwater passes through, but of course it is murky with suspended fine and clayey sediment.
Second, the water enters a shallow Open Water Zone . An expansive pond, it serves to spread and slow the gush from the GPT's exit pipe.
The slowing allows some of the suspended material to settle onto the sediment of the pond's bottom. The open pond also allows the sun's UV (ultraviolet) rays to partially disinfect the water of micro-organisms.
Third , the water then drifts towards the main sector, the Wetland Zone , sometimes termed the Reed Bed. It consists of plants (reeds, rushes) that flourish with their roots in the bottom sediment, sucking out dissolved pollutants such as nitrates and phosphates. The plants grow densely, obliging the water to thread its way through, and the bottom-deposit too draws in suspended solids, which it mixes with leaf litter falling from the plants. The sediment is soon converted to a rich humus in much the same way as a compost heap works.
Fourth , the cleaned water simply Exits from the wetland. Yet this final stage is not entirely simple, for the water-level of the wetland is controlled at this point - by a kind of adjustable weir over which the water flows, onto broken rock which spreads it, aerates it a little, and send it quietened into the local natural watercourse to be delivered into one of the rivers and finally into Botany Bay or Port Hacking.
Marvellous microbes at work.
The bottom of the wetland is not "just mud". It is a sediment that swarms with life, particularly in the reed zone. Not just the surface but the entire depth of the sediment swarms with organisms that a microscope reveals, bacteria galore, which are cells without a nucleus, and moulds and fungi all the way up to barely visible invertebrates such as mites and springtails and a host of tiny insects and worms. Most contribute in some degree to the cleaning process. A central aim of wetland management is to maximise the build-up of the enriched sediment and its microbial activity.
Cleaner but not clean.
The Shire's constructed wetlands are not large enough to completely clean the water. The Tudar Road wetland, for instance, only fully treats 40% of stormwater and partially treats the remaining 60%. But that's a great contribution. Though not drinkable, the water is certainly cleaner . To achieve this after storms, the stormwater must be held within the system for 48 hours; so the outflow is limited automatically by the exit weir, causing the water to rise high among the reeds and test the earthen banks that surround the wetland.
Maintenance is vital.
Because the four wetlands are small and have much work to do, they need to be managed carefully. They are inspected monthly to ensure that all sectors are functioning. For instance the reed bed is weeded when necessary. As to the bottom, when the deposit of sediment becomes excessive it has to be removed, but this is only needed at about 15-year intervals. After big storms the GPT is checked and cleaned. Annually, a wetland scientist carries out an overall inspection.
Treasured Towra.
For comparison of constructed with natural wetlands, there's the Shire's most famous natural wetland, at Towra Point on Kurnell Peninsula. You can check it on the Environment Centre's wonderful website http://ssec.org.au/towra Great in extent, this wetland includes the largest saltmarsh area in the Sydney region, and it has tidal and non-tidal areas. It attracts over a hundred bird species including migratory waders from the Arctic Circle. Deservedly it is protected by an international agreement (Ramsar). |
