Last stand of the Karaaf wetland?

ABC News is reporting that the Karaaf Wetland at Torquay on the surf coast of Victoria is threatened by stormwater runoff. Temperature and salinity are major ‘eco-barriers’. Many species will not survive outside of the salinity range in which they evolved. Obviously, coastal saltwater marshland ecosystems cannot tolerate chronic low salinity or large and sudden salinity fluctuations.

The ABC report mentioned a number of species, including the Orange-Bellied Parrot, that their sources claim may be threatened by further reduction of the wetland salinity by stormwater. Clearing grass and scrublands for urbanisation with hundreds of people and their motor vehicles, cats and dogs, noise and pollutants of all descriptions entering the drains also has a major impact on wildlife.

Constructed wetlands are fashionable but, as in this case, they are usually not well maintained. However, even if they were, nutrients such as nitrogen and phosphorus may be dissolved or bound to solids in the water. In a well-constructed wetland, nutrients are removed from the water by a gravel bed, or similar, in which aquatic plants are grown. Insects, larvae, fish algae and invertebrates of many kinds also take up nutrients.

All this works when stormwater flows are low, allowing solids to settle and the vegetation is growing vigorously. But plant growth and the wetland ecosystem generally is less productive and nutrient demand is lowest during the colder, wet months when stormwater flows are greatest.

Eventually the bed of the constructed wetland becomes saturated with nutrients which have to be removed. Failure to remove organic matter leaves it exposed to high stormwater flows which may dislodge the nutrient-rich sediment and transport it downstream. Other than for controlled experimental sites, the long-term performance of small, constructed wetlands is rarely reported.

It has been suggested that a large dam with questionable safety attributes could be used to attenuate stormwater flows and to supply water for irrigation. Assuming that sufficient storage could be provided, water would be collected at a level below the drains and would have to be pumped up for irrigation during the warmer months.

It is proposed to mix the stormwater with recycled water from Black Rock Water Reclamation Plant and use it to support high value agricultural activities in the Thompson Valley. The Thompson Valley Land System has soils which are most unsuitable for irrigation, either with reclaimed water containing elevated sodium levels or with low salinity stormwater.

It is a heroic assumption to make that there is enough land suitable for irrigation in the Thompson Valley and there will be sufficient demand for the water. Primary producers will quickly calculate the gross marginal value of the water in terms of additional crops or pasture. Very often, there is little interest in recycled water unless substantial assistance is offered in the form of finance or irrigation infrastructure.

Energy would be needed to lift the water to the level of the irrigation area. No doubt there will be suggestions about solar or wind power but, again, the cost of infrastructure would likely far outweigh the value of additional agricultural production with questionable marginal environmental benefit.

Raising the soil moisture content by irrigation will increase the volume of runoff to Thompson Creek. The quality of irrigation runoff would be poorer than the water supply, especially if the stormwater is ‘shandied’ with recycled water from Black Rock Water Reclamation Plant. Nutrient-rich runoff would increase the risk of algae blooms in the estuary. Impacts related to reduced water quality would be likely to include odour, restricted recreational opportunities and fish kills.

Another (expensive) approach would be to store the water for garden watering via a retrofitted dual water supply network, similar to the recycled water reticulation schemes adopted elsewhere, and funded by householders, perhaps with some government funding.

Returning the stormwater to the sea with minimal impact on the marine environment could be achieved by first mixing it with seawater. Not only does the seawater reduce the impact of freshwater in the marine environment, albeit localised, it can also promote settling of solids and mitigate pollutants such as free ammonia and pathogens.

A survey would be required to identify an area prone to flooding with seawater at high tide. It might be feasible to construct a dedicated mixing basin close to the sea with features appropriate to the natural surroundings. The basin would be designed to promote mixing which can be inhibited by temperature and salinity differences.