Control Options

Sediment Remediation

Wetland sediments can act as long-term reservoirs for nutrients and internal cycling, particularly of phosphorus, and can sustain eutrophic conditions and associated nuisance midge problems even after external sources are reduced. The major techniques used to remediate sediments with high nutrient loadings include dredging, artificial oxygenation and the addition of compounds to inactivate or bind phosphorus.

Dredging

Dredging, although effective because it permanently removes sediment from the wetland, is not a technique that has been widely used in wetlands on the Swan Coastal Plain, likely due to the high cost and issues with the disposal of dredge spoil (which may be contaminated with pesticides and heavy metals). The City of Cockburn estimated the cost of sediment removal at Bibra Lake to be approximately 30 million dollars and this technique is not likely to be viable for the majority of eutrophic wetlands. Disruption to benthic macroinvertebrates and other fauna such as the Swan River goby (Pseudogobius olorum) would need to be carefully managed if dredging were undertaken at wetlands on the Swan Coastal Plain.

Artificial Oxygenation

Artificial oxygenation has recently been used in Western Australia, by the Department of Environment to improve water quality in the Swan River and Canning River at the Kent St Weir. The process employs an oxygenation system which distributes oxygen to the bottom waters of the river. The main aim of the oxygenation process is to reduce the flux of nutrients from the sediments to the photic zone where they help to fuel phytoplankton blooms. Greenop (2001) et al conducted an oxygenation trial in the Canning River and reported the following results:

•Benthic dissolved oxygen concentrations were higher in the oxygenated area than the control area, except during extreme low oxygen events.
Nitrogen cycling processes were affected, and the number of nitrifying microbes appear to have increased due to oxygenation. However, the ability of oxygenation to reduce total nitrogen levels was inconclusive.
The release of phosphorus from the sediments was reduced on some occasions but the results were inconclusive regarding the success of the oxygenation process in reducing phosphorus concentrations.
There was some evidence to suggest that there was a beneficial impact on fish, freshwater prawns and benthic macroinvertebrates.

The disadvantages of this system are that the benefits of oxygenation only occur when the plant is in operation and cost is likely to be prohibitive for most managing authorities of wetlands on the Swan Coastal Plain. Some oxygenation of water may occur from the installation of fountains at wetlands however, the effects of these have not been quantified.

Phosphorus Inactivation

Phosphorus inactivation, through the capping of sediments, is a relatively new method of sediment remediation, and can potentially reduce the availability of phosphorus to phytoplankton in wetlands. Chen (2004), trialed five materials (fly ash, red mud, lime (CaCO3), precipitated calcium carbonate (PCC) and crushed limestone), to determine their suitability for sediment capping. The major findings of this study were:

Red mud and fly ash were deemed unsuitable for sediment capping as they had high concentrations of phosphorus within each material and would act as sources not sinks for phosphorus.
PCC was the most efficient of the remaining materials at removing phosphorus from solution with removal rates ca 97% at variable pH conditions. However this was not available in quantities suitable for commercial application and was not subject to further trials.
Lime removed 94% of phosphorus from solution at 1000?g/L and was chosen for
The capacity of lime to reduce phosphorus from highly enriched sediments was tested in microcosm experiments.
The results from these experiments indicated that the release of total phosphorus from sediment was significantly lower with the addition of lime.

Hart et al. (2003) looked at the suitability of precipitated CaCO3 for sediment capping and in a laboratory reactor experiment trialed three forms of CaCO3.
The major findings of this study were:

The three forms of CaCO3 trialed were: SoCal a German product; EsCal an Australian product; and a finely ground limestone from Lilydale, Australia.
SoCal and ESCal were both effective at reducing phosphorus release however the German product SoCal appeared to be more than four times as effective as the Australian product ESCal.
A 2% layer of SoCal reduced the amount of phosphorus release by almost 100 times over that occurring with no treatment and ESCal reduced phosphorus release by around 15 times that with no barrier.
Lilydale limestone appeared to be ineffective at reducing phosphorus release.
The estimates application cost of SoCal was $3,800 a tonne compared with $2000 per tonne for the Australian option.

Phoslock™

Phoslock™ is a capping material that shows substantial promise for sediment remediation. Phoslock™ is a modified clay substance which binds available phosphorus and was developed in Western Australia by CSIRO and the Department of Water (DoW) . Robb et al. (2003) trialed the efficacy of Phoslock™ in the Vasse River, Busselton and in the Canning River, Perth and the main findings from this study were as follows.

In the Vasse River a decrease of filterable reactive phosphorus (FRP) from 50 ?g 1-1 to 5 ?g 1-1 was recorded over a course of two treatments with Phoslock™. However, phytoplankton growth (as measured by chlorophyll a) did not show a corresponding trend and fluctuated throughout the study period eventually leading to a visible bloom.
In the untreated area nitrogen fixing cyanobacteria were dominant and in the Phoslock™ treated areas non-nitrogen fixing bacteria were common.
A phytoplankton bioassay indicated a reduction in bioavailable phosphorus of ca 96%.
In the Canning River FRP concentrations dropped in the treated area but the downward trend was not as strong as was exhibited in the Vasse and remained above the detection limit of 5 ?g 1-1. Chlorophyll a concentrations fluctuated and there was no difference between the treated and untreated sites.

In general the authors considered that Phoslock™ could effectively reduce phosphorus and subsequent phytoplankton growth. The application of Phoslock™ can also alter phytoplankton species composition. However, the timing of application and an understanding of the ecosystem dynamics of the system under remediation are essential to maximise the efficacy of this product.
Brett Crowley, the Managing Director of IMT Holdings, the current manufacturers of Phoslock™ has confirmed that a granulated form of Phoslock™ is now being produced in China and is available for commercial application. The cost of application is approximately $1800 a tonne and IMT provide a complete service whereby they provide and apply the product. Phoslock™ is applied as a slurry and the granules are mixed in a 1000 L tank on-site then sprayed onto the waterbody. Given the limitations of mixing large volumes of water, Phoslock™ is probably most suited for application to smaller wetlands such as constructed wetlands in urban developments and stormwater compensating basins. There is potential for Phoslock™ to cause acute toxicity to fish (LC50 = 4350 mg Phoslock™ L-1) (Martin and Hickey 2004) if the correct application rate is exceeded or if an accidental spill occurred and this should be considered before use.

Sediment capping technology is not a solution for all nutrient enrichment problems. Its use may be limited in systems with heavy-external loading, frequent wind re-suspension and short hydraulic retention times (Chen 2004). Hart et al. (2003) suggested that under aerobic conditions, sediments may only release negligible concentrations of phosphorus, rendering sediment capping ineffectual. However, under suitable conditions this technique has the potential to provide, as part of an integrated range of strategies, an effective means of inactivating sediment bound phosphorus. Although the Department of Environment and Conservation (DEC) has tested Phoslock™ in rivers, trials in wetland systems have not yet been conducted. Further research into these techniques should focus on a comparative trial of the efficacy of the various CaCO3 materials described and Phoslock™ under local conditions and the implementation of cost benefit analysis of each material. This should also include toxicity testing.

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Updated : 23 June, 2009