This investigation was recommended in the Contamination Management Plan that was developed by HLA for ADI and the Department of Defence to address the contamination of the groundwater to the southeast of the facility with excess nitrates and sulfates. The concentration of nitrates are of particular concern because in some parts of the plume, they are above the drinking water guidelines. Whilst the process discharges that caused the contamination have been stopped, there is still a significant residual contamination absorbed onto the clay beneath the areas where the discharges were. Rainwater percolating through these contaminated areas can leach out nitrates and sulfates which will prolong the life of the plumes.

The purpose of the investigation was to accurately determine where the areas of clay with high levels of nitrates and sulfates are, and to recommend a type of capping for each area of contamination to minimise the percolation of rainwater.

Delineation of the contaminated areas was done using a combination of sampling and an electromagnetic (EM) survey. The EM survey detected areas of high conductivity in the soil. Unfortunately this technology cannot differentiate between pieces of metal buried in the soil and areas of high nitrates / sulfates and so was of limited use during this investigation.

Plots of all of the major contaminated areas showing contours of concentration levels of nitrates and sulfates, were prepared.

The types of capping evaluated are summarised in the following table.

Summary of capping construction details

Cap type	Description	Final surface	Comments
Phytocap	0.6 m of sandy clay loam supporting native vegetation	Native vegetation of mixed size to simulate original vegetation.	Relies on vegetation to use all rainfall. Medium cost but low maintenance once established.
Clay cap	0.5 m compacted clay layer, 0.5 m of loam soil sub-base and topsoil with mulch.	Small plants/ grasses to stabalise the surface.	Relies on the clay layer to shed the water that gets past the grass. Medium cost, more ongoing monitoring / maintenance required.
Reference clay cap	0.6 m compacted clay, Bentonite layer, 0.3 m coarse sand (drainage layer), 1.0 m loam sand.	Small plants/ grasses to stabalise the surface	Relies on the sand layer to drain away any water that gets past the grass. The Bentonite layer is very impervious clay. High cost, low maintenance.

The evaluation of the different types of capping found that there was not a lot of difference in performance of the different styles in terms of rainwater leaching through, and so the recommendations in the report were based on other factors, such as cost, practicality and aesthetics. These recommendations are summarised below.

Summary of capping recommended for the different areas of contamination.

Area	Location	Recommended type of capping	Reason
A	Near existing and previous processing areas	Clay cap, if contamination cannot be treated with CSA.	Large mass of contaminants, but close to operating factory and so phytocap is unsuitable.
B	Effluent drain	Phytocap	Large mass of contaminants. Phytocap will blend with surrounding landscape.
C, D3 & D2	Old waste dumps close to Bayly street	Phytocap after removing surface contamination.	Large mass of contaminants. Phytocap will blend with existing vegetation to screen the factory.
D1	Old dump area 400 m north of Bayly Street.	Clay cap after relocating material from area C/D2/D3 to it.	Cost effective solution for waste storage. Phytocap not suitable over these wastes.
E	Old storage/ dump area 250 m north of Bayly Street	Clay cap	Area used for open air storage. Low priority area to be capped as no nitrates and not much sulfate.
F	Former Corowa Shire tip	Clay cap	Low priority area as low and decreasing impact on groundwater.

Following consultation with the ADI Mulwala Community Consultation Committee on 18 May 2005, a capping investigation will be finalised to investigate the merits or otherwise of capping technology.

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