Gas and Fuel Corporation. Initial operation of the site involved production of town gas by coal carbonisation. In 1962 a catalytic oil to gas plant was brought into operation at the site. Gas production ceased in 1970 and in 1974 the plant was demolished to ground level with in-ground structures being left in place. The site is now occupied by buildings used by Australian Customs, a container storage yard, a major arterial road and grassed areas either side of the road. The site is now being considered for residential redevelopment by the Victorian State Government. Geological Conditions The site is located on the Yarra River Delta which originally comprised an area of low-lying Quaternary age deposits (parts were originally referred to as the West Melbourne Swamp). Over the years the site and surrounding area has been filled to achieve a ground surface level of around RL 2 to 3 (AHD). The geological sequence at the site comprises: • Fill - consisting of ash, coke, other gas works waste, rubble and building foundations. • Coode Island Silt (CIS) - a soft silty clay extending to a depth of around 15 m. • Fishermens Bend Silt (FBS) - a stiff to very stiff sandy clay. • Moray Street Gravels (MSG) - sands and gravels interbedded with silts and clays. • Dargile Formation - variably weathered sandstone and mudstone. Groundwater beneath the site is encountered in the fill just above the CIS at about RL 1. Flow is generally towards the south-west (ie. towards the Yarra River). The CIS has low vertical permeability with lateral permeability possibly being much higher due to the occurrence of sand lenses. The MSG is an aquifer with moderately high permeability but has limited use as a groundwater resource due to moderately high salinity. Investigation Method Initial investigation of the West Melbourne Gas Works involved excavation of 18 test pits and drilling of 26 boreholes ranging in depth generally between 3 and 8 m. One deep borehole was drilled to a depth of about 25 m to investigate potential contamination of groundwater in the MSG. Drilling and sampling methods were selected to minimise cross-contamination between boreholes and sample locations. To assist with identification of contaminated areas any visual and odorous evidence of contamination was noted for each soil sample. A range of other field tests were adopted including "field head-space tests" and a limited program of shallow gas probe testing using a photoionisation detector. Groundwater monitoring wells were installed at 10 locations within the gas works site or in the vicinity of the site to enable monitoring of groundwater levels and recovery of groundwater samples for chemical analyses. The wells consisted of PVC pipe with a slotted screen at the base. At most of these locations pairs of wells were installed as follows: • One well was constructed so that the slotted length of pipe was installed across the water table to enable sampling of water within the fill and any light non-aqueous phase liquids. • A second well was constructed so that the slotted length of pipe was installed within the CIS so that groundwater from within the CIS could be sampled. A monitoring well was also constructed in the deep borehole which was drilled to the MSG. Several seals were installed in the borehole during well construction to minimise risk of crosscontamination between the upper contaminated areas and the MSG. This well was considered necessary since the many hundreds of piles which had been installed on the site were driven to the MSG and the risk of contamination migrating along the sides of the piles down into the MSG was considered to be high. Results Soil and groundwater samples were subjected to chemical analyses as listed in Table 2. Included in this table are the ranges of concentration obtained for each analyte. The results of the chemical analyses combined with field results were used to provide preliminary contamination assessment of the site taking account of hazards to future users of the site and 22 WATER June 1990 Table 2 - Results of Chemical Analysis - West Melbourne Gas Works Soil (mg/ kg) Range of Number Values of Constituent Groundwater (mg/ L) Range of Number Values of Results pH DCM Extractable Matter ('Tar') Total Phenolics Sulfate Ammonia as N Cyanide - free Cyanide - complex Benzene Toluene Xylene Et hyl benzene Oil and Grease Chemical Oxygen Demand Biochemical Oxygen Demand Total Organic Carbon Total PAH Results 4.2-9.5 160-350,000 124 119 4.4-7.7 <0.03-270 < I0-22,600 <2-2000