Monitored Natural Attenuation Program for Liquefied Natural Gas Production Facilities Acquisition

Problem

As part of a corporate asset purchase agreement our client was acquiring properties that in some instances had major groundwater impacts from petroleum hydrocarbon products. The property seller had implemented a limited scope environmental site assessment to determine the environmental liability and quantify the cost set-aside necessary to complete the sale. A consultant for the seller claimed that Monitored Natural Attenuation was the only remedial option necessary, as the hydrocarbon plumes were not migrating. Pelorus was hired to support the buyers consultant in performing a similar evaluation using the same modeling software developed by the sellers consultant.

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Solution

Pelorus developed a monitored natural attenuation sampling and analysis program to evaluate groundwater quality and extent of contamination on each of the 28 properties. Over a period of 90-days Pelorus analyzed 300 groundwater samples for MNA indicator parameters including inorganic electron acceptors and light hydrocarbon gases (i.e., dissolved methane, ethane, ethene, propane and butane levels).

Using the Bioscreen modeling software, the sites were modeled and evaluated to determine whether the plumes were migrating, static or retracting. In addition, indigenous biodegradation rate constants were estimated from historical data sets using the Buscheck & Alcanter regression analysis procedure. These rates were used to corroborate the Bioscreen model or in some instances to modify the Bioscreen input parameters in order to obtain an accurate assessment of plume dynamics. Pelorus further estimated the total source mass reduction that would be required for sites with migrating plumes to become MNA candidates.

Out of the 28 sites in question only 3 had plumes that were retracting and 12 had plumes that appeared to be static or non-migrating. The remaining 13 sites had major source contamination five of which had NAPL product thickness in excess of 1.5 feet. Using the supplemental site characterization data collected and the model output information a Corrective Measures Assessment of remedial alternatives was performed. The liability set-aside value increased to $ 23.5 million resulting in a potential cost savings to our client of $ 20 million dollars.

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