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Modeling Contaminant Transport and Remediation at the AKSA Acrylonitrile Spill Site

Sema Sevinç Şengör

2002/08

The August 1999 earthquake in Turkey damaged three "acrylonitrile" (AN) storage tanks at a plant producing synthetic fiber by polymerisation. As a result, 6500 tons of AN spilled into the concrete pools at the tank area. Some of the spilled AN was mainly lost to the sea through surface runoff, while some leaked into soil through the large cracks formed in the concrete pools, and subsequently reached the underlying shallow coastal aquifer at the site. To determine the areal extent of the chemical plume and monitor the contaminant concentrations in groundwater, a total of sixteen monitoring wells were installed shortly after the earthquake incident. Based on the field data, two pumping wells and four French Drain-type drainage trenches were put into operation to pump the contaminated on-site groundwater. The field work and data collection were undertaken by AKSA.

A numerical modeling study was carried out to analyse the groundwater flow and contaminant (AN) transport at the spill site. The simulations were performed using the software GMS (Groundwater Modeling System), a groundwater modeling package for flow and contaminant transport. The 2-D and 3-D transient flow and transport models were first calibrated using the first 266 days of observed head and concentration data and then verified using the remaining 540 day observed data set. The subsequent simulation results were analysed to evaluate the effectiveness of pump and treat system design and natural dilution processes. Off-site migration of the contaminant plume was kept under control within the site boundaries owing to the favorable geology of the site, the characteristics of the local groundwater flow regime and the pumping operations. The applied pump-and-treat system was effective at high permeability zones, but not fully effective at low permeability zones. The results of long term simulations for unconfined aquifer showed that the size of the plume in the high permeability zone shrinked significantly due to dilution by natural recharge. However, in the low permeability zone, it was not significantly affected. Based on a conservative approach to predicting AN transport under worst case conditions, long term simulations also indicated that AN concentration in the confined aquifer expected to be in 200-250 mg/L range.

Middle East Technical University, Department of Environmental Engineering

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