Reactive modelling of 1,2-DCA and DOC near the shoreline

摘要

1,2-Dichloroethane (1,2-DCA) was found to be the most abundant compound among chlorinated hydrocarbons detected in a petrochemical plant in southern Italy. This site is located near the coastline, and it is set above an unconfined coastal aquifer, where seawater intrusion is present. The presence of organic and inorganic contaminants at this site has required the implementation of remediation strategies, consisting of pumping wells (hydraulic barrier) and a horizontal flow barrier. The purpose of this work was to assess the influence of salt water intrusion on the degradation rate of 1,2-DCA. This was done on a three-dimensional domain relative to a limited portion of a well characterized field site, accounting for density-dependent flow and reactive transport modelling of 1,2-DCA and Dissolved Organic Carbon (DOC). The modelling procedure was performed employing SEAWAT-4.0 and PHT3D, to reproduce the complex three-dimensional flow and transport domain. In order to determine the fate of 1,2-DCA, detailed field investigations provided intensive depth profile information. Different, kinetically controlled degradation rates were simulated to explain the observed, selective degradation of pollutants in groundwater. Calibration of the model was accomplished by comparison with the two different sets of measurements obtained from the MLS devices and from pumping wells. With the calibrated model, it was possible to distinguish between dispersive non-reactive processes and bacterially mediated reactions. In the non-reactive model, 1,2-DCA sorption was simulated using linear sorption coefficient determined with field data and 1,2-DCA degradation was simulated using a first order decay coefficient using literature data as initial guess. Finally, on the reactive transport model, where a two-step approach with partial equilibrium approach was implemented, the effects of neglecting the cation exchange capacity, omitting density-dependent flow, and refining the vertical discretization of the model were investigated. Comparison of results from various scenarios shows that geochemical changes in inorganic constituents can be used to improve the site's conceptual model, and establishes that natural degradation processes can be suitable for 1,2-DCA as a remediation option.