MODELING LONG-TERM TRENDS OF CHLORINATED ETHENE CONTAMINATION AT A PUBLIC SUPPLY WELL

A mass-balance solute-transport modeling approach was used to investigate the effects of dense non-aqueous phase liquid (DNAPL) volume, composition, and generation of daughter products on simulated and measured long-term trends of chlorinated ethene concentrations at a public supply well tapping an unconsoladated coastal-plain aquifer. The model was built by telescoping a calibrated regional three dimensional MODFLOW model to the approximate capture zone of a public supply well that has a history of chlorinated ethene contamination. The flow-field of the local model was then used to simulate the interactions between naturally-occurring organic carbon that acts as an electron donor, and dissolved oxygen (DO), chlorinated ethenes (CEs), ferric iron, and sulfate that act as electron acceptors using the Sequential Electron Acceptor Model in three dimensions (SEAM3D) code. The modeling results indicate that asymmetry between rapidly rising and more gradual falling concentration trends over time suggests a DNAPL rather than a dissolved source of CEs. Peak concentrations of CEs are proportional to the volume and composition of the DNAPL source. The persistence of contamination, which can vary from a few years to centuries, is proportional to DNAPL volume but is unaffected by DNAPL composition. These results show that monitoring CE concentrations in raw water produced by impacted public supply wells over time can provide useful information concerning the nature of contaminant sources and the likely future persistence of contamination.