A groundwater flow and solute transport model of sequential biodegradation of multiple chlorinated solvents in the surficial aquifer, Palm Bay, Florida

The primary objective of this research was to analyze available field data and quantitatively examine the effects of advection, hydrodynamic dispersion, sorption, recharge dilution, and determine long-term field-scale anaerobic biodegradation rate constants for dissolved trichloroethene (TCE), cis 1,2 dichloroethene (DCE), and vinyl chloride (VC) in groundwater. In addition, the relative importance of pump-and-treat and natural biodegradation during contaminant remediation was examined. The Harris Palm Bay, FL facility was selected for this analysis because it is has a well characterized, relatively homogenous hydrogeologic setting, and is naturally anaerobic.;Using the MT3D99 multi-species reactive transport model, a manual calibration was performed and met the calibration criteria during both the calibration period (1981 to 1991) and prediction period (1992 to 2001). Within the central core of the plumes where most of the contaminant mass occurs, the simulated concentrations of TCE, DCE, and VC were generally within a factor of three of observed concentrations. This level of accuracy was less than the observed variability of the concentration data and was met over a 20-year period in which levels changed significantly from approximately 10,000 mug/L in 1981 to near nondetect (<1 mug/L) in 2001.;Results of this study indicate that biodegradation was the dominant removal mechanism for cleanup. The calibrated first-order field-scale biodegradation rate constants (half lives) for TCE, DCE (cis), and VC were 0.46 yr -1 (550 days), 0.53 yr-1 (480 days), and 0.43 yr-1 (590 days), respectively for the 25 EC groundwater temperatures at the site. For the period from commencement of pumping in 1985 until 2001, the recovery wells removed approximately 2.3%, 9.4%, and 24%, respectively, of the TCE, DCE, and VC mass removed from groundwater in the study area by 2001. Over this same period (1985--2001), natural biodegradation removed approximately 94%, 88%, and 73%, of the TCE, DCE, and VC mass removed from groundwater in the study area. Additional model simulations indicated that the cleanup standards are reached approximately 30 percent faster with pump-and-treat than with natural biodegradation as the sole remedy at this study area.