Monte Carlo vadose tone model for soil remedial criteria

Many vadose zone models are available for environmental remediation, but fe w offer the procedures for verifying model predictions with field data and for dealing with uncertainties associated with model input parameters. This article presents a modified model combining a one-dimensional vadose-zone transport model and a simple groundwater mixing model with a function of Mo nte Carlo simulation (MCS). The modified model is applied to determine soil remedial concentrations for methyl tertiary butyl ether (MTBE). The modifi ed model generates a distribution of MTBE ground-water concentrations at th e point of compliance. This distribution can be used to estimate the risk o f exceeding groundwater quality standard given soil remedial concentrations . In a case study, soil remedial concentration for MTBE is established to b e 5 mug/kg, with a 95% and 10 mug/kg with a 50% probability that groundwate r concentration will not exceed the water qualify objective of 13 mug/L. Fu rthermore, this study uses MOS to investigate uncertainties of model input parameter hydraulic conductivity (K). One set of data (KI) is based on the results of hydraulic conductivity laboratory tests, and the other (K2) is b ased on the results of slug tests conducted in the field. As expected, the laboratory data show :smaller K values than the field data. The comparison of the MCS results obtained from the two sets of K data indicates that the MTBE groundwater concentrations calculated based on KI are generally 160 to 625% greater than those calculated based on K2 at the same percentiles of the MCS distribution. A higher soil remedial concentration of 9 mug/kg is t hen calculated based on the MCS results from K2 at 95%ile and 19 mug/kg at 50%ile.