Cost-Effective Modeling of Contaminant Transport in a Complex Hydrogeologic Setting Using an Analytical Model

A frequent assumption made with respect to modeling groundwater flow and contaminant transport is that complex hydrogeologic settings or boundaries or highly dynamic systems can only be modeled accurately using numerical models. Development of such a model, however, is generally extremely expensive. More often than not, the expense is simply not warranted for the vast majority of projects. Analytical models should not be overlooked in these situations. Reasonable results can be obtained with careful implementation of an appropriate analytical model.

Successful application of a 2-D analytical model for contaminant transport will be described for a tidally-driven, complex glaciofluvial hydrogeologic setting. Simple geologic modeling was implemented to estimate hydraulic conductivity from slug-tests. Average flow direction was determined from a tidal study, and transverse dispersivity was assumed to be large due to the variation in flow direction. Seasonal temperature fluctuations and anticipated shut down of biodegradation was accommodated by developing a model from literature data for biodegradation of trichlorobenzene and its derivatives. Average, optimal first-order decay constants were estimated from the model based on seasonal ground water temperatures. Appropriate optimal decay constants for biodegradation daughter products were developed based on nuclear decay of parent and daughter products. Extension to the field was implemented using a fraction of the optimal decay constants. Source strength was estimated based on effective solubility from a DNAPL that drained into a borehole on the adjoining property. That borehole penetrated a confining layer allowing contamination to reach a deeper aquifer. Concentration brackets were developed based on the estimated range of hydraulic conductivity. The bracketed results included concentrations observed at the receptor well on our client’s property. The model results helped convince the regulator that our client was not responsible for the dichlorobenzene contamination that began appearing in a well on our client’s property in the late 1990s.