(256a) Biodegradation of Contaminants in Karst Groundwater, a Dual Continuum Model

The advection dispersion equation (ADE) as applied to pipe flow often successfully models solute transport and biodegradation along major karst groundwater features. However, the residence time distribution (RTD) generated by the transient solution of the ADE as applied to pipe flow often under estimates biodegradation of karst groundwater contaminants due to its failure to predict the long upper tail of tracer response curves.   The slower solute transport associated with flow in small fractures adjacent to karst conduits may explain the tendency of tracer response curves to have a long upper tail compared to the diminishing tail predicted by the ADE as applied to pipe flow.  It is in this context that the case is made for a dual continuum transport model for karst.  In the dual continuum model, the two adjacent continua share a common boundary.  Due to the extreme differences in the flow regimes near the common boundary, the velocity vectors at the boundary are characterized by very steep gradients. A dual continuum model was developed using an adaptive finite element mesh system that addresses these steep velocity gradients using fuzzy type switching between the flow equations for the adjacent continua.  This effectively interpolates the common boundary as a continuous polynomial and avoids the introduction of high frequency spatial components into the solution.  The model was developed as a staged solution in that it solves for the steady state velocity and pressure fields and then uses these fields as inputs for the second stage solution of the advection dispersion equation describing contaminant transport. Initial testing of the model demonstrates that it is capable of modeling a wide variety of karst tracer response behavior ranging from near Gaussian or symmetric responses to bimodal responses.