HR: 10:20h
AN: B31F-01 INVITED     [PDF]
TI: Coupled modeling of transport and biogeochemical processes in aquifers - Model requirements, strength and limitations
AU: * Mayer, K
EM: umayer@eos.ubc.ca
AF: Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AB: Microbially mediated geochemical changes in aquifers may trigger a series of secondary reactions that include aqueous and surface complexation, ion exchange, and mineral dissolution-precipitation. Due to the coupled nature and the multitude of processes involved it is often difficult to identify the reactions controlling the system's overall evolution. Numerical models can be a useful component for identifying gaps and inconsistencies in conceptual models and for performing a more quantitative investigation of these systems. \\ \\ Suitable computer codes must allow for a general description of transport and reaction processes to facilitate the investigation of site-specific conditions. In recent years significant advances have been made in terms of model generality and applicability. Major advances include the consideration of mass balance equations for reactants and reaction products, the integration of biodegradation and thermodynamic models, and the development of novel approaches for simulating biogeochemical processes and reactive transport under variably saturated conditions.\\ \\ MIN3P is one of the codes capable of simulating coupled biogeochemical and hydrological processes on an increasingly mechanistic level. The simulation of column experiments and a hypothetical case study at the field scale illustrate how reactive transport modeling can be used. Modeling column experiments can be particularly fruitful, because detailed data can be collected to support the mechanistic approach. However, analysis of conceptual models is also beneficial on the field scale. The case study considered here describes natural attenuation of a petroleum hydrocarbon spill in an unconfined aquifer by multiple electron acceptors. The simulations also consider geochemical reactions triggered by contaminant degradation including the re-oxidation of reaction products during transport away from the source area. Comparing the results to contaminant plumes described in the literature suggests that geochemical trends can be described well for most reactants and reaction products. However, this would not have been possible without considering re-oxidation reactions. Remaining discrepancies with observed plumes suggest that mixing occurs between mobile plume water and stagnant background water along the flow path away from the source area. Simulations that make use of a dual porosity formulation provide a more realistic evolution of the concentrations of components such as O$_2$, Fe(II) and CH$_4$. The example is also useful in highlighting some of the model limitations, which include extensive data requirements to provide model constraints and non-unique results for components involved in several reactions.
DE: 0400 Biogeosciences
DE: 1030 Geochemical cycles (0330)
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Biogeosciences [B]
MN: 2003 Fall Meeting