HR: 0830h
AN: H11G-0936    [PDF]
TI: The Role of Micromixing in Reactive Transport in Groundwater: Theory and Field Applications
AU: * Robinson, B A
EM: robinson@lanl.gov
AF: Earth and Environmental Sciences Division Los Alamos National Laboratory, Mail Stop T003, Los Alamos, NM 87545 United States
AU: Viswanathan, H S
EM: viswana@lanl.gov
AF: Earth and Environmental Sciences Division Los Alamos National Laboratory, Mail Stop T003, Los Alamos, NM 87545 United States
AB: Advances in computational resources and numerical codes make possible the simulation of complex reactive transport problems involving biogeochemical reactions, sorption, and mineral dissolution-precipitation reactions. In aqueous transport systems, a thorough understanding of both chemical and hydrodynamic processes is required to attain a predictive capability. To address the issue of mixing, we apply the theory of micromixing, first introduced in the chemical reaction engineering literature, to the topic of reactive transport in groundwater. For all but the simplest linear kinetic and sorption models, the fate and transport of a reactive solute depends on the residence times and the details of small-scale mixing. The latter phenomenon, called micromixing, is important because it brings into close proximity chemical species that react, and it controls the local concentrations in a flowing system. Solutes with reaction rates or sorption isotherms that depend on species concentration will therefore be affected by micromixing. In this new theory, two models for micromixing are introduced, the minimum and maximum mixedness models, that provide bounds on the extent of reaction or retardation behavior within the constraints imposed by the residence time distribution (RTD) of a conservative solute in the same flow system. These mixing models prescribe the latest or earliest permissible mixing of parcels of fluid of different residence times. This construct provides bounds on the degree of reaction of a reactive solute for nonlinear rate laws or sorption isotherms. Simulation results using the bounding models show that micromixing effects are most important for nonlinear reactions, solute pulses of short duration, and systems with broad RTD curves. Use of these models is a straightforward and practical way to investigate the importance of a phenomenon for which data are seldom available, and whose impact on groundwater reactive transport models has not been studied to date in a systematic, bounding manner. An important requirement of any such new theory is to demonstrate its applicability to field data. In this study we use the theory to interpret interwell sorbing tracer data in fractured volcanic tuffs. Using conservative tracer data to establish the RTD, micromixing models are then used to interpret tracers undergoing nonlinear sorption. In addition to providing bounds that are useful in field test interpretation, the theory developed here may provide a practical way to incorporate complex reactive transport processes in contaminant mass flux models in a manner that factors the complexities of mixing into the analysis.
DE: 1010 Chemical evolution
DE: 1803 Anthropogenic effects
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2003 Fall Meeting