Contaminant concentrations in aquifers can change as chemical reactions occur during groundwater transport through the aquifer. For instance, denitrification, in which the contaminant nitrate is converted to molecular nitrogen, reduces nitratecontaminant loads. It is useful to understand the rates at which denitrification and other reactions occur in an aquifer to improve understanding and prediction of contaminant migration.
However, estimates of denitrification and other reaction rates are often based on simplified transport models, typically by assuming all water in a sample has the same travel time and reaction history, an unrealistic assumption in many cases because of mixing of water in complex, geologically heterogeneous natural systems. To investigate the effects of mixing during transport in a heterogeneous environment on reaction rate estimates, Green et al. study an aquifer in the San Joaquin Valley using field observations and numerical models.
They find that apparent isotope fractionation and reaction rate estimates derived from field data using simple models are quite different from intrinsic (true) values from more realistic models accounting for heterogeneity. In fact, the apparent and true rates can differ by an order of magnitude or more. Moreover, the true parameter values for isotope fractionation and oxygen inhibition are in much better agreement with laboratory data than field-based estimates that do not account for mixing. They conclude that the effects of mixing during transport through a heterogeneous aquifer are important and that models accounting for these effects can improve forecasts of reaction progress.
Authors of the study include: Christopher T. Green, Barbara A. Bekins: U.S. Geological Survey, Menlo Park, CA, USA John Karl Böhlke: U.S. Geological Survey, Reston, VA, USA Steven P. Phillips: U.S. Geological Survey, Sacramento, CA, USA.
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