Aligning 1D and 2D axial symmetric transport simulations with observations: consequences for the reactive transport

With 1D and axial symmetric 2D finite element simulations of tracer transport in porous media columns, we conducted systematic studies of increasing heterogeneity on the resulting break-through curves (BTC).

While preferential flow can still have comparably little effect on the BTC of non-reactive tracer substances, for strongly adsorbing tracer preferential flow can significantly shift the break-through towards earlier arrival times by reducing the reactive surface area.

Motivation for conducting this systematic simulation study is our unique GeoPET method (Positron Emission Tomography) that allows the visualization of the spatio-temporal tracer distribution in geologic media (Richter et al., 2000; Gründig et al., 2007; Kulenkampff et al., 2008). The real complexity of thus directly observed flow fields is always surprising. Even for non-reactive tracers quantitative transport simulations typically remain a challenge due to the need for detailed knowledge of hydrodynamic parameter values in 3D with a certain critical spatial resolution.

Here we show simulation results obtained by the COMSOL Multiphysics code coupled with PHREEQC (Wissmeier and Barry, 2011) and compare them with measurement results (BTC) from reactive transport (herbicide MCPA on goethite/sand). CD-MUSIC adsorption model parameter were obtained from Kersten et al., (2014). The underlying structural heterogeneity of the simulated column experiments reflects preferential flow along the column boundaries caused by locally elevated permeability in otherwise homogeneous sand packing as verified by the GeoPET images from both conservative and reactive tracer experiments.

References
Gründig, M., Richter, M., Seese, A. and Sabri, O., 2007. Tomographic radiotracer studies of the spatial distribution of heterogeneous geochemical transport processes. Applied Geochemistry, 22: 2334-2343.
Kersten, M., Tunega, D., Georgieva, I. and Vlasova, N., 2014. Surface complexation modeling of herbicide adsorption by goethite: 1. 4-chloro-2-methylphenoxyacetic acid (MCPA). Environmental Science & Technology, submitted.
Kulenkampff, J., Gründig, M., Richter, M. and Enzmann, F., 2008. Evaluation of positron emission tomography for visualisation of migration processes in geomaterials. Physics and Chemistry of the Earth, 33: 937-942.
Richter, M., Gründig, M. and Butz, T., 2000. Tomographische Radiotracerverfahren zur Untersuchung von Transport- und Sorptionsprozessen in geologischen Schichten. Zeitschrift für Angewandte Geologie, 46(2): 101.
Wissmeier, L. and Barry, D.A., 2011. Simulation tool for variably saturated flow with comprehensive geochemical reactions in two- and three-dimensional domains. Environmental Modelling & Software, 26(2): 210-218.