MFIT 1.0.0: Multi-Flow Inversion of Tracer breakthrough curves in fractured and karst aquifers

Abstract

<jats:p>Abstract. More than half of the Earth's population depends largely or entirely on fractured or karst aquifers for their drinking water supply. Both the characterization and modeling of these groundwater reservoirs are therefore of worldwide concern. Artificial tracer testing is a widely used method for the characterization of solute (including contaminant) transport in groundwater. Tracer experiments consist of a two-step procedure: (1) introducing a conservative tracer-labeled solution into an aquifer, usually through a sinkhole or a well, and (2) measuring the concentration breakthrough curve (BTC) response(s) at one or several downstream monitoring locations, usually spring(s) or pumping well(s). However, the modeling and interpretation of tracer test responses can be a challenging task in some cases, notably when the BTCs exhibit multiple local peaks and/or extensive backward tailing. MFIT (Multi-Flow Inversion of Tracer breakthrough curves) is a new open-source Windows-based computer package for the analytical modeling of tracer BTCs. This software integrates four transport models that are all capable of simulating single- or multiple-peak and/or heavy-tailed BTCs. The four transport models are encapsulated in a general multiflow modeling framework, which assumes that the spatial heterogeneity of an aquifer can be approximated by a combination of independent one-dimensional channels. Two of the MFIT transport models are believed to be new, as they combine the multiflow approach and the double-porosity concept, which is applied at the scale of the individual channels. Another salient feature of MFIT is its compatibility and interface with the advanced optimization tools of the PEST suite of programs. Hence, MFIT is the first BTC fitting tool that allows for regularized inversion and nonlinear analysis of the postcalibration uncertainty of model parameters. </jats:p>

Delineation of discrete conduit networks in karst aquifers via combined analysis of tracer tests and geophysical data

“…We propose to assess the number of distinct tracer flow paths via an inverse modeling procedure implemented in MFIT software (Bodin, 2020). MFIT is a BTC fitting tool that combines different analytical transport models with PEST optimization routines (Doherty, 2019a(Doherty, , 2019b.…”

“…(diffusion) process in the SFDM. A complete mathematical description of these models has been provided by Bodin (2020) and is not repeated here for conciseness. While it is acknowledged that the actual geometry of the karst conduit system experienced by a tracer could be much more complex than that depicted in Fig.…”

“…Figure 1. Schematic layout of the multiflow modeling approach implemented in MFIT software (Bodin, 2020). The tracer transport from the injection site to the monitoring point is assumed to occur in a flow network comprising N independent onedimensional channels.…”

“…The multiflow SFDM was not adopted here because the typical duration of the considered tracer experiments, from a few hours to a few days, was insufficient for diffusion processes to be of influence. Actually, more precisely, this model could be fitted to the obtained HES tracer BTCs but without a better performance than that of the multiflow ADE model, which is simpler, or only if unrealistic diffusion parameter values were considered, e.g., Bodin (2020). The optimal (minimum) number of channels N * required to fit the above BTCs with the multiflow ADE and 2RNE models was determined as described in section 2.…”

“…Finally, interwell tracer test data are considered to maximize the information value of geophysical data. More precisely, tracer breakthrough curves (BTCs) are inverted with a recently developed software (Bodin, 2020) prior to the processing of geophysical data. BTC inversion allows us to identify the minimum number of flow paths between the tracer injection and monitoring locations, which is subsequently applied in the multiple path routing formulation.…”