Quantifying solute transport processes: Are chemically “conservative” tracers electrically conservative?

  • Singha K
  • Li L
  • Day-Lewis F
 et al. 
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Abstract

The concept of a nonreactive or conservative tracer, com- monly invoked in investigations of solute transport, requires additional study in the context of electrical geophysical mon- itoring. Tracers that are commonly considered conservative may undergo reactive processes, such as ion exchange, thus changing the aqueous composition of the system.As a result, the measured electrical conductivitymayreflect not only sol- ute transport but also reactive processes.We have evaluated the impacts of ion exchange reactions, rate-limited mass transfer, and surface conduction on quantifying tracer mass, mean arrival time, and temporal variance in laboratory-scale column experiments. Numerical examples showed that 1 ion exchange can lead to resistivity-estimated tracer mass, velocity, and dispersivity that may be inaccurate; 2 mass transfer leads to an overestimate in the mobile tracer mass and an underestimate in velocity when using electrical meth- ods; and 3 surface conductance does not notably affect esti- mated moments when high-concentration tracers are used, although this phenomenon may be important at low concen- trations or in sediments with high and/or spatially variable cation-exchange capacity. In all cases, colocated groundwa- ter concentration measurements are of high importance for interpreting geophysical data with respect to the controlling transport processes of interest.

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Authors

  • Kamini Singha

  • Li Li

  • Frederick D. Day-Lewis

  • Aaron B. Regberg

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