Aaron B. Regberg

Quantifying solute transport processes: Are chemically "conservative" tracers electrically conservative?

The concept of a nonreactive or conservative tracer, commonly invoked in investigations of solute transport, requires additionalstudyinthecontextofelectricalgeophysicalmonitoring. Tracers that are commonly considered conservative may undergo reactive processes, such as ion exchange, thus changingtheaqueouscompositionofthesystem.Asaresult, themeasuredelectricalconductivitymayreflectnotonlysolute 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 methods;and3surfaceconductancedoesnotnotablyaffectestimated moments when high-concentration tracers are used, although this phenomenon may be important at low concentrations or in sediments with high and/or spatially variable cation-exchange capacity. In all cases, colocated groundwater concentration measurements are of high importance for interpreting geophysical data with respect to the controlling transportprocessesofinterest.