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

January 4, 2011

The concept of a nonreactive or conservative tracer, commonly invoked in investigations of solute transport, requires additional study in the context of electrical geophysical monitoring. 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 conductivity may reflect not only solute 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; and (3) surface conductance does not notably affect estimated 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 transport processes of interest.

Publication Year 2011
Title Quantifying solute transport processes: Are chemically "conservative" tracers electrically conservative?
DOI 10.1190/1.3511356
Authors Kamini Singha, Li Li, Frederick D. Day-Lewis, Aaron B. Regberg
Publication Type Article
Publication Subtype Journal Article
Series Title Geophysics
Index ID 70074338
Record Source USGS Publications Warehouse
USGS Organization OGW Branch of Geophysics; Office of Ground Water; Toxic Substances Hydrology Program