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Publications > Singha and others, 2010.

Hydrogeophysical monitoring of transport processes — Are chemically "conservative" tracers electrically conservative?

K. Singha (ksingha@psu.edu)
Dept of Geosciences, Penn State University, University Park, PA, USA

L. Li (lili@eme.psu.edu)
Dept of Energy and Mineral Engineering, Penn State University, University Park, PA, USA

F.D. Day-Lewis (daylewis@usgs.gov)
Branch of Geophysics, U.S. Geological Survey, Storrs, CT, USA

A.B. Regberg (aregberg@geosc.psu.edu)
Dept of Geosciences, Penn State University, University Park, PA, USA

Abstract

The concept of a non-reactive 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 not only reflect solute transport but also reactive processes. Here, we evaluate the impacts of ion exchange reactions, rate-limited mass transfer, and surface conduction on quantifying tracer mass, mean arrival time, and temporal variance in lab-scale, column experiments. We show in numerical examples 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 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 phenomena may be important at low concentrations or in sediments with high and/or spatially variable cation-exchange capacity. In all cases, co-located groundwater concentration measurements are of high importance for interpreting geophysical data with respect to the controlling transport processes of interest.

Final copy as submitted to the American Geophysical Union for publication as: Singha, K., Li, L., Day-Lewis, F.D., and Regberg, A.B., 2010, Hydrogeophysical monitoring of transport processes — Are chemically "conservative" tracers electrically conservative?, [abs.], in 2010 Fall Meeting, San Francisco, California, 13-17 December 2010, proceedings: American Geophysical Union, Washington, D.C., abstract H13G-01 (invited).