Abstract

We conducted a field experiment to evaluate the ability of downhole electromagnetic induction (EM) measurements to track the migration of a salt plume in the vadose zone. A 6.9 g L−1 slug of NaCl was applied to a 3 by 3 m area for 85 d at a flux of 2.7 cm d−1. Electrical conductivity measurements were collected weekly, using 13 12-m boreholes installed in a 15 by 15 m area. The measured bulk soil electrical conductivity (ECa) was converted to soil water electrical conductivity (ECw) using water contents from neutron probe measurements and clay contents from soil cores. The calculated mass of salt in the profile agreed well with the known mass of salt infiltrated when the appropriate immobile water content was assumed. The low water content, θw, (<15% by volume) and low bulk soil electrical conductivity, ECa, (<100 mS m−1) measured at the test site presented more resistive conditions than previous studies of this type and were a cause of uncertainty in the results. Good agreement between calculated and known ECw resulted from the ability to measure θw and ECa in the same locations. Sensitivity analyses showed that the calculated ECw was strongly dependent on the assumed upper limit of immobile water content, and less sensitive to soil temperature and clay content. The results of this study demonstrate that downhole EM methods can accurately characterize water and solute distributions in the vadose zone.

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