Determination of Solute Distributions in the Vadose Zone Using Downhole Electromagnetic Induction

We used down-hole electromagnetic induction to determine the distribution of a salt pulse added to an unsaturated soil profile. Water was applied to the surface at a flux of 2.7 cm/d over a 3-m by 3-m area. After approximately 1100 days, a 6.9-g/L slug of NaCl was added to the infiltration water, increasing the irrigation water conductivity from 80 mS/m to 1300 mS/m. Electrical conductivity measurements were collected weekly with an EM39 probe (Geonics LTD., Mississauga, ON, Canada) in 13 12-m deep boreholes installed over a 16-m by 16-m area. Three-dimensional images based on these measurements showed salt distributions reflecting the site stratigraphy and the shape of the wetted bulb under the infiltrometer. The calculated mass of salt in the profile was compared to the known mass of salt infiltrated at the surface. Following models designed primarily for agricultural soils, the measured bulk soil conductivity (ECa) was converted to soil water conductivity (ECw) using estimates of the volumetric water content from monthly neutron probe measurements and the percent clay from lab analysis of continuous core samples. The calculations showed a close correspondence over time (r2 = 0.98) between the calculated total mass of salt in the soil water and the known mass of salt infiltrated at the surface. The low water content (<15% by volume) and low bulk soil conductivity (<100 mS/m) at the test site presented more resistive conditions than previous studies of this type and were sources of uncertainty in the calculated ECw values. Errors in the calculated mass of salt were particularly evident during the initial stages of the salt infiltration and following the salt pulse, when the average ECw was low. Sensitivity analyses showed that the calculated ECw was strongly dependent on the assumed cutoff between immobile and mobile water content. Calculations were relatively insensitive to soil temperature and percent clay. The results of this study indicate that downhole EM methods show much promise for characterizing water and solute distributions in the vadose zone.