Effects of salinity and water content on apparent conductivity in an alluvial setting in the Canadian Prairies

Electromagnetic induction surveys are commonly used to assess soil salinity. In this study, a DualEM 1S instrument was used to survey an area in southern Manitoba to characterize sulphate-dominated salinity in clay-rich alluvial soils. The efficacy of predicting apparent conductivity (corrected for departure from low induction number responses) was determined by regression analysis of parameters including saturated-paste salinity, pore-water salinity, volumetric water content, porosity and combinations of these terms determined from 542 soil samples at 65 sampling sites. Predictors were depth-weighted using the V–H mode (exploration depth of 0.5 m) and the V–V mode (exploration depth of 1.5 m) of the electromagnetic instrument. The analysis yielded strong correlations of apparent conductivity (EC_a) with saturated-paste or pore water conductivity, with Pearson r² correlation coefficients exceeding 0.75 for the V–H mode and 0.86 for the V–V mode. The strong correlations of EC_a with salinity are explained by moisture conditions being close to field capacity during the survey. These results are similar to those for chloride-dominated salinity. The study yielded less commonly observed results related to the clay-rich soils, including a threshold value of ~ 10% below which volumetric water content does not affect EC_a, and an improved prediction of EC_a by dividing the porosity data based on clay content. The study results demonstrate the efficacy of using EC_a to parameterize sulphate-dominated salinity in clay-rich soils. They also show that salinity can be estimated relatively accurately from saturated paste conductivity at near-field capacity without considering moisture content.