TDR estimation of electrical conductivity and saline solute concentration in a volcanic soil

Abstract

Relative to montmorillonitic or kaolinitic soils, volcanic soils have atypical dielectric characteristics that interfere with the applicability of the Time Domain Reflectometry (TDR) technique for soil moisture ( θ) determination when common, empirical calibration equations are used. This particular dielectric response affects estimation of salinity in volcanic soils. Six TDR-based methods to estimate bulk electrical conductivity ( σ a) on a range of KCl saline reference solutions were compared, with Nadler's method giving the best results ( R 1:1 2=0.988). Three models (linear, non-linear and empirical) for predicting soil solution electrical conductivity ( σ w) based on σ a and θ, were experimentally tested on 24 hand-packed soil columns varying in salinity (Br −) from 0.2 to 4.0 dS m −1, each in four θ levels (36–58%). Rhoades' linear model performed better, especially for large water contents, than the other two ( R 1:1 2=0.986 vs. 0.976 and 0.983, respectively). An interpretation in terms of mobile vs. immobile volumetric fractions of water present in volcanic soils is suggested as a possible explanation for these results. The empirical model resulted over-parameterized and an alternative equation with fewer non-correlated parameters, σ a=( aθ 2+ bθ) σ w+ cθ 2, is proposed and tested with good results in volcanic soils from the Canary Islands and New Zealand. The equation encompasses both the relative dielectric dominance of the mobile water fraction at high water content typical of volcanic soils, and of the immobile fraction at low water contents. Simultaneous measurements made with a standard four-electrode probe and TDR gave good correlation ( R 2=0.964). A good linear correlation was also found between tracer concentration in the soil solution and σ w ( R 2=0.960). Nadler's and the new empirical model also tested with good results under dynamic (flow) conditions during a miscible displacement experiment in a large monolith using bromide as a tracer. The method reveals itself as a robust tool for solute transport studies under controlled salinity conditions in a volcanic soil.