Field study on the local variability of soil water content and solute concentration

We characterized temporal and spatial variables describing hydraulic and so lute transport in a heterogeneous soil. Under field conditions, bulk densit y, water content and concentration of three applied tracers (conservative B r-, nitrate NO3-, and a pesticide BENTAZON) were measured after destructive sampling and compared to continuous measurements. A comparison of destruct ive sampling and continuous measurements were used to investigate the held. Soil volumetric samples were obtained from trenches on a grid of 1.2 x 1.1 x 0.1 m in a fallow area of approximately 1300 m(2) to generate two dimens ional (2D) interpolated maps. Bulk density, water content, and solute conce ntration data sets were evaluated by geostatistical analysis in order to ch oose the best method to build interpolations. Semivariogram analyses for vo lumetric water content (theta), bulk density (rho(d)) and solute concentrat ions (NO3- and BENTAZON) showed no autocorrelation possessing only a pure n ugget effect. 2D interpolated maps of these soil parameters were constructe d using the Radial Basis Function method that allowed calculation of vertic al water content and solute concentration profiles and the associated mean (mu) and variance (sigma) distributions. Vertical neutron and TDR water con tent profiles were successfully simulated with a discrete geometry approach using the 2D interpolated maps. Destructive sampling for water content and continuous measurements using neutron and TDR probes, revealed consistent results. Comparison of solute concentration from destructive sampling and c eramic solution samplers is less satisfactory at this site owing to both th e heterogeneity and the inadequacy of ceramic solution samplers to sample t he total soil concentration. Finally, preferential flow pathways can be vis ualized on the 2D interpolated maps of the concentration of Br-, BENTAZON, and NO3-. The observed distribution of tracers provides clear evidence of t he influence of both soil hydraulic properties and geochemical heterogeneit ies which must be incorporated to predict water and solute fluxes. (C) 1999 Published by Elsevier Science B.V. All rights reserved.