Effect of extraction techniques on soil pore‐water chemistry

Abstract

The retention of contaminants in soil and overburden is often estimated using a solid/liquid partition coefficient, Kd, which lumps all the processes into an empirical value. Determination of this value in unsaturated porous media requires the separation of the pore water from the solid phase. Soil pore‐water recovery and composition were investigated in three chemically and texturally different mineral soils and one organic soil. The removal of pore water was achieved through centrifugation at low (1000 to 2500 rpm) and ultra (10,000 to 20,000 rpm) speeds, ceramic plate extraction and immiscible displacement. Pore‐water recovery was highest using ceramic plate extraction and lowest with displacement. Pore‐water quality was not affected by centrifugation time. However, the pore‐water concentrations of F, Cl, NO₃, Fe, and Na suggest that the effect of centrifuge speed on the element or ion of interest should be determined prior to extraction. Ceramic plates retained both cations and anions, and the immiscible displacent depressed the pH of the soil slurry affecting the pore‐water composition. Comparisons between distilled water extracts, standardized to field capacity moisture, and the centrifugate for a sand and an organic soil indicated that with low solid/liquid ratios, pore‐water concentrations are influenced by dissolution or desorption. Therefore, Kd values based on centrifuged pore water will be lower than those based on extraction/ desorption.