MEASUREMENT OF VARIATION IN SOIL SOLUTE TRACER CONCENTRATION ACROSS ARANGE OF EFFECTIVE PORE SIZES

Solute transport concepts in soil are based on speculation that solute s are distributed nonuniformly within large and small pores. Solute co ncentrations have not previously been measured across a range of pore sizes and examined in relation to soil hydrological properties. For th is study, modified pressure cells were used to measure variation in co ncentration of a solute tracer across a range of pore sizes. Intact co res were removed from the site of a field tracer experiment, and soil water was eluted from 10 or more discrete classes of pore size. Simult aneous changes in water content and unsaturated hydraulic conductivity were determined on cores using standard pressure cell techniques. Bro mide tracer concentration varied by as much as 100% across the range o f pore sizes sampled. Immediately following application of the bromide tracer on field plots, bromide was most concentrated in the largest p ores; concentrations were lower in pores of progressively smaller size s. After 27 days, bromide was most dilute in the largest pores and con centrations were higher in the smaller pores. A sharp, threefold decre ase in specific water capacity during elution indicated separation of two major pore size classes at a pressure of 47 cm H2O and a correspon ding effective pore diameter of 70 mum. Variation in tracer concentrat ion, on the other hand, was spread across the entire range of pore siz es investigated in this study. A two-porosity characterization of the transport domain, based on water retention criteria, only broadly char acterized the pattern of variation in tracer concentration across pore size classes during transport through a macroporous soil.