Unsaturated solute transport through a forest soil during rain storm events

Abstract

The unsaturated transport of Br − through two distinct pore classes of an undisturbed pedon was investigated as a function of time and profile depth for a series of intermittent rain events. Water movement was monitored with tensiometers and a neutron probe and solute was collected in coarse and fine fritted glass solution samplers maintained at 2 and 10 kPa suction, respectively. The coarse samplers collected soil solution held at tensions of 0 to 2 kPa and are believed to have monitored predominately macropore and mesopore channels (large pores). The fine samplers are thought to have collected soil solution from predominately matrix mesopores (small pores) which are assumed to be approximately the lower pore size limit for gravitationally mobile water in this soil. The rate of tracer movement through large and small pores was dependent on the intensity and duration of a particular rain event as well as the antecedent soil water content. Small pores retained water and solutes longer and had a greater operational storage for ions than large pores. The data suggested that during the vertical flux of water through the soil profile, solutes were transported by convection and diffusion from small-pore regions to large-pore regions via hydraulic and concentration gradients, respectively, with small pores being a major source for solute transport in large pores. Because of physical and chemical nonequilibrium during rain events, solutes diffused from micropores (believed to have matric potentials > 10 kPa) into large and small pores. Solutes which were rapidly transported to greater depths through large pores increased the solute reserves in adjacent small pores at the same depths through a reversal of the mechanism described above. Solute reserves in small pores were also fed by significant vertical transport processes through this pore class.