Anion and cation leaching through large undisturbed soil cores under different flow regimes. 1. Experimental results

Abstract

The movement of anions and cations under different unsaturated flow regimes was studied in 2 large undisturbed cores from a soil under pasture. Sequential leaching of calcium sulfate (CaSO₄) and potassium bromide (KBr) solutions was carried out at water fluxes which ranged from a normal 5–20 mm/h rate in one core (the slow flow core), to unusually high values up to 350 mm/h in the other (the fast flow core). The objective was to examine the leaching behaviour of the applied cations and anions under these contrasting flow conditions, and determine the relative influence of soil physical and chemical processes in governing ion movement. A better understanding of this behaviour should help in the development of improved practices to reclaim saline soils, ameliorate sodic soils, and minimise unwanted leaching of nutrients.

In the slow flow core, miscible displacement breakthrough curves (BTCs) were observed for bromide, with sulfate movement retarded relative to the bromide. Cation exchange reactions occurred in the slow flow core, with calcium generally the dominant cation balancing anions during their movement through the soil. When KBr solution was substituted for CaSO₄, potassium ions replaced calcium ions on the exchange sites, resulting in an increase in the calcium concentration in the effluent. In the fast flow core, however, flow was highly preferential and the water flux very variable. The surface-applied solutes appeared very rapidly in the effluent, irrespective of whether they were cations or anions. Under these conditions, solution cation/anion interactions with the soil surfaces did not appear to approach equilibrium, so that the leaching process was dominated by the soil’s water flow characteristics and showed little influence of surface chemical reactions.