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

The movement of anions and cations under different unsaturated flow regimes was studied in 2 large undisturbed cores from a soil under pasture. Sequen tial leaching of calcium sulfate (CaSO4) and potassium bromide (KBr) soluti ons was carried out at water fluxes which ranged from a normal 5-20 mm/h ra te 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 leach ing behaviour of the applied cations and anions under these contrasting flo w conditions, and determine the relative influence of soil physical and che mical processes in governing ion movement. A better understanding of this b ehaviour should help in the development of improved practices to reclaim sa line soils, ameliorate sodic soils, and minimise unwanted leaching of nutri ents. In the slow flow core, miscible displacement breakthrough curves (BTCs) wer e observed for bromide, with sulfate movement retarded relative to the brom ide. Cation exchange reactions occurred in the slow flow core, with calcium generally the dominant cation balancing anions during their movement throu gh the soil. When KBr solution was substituted for CaSO4, potassium ions re placed 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-appl ied 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 charac teristics and showed little influence of surface chemical reactions.