Lk. Heng et al., Anion and cation leaching through large undisturbed soil cores under different flow regimes. 1. Experimental results, AUST J SOIL, 37(4), 1999, pp. 711-726
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.