PHOSPHATE AND TRITIUM TRANSPORT THROUGH UNDISTURBED SUBSOIL AS AFFECTED BY IONIC-STRENGTH

Storm flow can cause serious subsurface losses of dissolved orthophosp hate from soils that under low now conditions effectively retain ortho phosphate (P-i). To test if storm flow loss of P-i can be related to p referential flow and/or decrease in ionic strength of mobile solution we applied combined pulses of (H2PO4-)-P-32 ((32)p) and tritium ((H2O) -H-3) to an undisturbed column of clayey subsoil (diam, 0.5 m, height 0.73 m) subjected to water saturated steady flow, with either distille d water (0.003 mS cm(-1)) or a dilute salt solution (0.2 mS cm(-1)). T he pulse applications resulted in narrow breakthroughs of P-32 with pe ak arrivals after displacement of only 2 to 4% of the total water cont ent. In comparison, the (H2O)-H-3-breakthrough curves had peak arrival s after displacement of 8 to 18% of the total water content and showed extensive tailing, In distilled water approximately twice as much pho sphate was transported through the column than in the diluted salt sol ution, although the recovery percentage in both cases was <1% of the a pplied P-32-mass. Slicing of the column after dye application confirme d the presence of heterogeneous flow paths, The results suggest that t he positive correlation observed between concentration of dissolved Pi and field effluent discharge rate is restricted to rainfall intensiti es that initiate preferential flow, and that the P-i-transport is incr eased the more dilute the percolating solution remains.