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Phosphate and Tritium Transport through Undisturbed Subsoil as Affected by Ionic Strength

Department of Agricultural Sciences, Laboratory of Soil Fertility and Plant Nutrition, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark.
Chemistry Department, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark.
The Danish Geotechnical Institute, Maglebjergvej 1, DK-2800 Lyngby, Denmark.

^* Corresponding author (mbj{at}fsl.dk).

ABSTRACT

Storm flow can cause serious subsurface losses of dissolved orthophosphate from soils that under low flow conditions effectively retain orthophosphate (P_i). To test if storm flow loss of P_i can be related to preferential flow and/or decrease in ionic strength of mobile solution we applied combined pulses of H₂³²PO^–₄ (³²p) and tritium (³H₂O) to an undisturbed column of clayey subsoil (diam. 0.5 m, height 0.73 m) subjected to water saturated steady flow with either distilled water (0.003 mS cm^–1) or a dilute salt solution (0.2 mS cm^–1). The pulse applications resulted in narrow breakthroughs of ₃₂P with peak arrivals after displacement of only 2 to 4% of the total water content. In comparison, the ³H₂O-breakthrough curves had peak arrivals after displacement of 8 to 18% of the total water content and showed extensive tailing. In distilled water approximately twice as much phosphate was transported through the column than in the diluted salt solution, although the recovery percentage in both cases was <1% of the applied ₃₂P-mass. Slicing of the column after dye application confirmed the presence of heterogeneous flow paths. The results suggest that the positive correlation observed between concentration of dissolved P_i and field effluent discharge rate is restricted to rainfall intensities that initiate preferential flow, and that the P_i-transport is increased the more dilute the percolating solution remains.