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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Transport of Phosphate through Artificial Macropores during Film and Pulse Flow

^a The Royal Veterinary and Agricultural University, Department of Agricultural Sciences, Laboratory for Agrohydrology and Bioclimatology, Højbakkegård Allé 9, DK-2630 Taastrup, Denmark
^b The Royal Veterinary and Agricultural University, Department of Natural Science, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark

^* Corresponding author (bgj{at}kvl.dk)

Received for publication November 10, 2003. Flow through artificial macropores may occur as a water film along the macropore walls (film flow) or as moving water segments separated by air bubbles (pulse flow). To investigate the effect of macropore flow pattern (i.e., film and pulse flow) on the interaction of solutes with macropore walls, we studied orthophosphate (P) transport and sorption in artificial macropores. The experimental setup consisted of a column (height = 20 cm, diameter = 20 cm) homogenously packed with glass beads and fitted at outflow with a vertical artificial macropore placed below the column. The artificial macropore consisted of ceramic tubes (3 or 8 mm i.d.; 31.5 cm long) coated on the inside with iron oxide serving as phosphate sorbents. An orthophosphate solution containing 0.04 mg P L^–1 was applied at a rate of 9 to 12 mm h^–1 to the column, eventually causing macropore flow. In the 8-mm-i.d. tubes only film flow occurred. Pulse flow was dominating in the 3-mm-i.d. tubes. Generally, the flow patterns were reproducible and seldom did pulse flow replaced film flow or vice versa. During film flow, a significantly larger decrease in macropore P concentration per tube was observed relative to that with pulse flow events. However, pulse and film flow lead to almost the same amounts of P sorbed per unit surface area when exposed to the same solute P concentration. Comparison with P sorption capacity experiments indicated that the sorption rate, rather than the sorption capacity, controls the amount of sorbed P during macropore flow in the studied system.

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