Transport and retention of colloids are important issues when addressing the risk of contamination of the aquatic environment. A field study tracer experiment was performed allowing a quantification of solutes and colloids along macropores in a sandy loam soil with a tile drain located at 1.2 m depth. Using a field sprayer, a 6-m2 plot was irrigated with 50 mm of water containing a mixture of 1-μm fluorescent microspheres (1.34 × 1010 melamine-resin microspheres [MS] L−1), bromide (0.14 g Br L−1), and the dye Brilliant Blue (2.2 g BB L−1) during a 4-h period. Before irrigation, the groundwater table was more than 0.7 m below drain depth. The tracer concentrations were measured in drain water and in soil collected along 15 stained preferential flow paths located in the drain trench and the adjacent till. It was found that the tracer concentration along biopores do not necessarily reflect the concentration of tracers transported through the biopore. Furthermore, it was recognized that at drain depth the proportion of soil samples from the drain trench with concentrations of Br and MS greater than two times the detection limit was significantly higher than in samples from the till. This implies that substances can accumulate along the drain pipe during heavy precipitation events. Finally, water transported through a few biopores well connected to the drain pipe resulted in drainage. Compared with the concentrations in the added tracer solution, the sampled drain water showed undiluted concentrations of BB and Br, whereas MS was reduced 150 times.

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