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Within a 10-ha field site, annual surface applications of atrazine were made to a 0.37-ha bermed plot from 1999 to 2001, and a potassium-bromide tracer was applied in 1993 and 2000. The field site includes an intensive network of 40 shallow monitoring wells, 12 deep monitoring wells, a precipitation gauge, and a H-flume with an automated sampler to quantify and sample runoff. In the spring of 2003, twenty-one continuous soil cores were collected to a depth of approximately 3.4 m and analyzed for atrazine, bromide, and water content. Using a mass balance approach, the relative amounts of water transported from the site as runoff, subsurface plug-flow, and subsurface preferential flow were calculated.

When detected, atrazine concentrations were typically just above the analytical detection limit of 7.5 g per liter of soil pore water. Although atrazine appeared relatively persistent within the soil profile, a quantitative analysis of its transport proved difficult due to the low concentrations. The bromide distributions demonstrated apparent classic plug flow with the exception that up to 82% (mean of 58%) of the applied bromide was absent from the soil profiles. The missing bromide was attributed to subsurface preferential flow, which is supported by the detection of atrazine and bromide in the shallow monitoring wells 150 m from the application area soon after application. One-dimensional plug-flow modeling of the bromide transport adequately predicted the measured bromide profiles when the applied bromide mass was left as an unknown variable. Results of the modeling suggest that without prior knowledge of the mass of a released solute, quantifying the true mass of solute released from soil core analysis alone may be difficult if preferential flow is occurring.