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DIVISION S-1—SOIL PHYSICS

Measured and Predicted Solute Leaching from Multiple Undisturbed Soil Columns

^a NSTL, 2150 Pammel Dr., Ames, IA 50011-3120
^b BASF, Ag. Prod. Center, 26 Davis Dr., P.O. Box 13528, Res. Triangle Park, NC 27709-3528

* Corresponding author (logsdon{at}nstl.gov)

Preferential flow may cause rapid leaching of solutes. The preferential flow model MACRO is a flexible, transient-state preferential flow model. The objective of this study is to compare measured and predicted Br and tritium leaching for multiple soil columns. We collected 48 undisturbed columns (0.35 m long and 0.20 m in diameter), that represented three soils (Mollisols), two tillage treatments (no-till and chisel), and two rainfall application rates for the first rain. We applied Br to the soil 2 d before the first rain, which contained tritium. There were four rain events, each 1 wk apart with drainage and evaporation between rains. We determined effluent breakthrough curves. We determined input parameters from soil properties measured on three extra columns for each soil and tillage combination and from calibration. For the first rain event, measured Br loss was 175% larger than tritium loss. Predicted bromide leaching for the first rain was 2% smaller than tritium loss. Tritium retention could have reduced tritium leaching. Concentrations of tritium and bromide were usually overpredicted for the first flush and underpredicted for the end of the first rain event through the start of the third rain event. The root mean square error values were as much as four times as high as a treatment mean for bromide, and up to eight times as high for tritium. Lack of agreement between measured and predicted bromide and tritium leaching may suggest further important mechanisms should be incorporated into MACRO.

Abbreviations: AE, anion exclusion • CDE, convection-dispersion equation • d, half spacing between equivalent parallel fractures • D_b, bulk density • D_v, dispersivity • h, pressure head • |h_b|, the boundary tension between the macropore and micropore regions • K, hydrualic conductivity • K_b, K between the macropore and micropore boundaries • K_s, saturated K • n*, exponent of the K– relation for the macropore region • RMSE, root mean squared error • SA, surface area • , water content • _sma, macropore volume fraction • , micropore scale index

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