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Field Study of Bromacil Transport under Continuous-Flood Irrigation

National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011

* Corresponding author.

ABSTRACT

The transport processes of sorbing chemicals in field soils are poorly understood. This study characterized the leaching behavior of the weakly sorbing herbicide bromacil (5-bromo-3-sec-butyl-6-methyluracil) in comparison to Br^- during continuous-flood irrigation of a small field plot. Twenty-four solution samplers were used to periodically collect in situ samples from seven depths within four 1.83 by 1.83 m subplots. Estimates of the pore water velocity (v_s) and dispersion coefficient (D) were made by fitting an analytical solution of the convection-dispersion (CD) equation to the Br^- data. Estimates of retardation (R) were made by fitting the CD equation to the bromacil data using the v_s and D estimates from the Br^- data and letting R be a fitting parameter. Estimates of R were also made from the results of batch equilibration studies using soil from seven depths. Best-fit v_s and D values exhibited considerable variability from sampler to sampler (CV = 1.25 and 1.30, respectively) and showed no significant trends with depth. Retardation values estimated from the CD equation averaged 1.88, but varied from 1.21 to 3.35 and also showed no significant trend with depth. In contrast, batch equilibration studies showed the absorption properties of the surface 0.6 m of soil to be significantly different than the 0.6- to 3-m depth, with R values decreasing from 1.62 for the surface 0.6 m to 1.31 at 3 m. Although the two methods gave the same estimate of R in the surface soil, neither the lower R values at deeper depths nor the tendency to decrease with depth as predicted from the batch studies was apparent in the transport data. Using D as an additional fitting parameter to the bromacil data resulted in an average increase of 1.96 for this parameter, indicating more dispersion for the sorbing solute. Increased dispersion and increased tailing of the sorbed solute may be attributed to spatially variable adsorption and to a negative correlation between v_s and R (r = –0.524).

Contribution from the USDA-ARS National Soil Tilth Lab. and U.S. Water Conservation Lab.

Received for publication July 23, 1990.

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