PREFERENTIAL BROMIDE TRANSPORT THROUGH A CLAY LOAM UNDER ALFALFA AND CORN

Cropping systems can affect subsurface water quality through modificat ions of soil hydraulic properties of the unsaturated, near-surface soi l, The purpose of this study was to assess the potential impact of two contrasting cropping systems on groundwater contamination by followin g the distribution in the soil profile of an inert solute under natura l rainfall conditions. The experiment consisted of successive applicat ions of 45 kg ha(-1) of KBr, on May 10 and July 25, 1991, to the surfa ce of a St-Laurent clay loam (Typic Haplaquept) which had been cropped with either corn (Zea mays L.) or alfalfa (Medicago sativa L.) for 4 years. Sequential soil sampling was then carried out to determine the depth-wise bromide concentration at various time intervals. Soil sampl es were taken at three different locations within each plot at 0-8, 8- 16, 20-28, 44-52, 68-76, 92-100 and 116-124 cm depths on May 17, May 2 8 and July 4 after the first application made on May 10. For the secon d application made on July 25, the samples were taken at the same dept hs on August 15. For August 15 data, the observed bimodal distribution of Br concentration with depth indicated the preferential flow of sol utes. Preferential flow appeared to be more important under alfalfa th an under corn. Estimates from all observed data suggested that 39% mor e bromide was leached out of the 0-30 cm zone in the alfalfa as compar ed to the corn plots. It is also shown in this study that changes in c ropping practices from corn to alfalfa significantly affect the hydrau lic conductivities of the soil and that higher values of hydraulic con ductivities were observed at the 50 cm depth in the alfalfa plots rela tive to the corn plots. These changes appeared to result in increased solute leaching in alfalfa relative to corn plots. Simulation of stead y-state stochastic-convective bromide transport from the measured satu rated hydraulic conductivity data were consistent with the observed br omide transport data, predicting that, on average, 34% more solute wou ld leach out of the 0-30 cm zone in the alfalfa plots as compared to t he corn plots.