FIELD-SCALE SUBSURFACE TRANSPORT OF SURFACE-APPLIED BROMIDE IN A SOUTHWEST FLORIDA CITRUS GROVE

A tracer test was conducted at a citrus grove in Manatee County, FL, i n which the leaching of surface-applied potassium bromide (KBr) was mo nitored through the vadose zone. Subsequent transport in the shallow a quifer was also monitored for approximate to 2 yr. Vadose-zone results yielded: 1) reasonable held-averaged mass-recovery rates, 2) linear m ovement of the field-averaged depth of the center of mass as a functio n of net applied water, 3) steady increase in vertical spread of the f ield-averaged plume with net applied water while the plume remained in the unsaturated zone, and 4) significant spatial and temporal variabi lity in point measurements of Br concentration. Bromide (Br) was first detected in the surficial aquifer (approximate to 2.5 m below ground surface) approximately 70 d after application, and continued to be det ected in groundwater at this site more than 700 d post-application. Ve locity and dispersion parameters estimated from the data were successf ully used to characterize behavior of the field-averaged Br plume in t he unsaturated zone as a function of net applied water However, while field-averaged concentrations in the vadose zone were fairly predictab le, site-specific variability around the field-averaged mean was signi ficant. iis a result, mass loading-to the saturated groundwater zone v aried significantly over both space and time. This makes predictions o f arrival times and solute concentrations at particular compliance bou ndaries highly uncertain.