ESTIMATION OF PREFERENTIAL MOVEMENT OF BROMIDE TRACER UNDER FIELD CONDITIONS

Leaching of agricultural chemicals from the root and vadose zones into groundwater is an important environmental concern. To procure a bette r understanding of the movement and transport of agricultural chemical s through the soil profile, a field research study was conducted to es timate bromide leaching losses under saturated conditions where prefer ential flow is occurring. The field data were then used to evaluate th e LEACHM model. Eighteen double-ring infiltrometers were used to apply a pulse (100 mm depth) of bromide tracer on two previously saturated soils located in a karst region of southeastern Pennsylvania. Internal drainage over the next seven days resulted in nearly 51 % of the appl ied Br- being leached to a depth below 0.80 m. The LEACHM model was us ed to simulate the amount of bromide leached in each infiltrometer. Th e model predicted, accurately, an average of 46% of the applied Br- le ached below the 0.80 m depth. Mcan values of bromide concentration in the soil profile were predicted within two standard deviations of the measured mean for all depths except for the 0.20-0.40 m depth incremen t where the model overpredicted the bromide concentration. The model p redictions of Br- leached were tested against field measurements using several statistical tests. The LEACHM model performed adequately unde r preferential flow conditions, perhaps because the infiltration rate at each site was used as a model input. This, actually, is some measur e of the macropore flow process and suggests that simple models such a s LEACHM can be used in the field, as long as a distribution of infilt ration rates is used as an input.