Evaluating transport risk in soil using dye and bromide tracers

Abstract

Dye and bromide tracers are established methods of assessing the presence, function, and extent of hydrological pathways in soil. Prediction of E. coli transport pathways in soil, using brilliant blue (BB) dye and bromide tracers, was investigated using in situ field trials on three grassland soil types, under different moisture regimes. Passive transport through preferential flow routes was the dominant mechanism of vertical E. coli transport in the soils studied. However, lateral movement of E. coli from macropores to the soil matrix was also observed. E. coli transport was mainly associated with visualized infiltration patterns but there was some evidence of differential transport of BB and E. coli. Maximum E. coli depth was found not to co-occur with BB and bromide tracers in 44 and 71% of samples, respectively. Soil type and season of application were important in the distribution and maximum depth of E. coli, and the relationship between the bacterium and its tracers. Moisture content was found to be important for the relationship between E. coli and BB, and the extent of this effect varied with soil type. There was a trend of increasing E. coli concentrations to a peak sample moisture concentration of 0.3 to 0.4 g g(-1) dry soil followed by a decrease. Overall BB was found to have greater predictive value than Br. Correlation and co-occurrence analysis found that shortly after land application both BB and Br were good predictors of E. coli transport pathways and distribution under certain conditions, but underestimate risk to shallow groundwater.