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Transport and attenuation of bacteria and bacteriophages in an alluvial gravel aquifer

Abstract The relative attenuation of rhodamine WT dye, two strains of Escherichia coli, Bacillus subtilis endospores, and the F‐RNA bacteriophage MS2 in an alluvial gravel aquifer was investigated in two tracing experiments at Burnham, near Christchurch, New Zealand. A simulated concentration curve was fitted to the observed breakthrough curves using the contaminant transport model AT123D, by optimising hydraulic conductivity (K), longitudinal dispersivity (αx), and a removal constant (λ) (which includes die‐off and physical removal processes) with the Parameter Estimation (PEST) optimisation routine. When comparing the parameters, the hydraulic conductivity was converted to velocity (V). The V ranking was E. coli 2690 > B. subtilis endospores > rhodamine WT in Experiment 1 and E. coli J6–2 > phage MS2 > rhodamine WT in Experiment 2. These rankings are consistent with the concept of pore size exclusion, whereby larger particles are preferentially transported in the larger interconnected pores where water velocities are higher. The longitudinal dispersivity (αx) rankings were consistent with pore size exclusion in Experiment 1, and broadly consistent in Experiment 2. Of the two parameters, V is considered to provide the more reliable result, because it is easier to determine peak position in time than peak height. Little useful information could be derived from the λ values in our study, because of high levels of uncertainty associated with determining peak heights, particularly in Experiment 1. Overall, the curve fits were better in Experiment 2, because of a greater number of bores and observations. Although this complicated between‐experiment comparisons, an overall retardation (R) ranking of rhodamine WT > phage MS2; B. subtilis endospores > E. coli J6–2 > E. coli 2690 is broadly consistent with pore size exclusion. Overall, our study showed that the application of the AT123D model to the observed velocities of the tracer curves demonstrated an effect consistent with pore size exclusion.