Transport of Bacteria and Bacteriophages in Irrigated Effluent into and Through an Allivial Gravel Aquifer

The movement of bacteria and bacteriophages into and through an alluvial gravel aquifer was investigated at a bordered strip effluent irrigation scheme near Christchurch, New Zealand. Irrigation of one set of strips resulted in the contamination, by faecal coliform bacteria, and somatic and F-RNA coliphages, of two bores, approximately 60 m and 445 m downstream of the centre of the strips. F-RNA coliphages showed the greatest attenuation between the soil surface and the first bore, and faecal coliforms the least. Estimates of percolation times through the 13 m vadoze zone (based on times to peak concentration in the groundwater) ranged from 1.6 to 10.5 hr, with travel times for the bacteriophages being 1.4-3.4 times longer than for the bacteria. Injection of oxidation pond effluent containing rhodamine WT dye into the first bore resulted in contamination of the second bore (385 m downstream) by the dye, F-RNA coliphages, and faecal coliforms. In a second experiment, injection (into the same bore) of a mixture of phage MS-2, the bacterial tracer Escherichia coli J6-2, and rhodamine WT dye, produced a similar result in the downstream bore and in a newly-installed bore, 401 m downstream. In both injection experiments, the phages exhibited the shortest times to peak concentrations in the downstream bore(s), followed by the bacteria, and then the dye. Attenuation of the bacteria and phages was similar, but the microbes exhibited 100-fold greater reduction than the dye. Flow direction and longitudinal dispersivity were determined in a preliminary analysis using an idealised 2-D dispersion model. This information, and other measured and reported data, were then used as inputs in a 3-D dispersion model. The predicted concentration curves were matched to the observed curves by trial and error adjustment of the decay constant (l). The best curve fits were obtained with l values higher than those reported elsewhere. It is suggested that many of the reported microbial decay values underestimate microbial reductions in groundwater because they do not account for other removal mechanisms, such as filtration, sedimentation and irreversible adsorption.