TRANSPORT OF BACTERIA AND BACTERIOPHAGES IN IRRIGATED EFFLUENT INTO AND THROUGH AN ALLUVIAL GRAVEL AQUIFER

The movement of bacteria and bacteriophages into and through an alluvi al gravel aquifer was investigated at a bordered strip effluent irriga tion scheme near Christchurch, New Zealand. Irrigation of one set of s trips 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 bare, and faecal coliforms the least. Estimates of percolation times through th e 13 m vadoze zone (based on times to peak concentration in the ground water) ranged from 1.6 to 10.5 hr, with travel times for the bacteriop hages being 1.4-3.4 times longer than for the bacteria. Injection of o xidation pond effluent containing rhodamine WT dye into the first bore resulted in contamination of the second bore (385 m downstream) by th e dye, F-RNA coliphages, and faecal coliforms. In a second experiment, injection (into the same bore) of a mixture of phage MS-2, the bacter ial tracer Escherichia coli J6-2, and rhodamine WT dye, produced a sim ilar 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), follo wed by the bacteria, and then the dye. Attenuation of the bacteria and phages was similar, but the microbes exhibited 100-fold greater reduc tion than the dye. Flow direction and longitudinal dispersivity were d etermined in a preliminary analysis using an idealised 2-D dispersion model. This information, and other measured and reported data, were th en used as inputs in a 3-D dispersion model. The predicted concentrati on curves were matched to the observed curves by trial and error adjus tment of the decay constant (lambda). The best curve fits were obtaine d with lambda values higher than those reported elsewhere. It is sugge sted that many of the reported microbial decay values underestimate mi crobial reductions in groundwater because they do not account for othe r removal mechanisms, such as filtration, sedimentation and irreversib le adsorption.