3.2 Viruses
Viruses are an order of magnitude smaller in size compared to bacteria, (~0.1 vs 1 µm) and consequently, are less impeded by physical filtration during subsurface flow. However, they remain strongly affected by sorption reactions as a result of their net negative surface charge at normal pH ranges (Bales et al., 1995). Virus sorption (attachment) is influenced by factors such as sediment particle size, organic carbon content, water content, solution pH and ionic strength. Attachment is considered the most important depletion process for viruses because it leads to inactivation (die-off) and is irreversible in some cases (Betancourt et al., 2019).
In large-scale laboratory sand columns, Betancourt and others (2019) observed variable transport of fecal viruses, with die-offs ranging from 3000-fold over 4 meters to 1000-fold over 0.3 m, at rates that were non-linear (first order) with rate constants ranging from 200 to 1000 day–1. Such rapid depletion implies removal by attachment (sorption).
Deborde and others (1998a), reported a high degree of fecal coliphage virus attenuation in the groundwater plume from a school septic system at a site that had rapid groundwater velocities. The unsaturated zone was 1.5 m thick and the permeable sand and gravel aquifer had groundwater velocities of 1 to 3 m/day. Coliphage virus decreased 100,000-fold, within 7 m of the drainfield and further declined to below detection (< 1 CFU/100 mL) beyond 38 m from the drainfield. However, in a subsequent test in which both tracer viruses and bromide were injected into the core of the plume, a portion of the tracer virus arrived at monitoring locations 7 to 17 m downgradient, at about the same time as Br–, after only 2 to 6 days (Deborde et al., 1998b). However, concentrations were 100,000 times lower over this distance, similar to the die-off observed in the ambient plume. It was also determined that the virus retained in the aquifer sediments near the injection well, remained viable for more than nine months after injection. In another test in the same aquifer, a tracer virus was injected at a location 22 m away from a large-scale pumping well and a large fraction of the injected virus mass (up to 55%) was captured by the well within 47 hours (Deborde et al., 1999; Woessner et al., 2001), illustrating the potential for rapid viral transport under certain conditions.
Rapid viral transport was also observed in the Cape Cod tracer experiment discussed above. During that experiment, a bacteriophage virus co-tracer arrived at the monitoring location 6 m downgradient from the injection site, about 12 days after the injection, coincident with Br– and the tracer bacteria. However, the tracer virus was more highly attenuated compared to the tracer bacteria (100,000-fold versus 10-fold depletion) because of its greater susceptibility to removal by adsorption. However, during a second phase of the same tracer test, when higher pH water was injected, a substantial amount of the adsorbed virus mass was remobilized (Bales et al., 1995).
Overall, field observations indicate that pathogens, including both bacteria and viruses are relatively immobile in septic system plumes. However, under certain conditions they can be more mobile, particularly when fast groundwater velocities occur due to the presence of high permeability sand and gravels, fractures or macropores; or to close proximity of high-yield production wells.