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Introduction to the modelling of virus transport in groundwater in the Netherlands

Inleiding tot modellering van virustransport door het grondwater in Nederland


Protection policy of ground water supplies in the Netherlands is currently based on the assumption that a travel time of 60 days of the ground water to the well head is sufficient for reducing pathogenic micro-organisms to levels that do not lead to unacceptable public health risk (CBW, 1980). In the United States the Environmental Protection Agency (USEPA) has developed the Ground Water Disinfection Rule (GWDR, 1992) that gives guidance for determining whether a ground water system is at risk from virus contamination. Protection is sufficient if through natural disinfection the level of the maximum allowable virus concentration of 2 x 10-7 pfp/l at the wellhead is not exceeded. According to the GWR a ground water travel time of 9 to 24 months is needed, depending on the rate of virus inactivation. This is much longer than the travel time of 60 days currently applied in the Netherlands. Moreover, the travel fimes given as natural disinfection citeria in the GWDR are possibly underestimated (Yates, 1993). In view of both the limited scientific basis for the travel time of 60 days used in the Netherlands and of the uncertainties in the disinfection criteria according to the GWDR, further investigations of the vulnerability of Dutch ground water supplies to virus contaminations is necessary. In this report the following investigation is proposed: some phreatic wells situated in urban areas will be selected. At these sites leaking sewage pipes in the saturated zone that are releasing viruses at an initial concentration of 10x4 pfp/l will be simulated. With the use of the "Landelijk Grondwater Model" (LGM ; Kovar et al., 1992), VIRALT (Park et al., 1992 en 1994a), CANVAS (Park et al., 1994b) and other modeling programs virus breakthrough at the well will be calculated. The calculations will be based on the same viral transport equation (Yates et al., 1991) for all modeling programs. In the Netherlands a provisional point of view of a maximum allowable average virus concentration at the well head of 2 x 10-7 pfp/l based on rotavirus infection is in preparation. Thus, based on an initial virus concentration of 10x4 pfp/l at the source of contamination a reduction in virus concentration of 11 units on log-scale is needed. As a result of these simulations a protection zone will be delineated that guarantees virus reduction through natural disinfection of at least 11 log. A survey of phreatic wells together with the number of potential sources for virus contamination (sewage pipes) within the protection zone will give an impression of the extent of the vulnerability-problem in the Netherlands. These simulations will also include a sensitivity analysis for charges in the transport parameters and an uncertainty analysis, of both the different parameters and the delineation of the protection zone. For calibration of the models tracing experiments using bacteriophages as microbial markers will be proposed. Data from cases of virus breakthrough known from literature will also be used for calibration

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