Streamline-based simulation of virus transport resulting from long term artificial recharge in a heterogeneous aquifer

The likelihood for viruses, protozoan oocysts, and other human pathogens to enter groundwater, and in particular, sensitive or vulnerable water supplies, has increased as the numbers of anthropogenic sources such as septic systems, leaking sewers, animal farming operations, and artificial recharge of treated wastewater have proliferated. In this paper, we utilize a detailed numerical model of groundwater flow in a region encompassing a large artificial groundwater recharge operation in Orange County, California to evaluate the potential for transport of viruses and protozoan oocysts in such a system, as dictated by a transport model that includes colloid filtration and microbial inactivation components. The purpose of the model is not oriented towards the analysis of any perceived or real microbial contamination, but rather is directed at understanding the influence of aquifer heterogeneity within the modeled system. The transport model is based upon a novel representation of geologic heterogeneity, a high-resolution flow simulator, and an efficient streamline-based transport algorithm. Example virus transport simulations illustrate a large degree of variability in virus breakthrough across water supply pumping wells, with shallower wells providing less than two orders of magnitude of virus removal, and deeper wells indicating many orders of magnitude of virus removal. Simulation results also show variability among pathogens modeled, with Cryptosporidium parvum filtered to a much greater degree than other pathogens. Comparison to transport of an abiotic colloid and a conservative chemical tracer are provided to illustrate the influence of filtration and inactivation on the transport process. The results emphasize the need for improved microbial transport models in realistic aquifer systems, more reliable virus characterization methods and monitoring networks, and their ultimate integration into a broader epidemiological and regulatory framework for aquifer management.