Water Research 37 (2003) 4719–4729 Modeling leaching of viruses by the Monte Carlo method Barton R. Faulkner a, *, William G. Lyon b , Faruque A. Khan c , Sandip Chattopadhyay d a Subsurface Protection and Remediation Division, National Risk Management Research Laboratory, US EPA Office of Research and Development, US Environmental Protection Agency, Ada, OK 74820, USA b ManTech Environmental Research Services Corp., Ada, OK 74820, USA c US EPA Headquarters, Washington, DC 20460, USA d Environmental Restoration Department, Battelle Memorial Institute, Columbus, OH 43201, USA Received 7 June 2002; received in revised form 8 July 2003; accepted 9 July 2003 Abstract Apredictivescreeningmodelwasdevelopedforfateandtransportofvirusesintheunsaturatedzonebyapplyingthe final value theorem of Laplace transformation to previously developed governing equations. A database of input parametersallowedMonteCarloanalysiswiththemodel.Theresultingkerneldensitiesofpredictedattenuationduring percolation indicated very small, but finite probabilities of failure for all homogeneous USDA classified soils to attenuatereovirus3by99.99%inone-halfmeterofgravitydrainage.Thelogarithmofsaturatedhydraulicconductivity andwatertoair–waterinterfacemasstransfercoefficientaffectedvirusfateandtransportabout3timesmorethanany other parameter, including the logarithm of inactivation rate of suspended viruses.Model results suggest extreme infiltration events may play a predominant role in leaching of viruses in soils, since such events could impact hydraulic conductivity. The air–water interface also appears to play a predominating role in virus transport and fate. Although predictivemodelingmayprovideinsightintoactualattenuationofviruses,hydrogeologicsensitivityassessmentsforthe unsaturated zone should include a sampling program. r 2003 Elsevier Ltd. All rights reserved. Keywords: Virus attenuation; Marginal quality waters; Water reuse; Septic system leachate; Ground Water Rule 1. Introduction Althoughfunctionalsepticsystemsandirrigationwith marginal quality water can provide safe and efficient conservation of fresh water, percolation of marginal quality waters can be a cause of nonpoint source contamination of drinking water supplies in some cases. Pathogens, carried with percolating water are most likely to be attenuated in soils above the saturated zone [1]. The probability that a homogeneous soil with a thickness on the order of meters will leach pathogens under normal flow conditions is very low. Viruses may be of concern however due to their small size and high survival rates in natural systems. Although the prob- ability of leaching is low, it is known that as few as one or two virus particles may cause illness in humans, thus even a low probability must be carefully evaluated. The processes that affect virus attenuation are numerous and poorly understood. The inherent uncer- tainty in soil transport characteristics that may retard water and virus movement, and the sparsely understood virus inactivation processes suggest predictions of attenuation must contain significant uncertainty. Never- theless,regulatorsarecallingformodelsthatcanpredict virus attenuation in soils. Such models may be useful to planners considering new septic system installations, or irrigation with reclaimed sewage effluent. Regulators are currently developing pilot Compre- hensive Performance Evaluations for groundwaters of publicwatersystems [2].Thesecontainvirusattenuation ARTICLE IN PRESS *Corresponding author. Tel.: +1-580-436-8530; fax: +1- 580-436-8703. E-mail address: faulkner.bart@epa.gov (B.R. Faulkner). 0043-1354/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00419-6