Introduction In the last two decades, a number of studies have been done with batch, flowing column, and field experiments on viral behavior in the subsurface. Several processes may affect viral fate and transport in ground water, irreversible attachment, reversible attachment, and inactivation. Revers- ible attachment is defined as a process where the pre- attached virus re-enters the water in the time period of interest. Irreversible attachment is defined as the case where the attached viruses do not enter the water in the time period of interest. Only irreversible attachment can result in per- manent removal of viruses from water. The viral behavior in ground water appears to be controlled by the properties of viruses (Dowd et al. 1998; Deborde et al. 1999; Schijven et al. 2001; Woessner et al. 2001), the properties of the porous medium (Loveland et al. 1996; Pieper et al. 1997; Ryan et al. 1999), and the properties of water transporting the virus (Loveland et al. 1996; Bales et al. 1993, 1997; Redman et al. 1999). An incomplete understanding of the processes governing virus fate and transport is achieved if the study does not consider all controlling factors from the three aspects listed. The electrostatic attraction and repul- sion, van der Waals forces, and hydrophobic effects are three major forces responsible for interaction between the virus and the porous medium (Jin et al. 2000). In particular, viral attachment was observed to be a function of water pH (Bales et al. 1993, 1997), the isoelectric point (pH iep ) of the porous medium (Loveland et al. 1996), and the isoelectric point of the virus (Deborde et al. 1999; Dowd et al. 1998; Woessner et al. 2001). This suggests that electrostatic inter- action is an important factor controlling virus attachment and detachment. Loveland (1996) reported a pH edge for electrostatic interaction between PRD1 and porous medium, which is 2.5 to 3.5 pH units above the pH iep of the mineral surface. PRD1 attachment is nearly complete below this pH, while minimal above that pH. More recently, Schijven et al. (2001) reported that bacteriophage PM2 with an isoelectric point of 7.3 shows different adsorp- tion characteristics from four other viruses (MS2, Qβ, PRD1, and φX174) with lower isoelectric points. These Abstract Several viral transport experiments were conducted in a model aquifer 1 m long, using bacteriophages MS2 and φX174 at various pH (4.6 to 8.3) conditions, to increase our understanding of virus behavior in ground water. The results indicate the existence of a critical pH at which the virus behavior changes abruptly. This is supported by data from field and batch experiments. The critical pH is determined to be 0.5 unit below the highest isoelectric point of the virus and porous medium. When water pH is below the critical pH, the virus has an opposite charge to at least one component of the porous medium, and is almost completely and irreversibly removed from the water. This suggests that electrostatic attraction at a subcritical water pH condition is an important factor controlling virus attenuation in ground water. The concept of critical pH can assist in the design of geologic barriers for preventing viral contamina- tion in ground water. 701 The Effect of Critical pH on Virus Fate and Transport in Saturated Porous M edium by Huade Guan 1 , Dirk Schulze- Makuch 2 , St eve Schaf f er 3 , and Suresh D. Pillai 4 1 Department of Earth and Environmental Science, New Mex- ico Institute of Mining and Technology, Socorro, NM 87801; (505) 835–5465, fax (505) 835–6436 2 Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968; (915) 747–5168, fax (915) 747–5073; dirksm@ geo.utep.edu 3 Department of Mathematics, New Mexico Institute of Mining and Technology, Socorro, NM 87801; (505) 835–5811, fax (505) 835–5366 4 Department of Poultry Science, Texas A & M University, Col- lege Station, TX 77843; (979) 845–2994; fax (979) 845–1921 Received February 2002, accepted February 2003. Vol. 41, No. 5—GROUND WATER—September–October 2003 (pages 701–708)