...inactivation. Reversible attachment defined a process where the preattached 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 permanent 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 repulsion, 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.sub.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 interaction 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.sub.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 adsorption characteristics from four other viruses (MS2, Q[beta], PRD1, and [phi]X174) with lower isoelectric points. These results indicate that there are probably water pH ranges, depending on isoelectric points of the virus and the porous medium, between which the virus behavior in ground water changes abruptly. To verify this hypothesis, two bacteriophages with different isoelectric points were studied in a model aquifer. The pH was varied and monitored between experiments to evaluate the effects of pH on virus fate and transport in ground water.

Materials and Methods

Model Aquifer

A 109 cm X 40 cm X 2 cm model aquifer was constructed and filled with sand (Figure 1). Ninety percent of the sand grains were 0.5 to 1.0 mm in diameter with the remaining 10% less than 0.5 mm in diameter. One injection port (W0) and five major sampling ports (W1, 2, 3, 4, and 5) were installed in the model, all at the same depth. The distance between major ports was 18 cm. Four additional monitoring ports were installed beside the major ports, but ~6 cm above (W2a and 4a) or below (W3b and 5b) them. The sand consisted of 71% quartz, 9% feldspar, and 20% igneous rock fragments, and was free of iron oxide coating under optical microscope observations. The isoelectric point of quartz is estimated to be 2.9 to 3.0, and that of feldspar 5.2 to 6.1 (Sverjensky and Sahai 1996). The igneous rock fragment component had an elemental composition similar to feldspar when analyzed under the electron microscope. Its isoelectric point is estimated to be similar to that for feldspar. Thus, the sand has two major components in terms of the isoelectric point, one (quartz hereafter) at pH 3 and the other (feldspar hereafter) at pH 5.5. The porosity of the sediment was ~0.45, and the hydraulic conductivity 1.3 X [10.sup.-3] m/sec. The hydraulic gradient was controlled by manipulation of the water level in the inlet reservoir and outlet reservoir.

Experiments

The model was flushed with deionized water (MiliQ) until the background ionic concentration sampled from injection and sampling ports was constant and below 0.2 mM. Two bacteriophages were selected for the study, MS2 and [phi]X174. The two phages have similar sizes (24 and 27 nm, respectively), but significantly different isoelectric points (3.9 and 6.6, respectively), which allows possible effects of the viral...

NOTE: All illustrations and photos have been removed from this article.



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