A two-well forced-gradient experiment involving virus and microsphere transport was carried out in a sandy aquifer in Borden, Ontario, Canada. Virus traveled at least a few meters in the experiment, but virus concentrations at observation points 1 and 2.54 m away from the injection well were a small fraction of those injected. A simplified planar radial advection-dispersion equation with constant dispersivity, coupled with equilibrium and reversible first-order mass transfer, was found to be adequate to simulate the attachment and transport process. During the experiment a short-duration injection of high-pH water was also made, which caused detachment of previously attached viruses. For simulating this detachment and associated transport, the same transport and mass-transfer equations were used; but all rate parameters were varied as groundwater pH changed from 7.4 to 8.4 and then back to 7.4. The physicochemical parameters obtained from fitting breakthrough curves at one sampling well were used to predict those at another well downstream. However, laboratory-determined parameters overpredicted colloid removal. The predicted pattern and timing of biocolloid breakthrough was in agreement with observations, though the data showed a more-disperse breakthrough than expected from modeling. Though clearly not an equilibrium process, retardation involving a dynamic steady state between attachment and detachment was nevertheless a major determinant of transport versus retention of virus in this field experiment.¿ 1997 American Geophysical Union