Modeling biocolloid transport in saturated porous media.

Abstract

A forced-gradient experiment of virus and carboxylated microsphere transport was carried out at a two-well system in saturated aquifer at Borden, Ontario. The purpose of experiment was to qualitatively and quantitatively investigate bacteriophage transport relative to that of a conservative solute in porous media. A simplified plane radial advection dispersion equation coupled with reversible first-order and equilibrium mass transfer was found to be adequate to simulate the attachment and transport process. For simulating detachment and transport, all rate parameters were varied with time up/down (depending on the parameter) to reflect the changes in pH of groundwater with time from 7.4 to 8.4 then back to 7.4. Both constant and scale-dependent dispersivity were used in the modeling of the transport process. Time-moment analysis of the conservative-tracer breakthrough curves produced dispersivity values of 0.1-0.6 m, close to the macrodispersivity of 0.6 m obtained using a stochastic model to describe a previous larger-scale experiment at the site. The multiple-peak feature of all the breakthrough curves suggests that the aquifer heterogeneity may be more important than local dispersion in affecting the appearance of both electrical conductivity and phage breakthrough curves. Strack's model was found quite well to describe the hydraulic head profile during the whole period of experiment if proper values for transmissivity and cone radius are chosen. Virus traveled at least a few meters in the experiment, but virus concentrations at observation points 1-m to 2-m away were a small fraction of those injected. Though clearly not an equilibrium process, retardation involving a dynamic steady state between attachment and detachment is nevertheless a major determinant of transport versus retention of virus.