Influence of flow rate on transport of bacteriophage in a column of highly weathered and fractured shale

An undisturbed soil column of highly weathered and fractured shale saprolite was collected from SWSA-7 on the Oak Ridge Reservation near Oak Ridge, TN. The 21 cm X 29 cm soil column (bulk hydraulic conductivity = 6.6 x 10^-5 m/sec) was used in a series of four miscible displacement experiments to evaluate the influence of flow rate on the transport of microorganisms under saturated conditions. The flow rates for the experiments ranged from 0.18 mL/min to 23.0 mL/min (hydraulic gradients of 0.001 to 0.2). During each experiment 2 pore volumes (PV) of a tracer solution containing bacteriophage, MS-2 and PRD-1, and a non-reactive solute, bromide, was injected into the column at a constant flow rate and was immediately followed by a 2 PV flush with a 0.005 M CaCl₂ carrier solution. The data clearly shoed that bacteriophage transport in fractured saprolite was strongly influenced by flow rate. For each experiment, first arrival of the bacteriophage occurred within 0.01 to 0.037 PV, which corresponds to transport rates of 0.5 to 208 m/day. After first arrival, the concentrations in the effluent increased abruptly, indicating very little dispersion of the bacteriophage. The bromide breakthrough curves showed much greater dispersion than the bacteriophage, especially at the lowest flow rate, 0.18 mL/min, apparently because of diffusion of the bromide into the small matrix pores in the soil. Bacteriophage concentrations in all but the slowest flow rate experiment very quickly leveled off at a constant "plateau" level concentration. The relative concentration level of the plateau for each bacteriophage was directly related to flow rate, with higher levels observed at higher flow rates. The elution portion of each experiment showed very rapid declines in bacteriophage concentration relative to bromide concentration during flushing with the CaCl₂ carrier solution. This shows that a large portion of the injected bacteriophage were irreversibly attached to the soil, whereas the bromide slowly diffused out of the matrix and back into the fractures before being flushed out of the column. Increasing flow rate during the flush caused some increase in bacteriophage concentration, but had little effect on the total mass of bacteriophage flushed out of the column. Mass balances calculated for each experiment showed much greater loss of bacteriophage at lower flow rates, while recovery of bromide was only weakly effected by flow rate. In all of the experiments, losses of MS-2 were greater than for PRD-1 and this was believed to be due to grater movement of the smaller bacteriophage, MS-2, into the smaller pores. Based on this study, it is expected that seasonal or short term variations in flow rate, which commonly occur in fractured shales, will greatly influence transport of microorganisms. These variations in flow can greatly influence attachment/loss rates and remobilize microorganisms attached to soils surfaces, resulting in further and faster transport of microorganisms than might occur in homogeneous granular deposits of the same hydraulic conductivity.