Field-scale migration of colloidal tracers in a fractured shale saprolite

A field-scale tracer experiment carried out under natural gradient ground w ater flow conditions showed that colloids can be highly mobile in a fractur ed and highly weathered shale saprolite. Four colloidal tracers (0.100 mu m fluorescent latex microspheres, bacteriophage strains PRD-1 and MS-2, and INA, a dead strain of Pseudemonas syringae), were introduced to a 6.4 m dee p well, and concentrations of the tracers were monitored in the source well and in downgradient monitoring wells at distances of 2 to 35 m, till of th e colloidal tracers were detected to distances of at least 13.5 m and two o f the tracers (microspheres and INA) were detected in all of the downgradie nt wells. In most wells the colloidal tracers appeared as a "pulse," with r apid first arrival (corresponding to 5 to 200 m/d transport velocity), one to six days of high concentrations, and then a rapid decline to below the d etection Limit. The colloids mere transported at velocities of up to 500 ti mes faster than solute tracers (He, Ne, and rhodamine-WT) from previous tes ts at the site. This is believed to be largely due to greater diffusion of the solutes into the relatively immobile pore water of the fine-grained mat rix between fractures. Peak colloid tracer concentrations in the monitoring wells varied substantially,,vith the microspheres exhibiting the highest r elative concentrations and hence the least retention. Rates of concentratio n decline with distance also varied, indicating that retention is not a uni form process in this heterogeneous material, Two of the tracers, INA and PR D-1, reappeared in several monitoring wells one to five months after the in itial pulse had passed, and the reappearance generally corresponds with inc reased seasonal precipitation. This is consistent with subsequent laborator y experiments that showed that colloid retention in these materials is sens itive to factors such as flow rate and ionic strength, both of which are ex pected to vary with the amount of precipitation.