Field-Scale Migration of Colloidal Tracers in a Fractured Shale SaproliteGround Water (2000)
AbstractA field-scale tracer experiment carried out under natural gradient ground water flow conditions showed that colloids can be highly mobile in a fractured and highly weathered shale saprolite. Four colloidal tracers (0.100 μm 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 deep well, and concentrations of the tracers were monitored in the source well and in downgradient monitoring wells at distances of 2 to 35 m. All of the colloidal tracers were detected to distances of at least 13.5 m and two of the tracers (microspheres and INA) were detected in all of the downgradient wells. In most wells the colloidal tracers appeared as a “pulse”, with rapid 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 detection limit. The colloids were transported at velocities of up to 500 times faster than solute tracers (He, Ne, and rhodamine-WT) from previous tests 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 matrix between fractures. Peak colloid tracer concentrations in the monitoring wells varied substantially, with the microspheres exhibiting the highest relative concentrations and hence the least retention. Rates of concentration decline with distance also varied, indicating that retention is not a uniform process in this heterogeneous material. Two of the tracers, INA and PRD-1, reappeared in several monitoring wells one to five months after the initial pulse had passed, and the reappearance generally corresponds with increased seasonal precipitation. This is consistent with subsequent laboratory experiments that showed that colloid retention in these materials is sensitive to factors such as flow rate and ionic strength, both of which are expected to vary with the amount of precipitation.
Publication DateJanuary, 2000
Citation InformationLarry McKay, W. E. Sanford and J. M. Strong. "Field-Scale Migration of Colloidal Tracers in a Fractured Shale Saprolite" Ground Water Vol. 38 Iss. 1 (2000)
Available at: http://works.bepress.com/larry_mckay/29/