Hydrogeology Research Group

Colloid Tracer Experiments

A series of field and laboratory experiments is underway to investigate processes controlling migration and retention of colloidal particles and microorganisms in fractured, highly weathered mudrock (saprolite). Experiments are carried out at an instrumental field site (Figure 1) at Oak Ridge National lab (ORNL) or in undisturbed columns of saprolite (Figure 2). Results of the first field experiment show that colloids are highly mobile in these soils, with transport rates of 5-200 m/day, monitored over distances of 2-35 m. However, retention rates are also very high, ranging from 0.3 to 2.3 log cycles of concentration loss per m of travel (Figure 3). Subsequent laboratory experiments have shown that retention rates are strongly influenced by flow rate (Figure 4) particles diameter, ionic strength of the groundwater and valence state of the dominant cations. The experiments show that under conditions of high flow rate and low ionic strength, which typically occurs during seasonally wet periods, colloids can migrate with very little retention.

Recent journal publications and MS theses resulting from this research include:

• McKay, L.D., W.E. Sanford, and J.M. Strong, Field-scale migration of colloidal tracers in a fractured shale saprolite, Ground Water, accepted pending minor revisions.
• Cumbie, D.H., and L.D. McKay, Influence of diameter on particle transport in a fractured shale saprolite, J. Contaminant Hydrology, 31(1-2), 2-20, 1999.
• Harton, Angela, MS, 1996, Influence of flow rate on transport of bacteriophage in fractured shale saprolite. Manuscript submitted to Water Resour. Res. in 1998, currently being revised.
• Haun, Deirdre, MS, 1998, Influence of chemical perturbations on mobility of colloids in a fractured shale saprolite. Manuscript in preparation.