Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/10/2004
Publication Date: 2/10/2005
Citation: Bradford, S.A., Bettahar, M. 2005. Straining, attachment, and detachment of Cryptosporidium oocysts in saturated porous media. Journal of Environmental Quality. 34:469-478. Interpretive Summary: Knowledge of the processes that control the movement and fate of pathogenic microorganisms, such as Cryptosporidium parvum oocysts, is required to protect food and water supplies. Laboratory and mathematical modeling studies were undertaken to examine mechanisms of oocyst transport and retention in groundwater systems. Previous studies and models have assumed that oocyst migration was controlled by chemical interactions between the oocysts and soil particles. Results for this study indicate that oocysts can also be retained in small pores at the soil surface. Oocyst retention increased and transport decreased in finer textured sands due to these small pores. Inclusion of chemical and pore mechanisms of oocyst retention into models improved their ability to describe the oocyst transport behavior. This information should aid researchers that are concerned with predicting pathogen transport and fate.
Technical Abstract: Experimental and modeling studies were undertaken to examine the roles of attachment, detachment, and straining on Cryptosporidium parvum oocyst transport and retention. Saturated soil column studies were conducted using Ottawa aquifer sands that encompassed a range of median grain sizes; 710, 360, and 150 micron. Decreasing the median sand size tended to produce lower effluent concentrations, greater oocyst retention in the sand near the column inlet, and breakthrough of oocysts at later times. Oocyst transport data also exhibited concentration tailing. Mathematical modeling of the oocyst transport data using fitted first-order attachment and detachment coefficients provided a satisfactory description of the observed effluent concentration curves, but a poor characterization of the oocyst spatial distribution (especially in the finer textured sands). Modeling of these data using an irreversible straining term that is depth dependent provided a better description of the oocyst spatial distribution, but could not account for the observed effluent concentration tailing or late breakthrough times. A more physically realistic description of the data was obtained by modeling attachment, detachment, and straining. The percentage of the total oocysts that were retained by straining was first estimated from effluent mass balance considerations to be 68, 79, and 87% for the 710, 360, and 150 micron sands, respectively. Straining coefficients were then selected to achieve these percentages of total oocyst retention, and attachment and detachment coefficients were fitted to the effluent concentration curves. Consideration of these factors in the simulations provided a better description of effluent and retention data. Justification for inclusion of straining in oocyst transport models was provided by trends in the transport data, simulation results, pore size distributions and residual saturations, and published literature.