Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2009
Publication Date: 2/1/2010
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2300.pdf
Citation: Kim, H., Walker, S., Bradford, S.A. 2010. Coupled Factors Influencing the Transport and Retention of Cryptosporidium Parvum Oocysts in Saturated Porous Media. Water Research. 44(4):1213-1223. Interpretive Summary: Cryptosporidium Parvum oocysts have caused major water-borne disease outbreaks in the United States. The objective of this study was to investigate the coupled influence of solution chemistry, water velocity, and grain size on oocyst transport and retention. Oocyst retention was found to be controlled by a combined role of low velocity regions and chemical interactions. Two major findings were observed: (i) oocyst retention was enhanced in low velocity regions near grain-grain contacts when chemical conditions were unfavorable for oocyst-sand interactions; and (ii) reversible oocyst retention occurred under conditions that were favorable for oocyst-sand interaction due to the presence of macromolecules on the surface of the oocysts. This study helps us to better understand mechanisms that control the fate of oocysts in groundwater environments, and will be of use to scientists, engineers, and regulators who are concerned with protecting water quality.
Technical Abstract: The coupled role of solution ionic strength (IS), system hydrodynamics and pore structure on the transport and retention of viable Cryptosporidium parvum oocyst was investigated via batch, packed-bed column, and micromodel systems. The experiments were conducted over a wide range of IS (0.1-100 mM), at two Darcy velocities (0.2 and 0.5 cm/min), and in two sands (median diameters of 275 and 710 µm). Overall, the results suggested that oocyst retention was a complex process that was very sensitive to solution IS, the system velocity, and the grain size. Increasing IS led to enhanced retention of oocysts in the column, which is qualitatively consistent with predictions of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Conversely, increasing velocity and grain size resulted in less retention of oocysts in the column due to the difference in the fluid drag force and the rates of mass transfer from the liquid to the solid phase and from high to low velocity regions. Oocyst retention was controlled by a combined role of low velocity regions, the secondary energy minimum, and/or steric interaction. The contribution of each mechanism highly depended on the solution IS. In particular, micromodel observations indicated that enhanced oocyst retention occurred in low velocity regions near grain-grain contacts under highly unfavorable conditions (IS=0.1 mM). When the IS=1 mM the secondary minimum was also found to play a role in oocyst retention. Reversible retention of oocysts to the sand in batch and column studies under favorable attachment conditions (IS=100 mM) was attributed to steric interactions between the oocysts and the sand surface due to the presence of oocyst surface macromolecules. The sensitivity of the oocyst interaction to velocity and grain size in column studies indirectly supported the presence of this weak steric interaction.