Ž . Journal of Contaminant Hydrology 50 2001 287–305 www.elsevier.comrlocaterjconhyd Effect of surface coatings, grain size, and ionic strength on the maximum attainable coverage of bacteria on sand surfaces Carl H. Bolster ) , Aaron L. Mills, George M. Hornberger, Janet S. Herman Department of EnÕironmental Sciences, UniÕersity of Virginia, CharlottesÕille, VA 22903, USA Received 3 July 2000; received in revised form 10 January 2001; accepted 30 January 2001 Abstract The injection of bacteria in the subsurface has been identified as a potential method for in situ cleanup of contaminated aquifers. For high bacterial loadings, the presence of previously deposited bacteria can result in decreased deposition rates—a phenomenon known as blocking. Ž . Miscible displacement experiments were performed on short sand columns ;5 cm to determine how bacterial deposition on positively charged metal-oxyhydroxide-coated sands is affected by the presence of previously deposited bacteria. Approximately 8 pore volumes of a radiolabeled bacterial suspension at a concentration of ;1 =10 9 cells ml y1 were introduced into the columns followed by a 2-pore-volume flush of cell-free buffer. It was found that the presence of Al- and Fe-coated sand increased both deposition rates and maximum fractional surface coverage of bacteria on the sediment surfaces. The effect of grain size on maximum bacterial retention capacity, however, was not significant. Decreasing ionic strength from 10 y1 to 10 y2 M KCl Ž . Ž . resulted in noticeable decreases in sticking efficiency a and maximum surface coverage u max for clean silica sand—results consistent with DLVO theory. In columns containing positively charged Al- and Fe-coated sands, however, changes in a and u due to decreasing ionic max strength were minimal. These findings demonstrate the importance of geochemical controls on the maximum bacterial retention capacity of sands. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Bacteria; Transport; Bioremediation; Metal hydroxides ) Corresponding author. Current address: Department of Natural Resources, University of New Hampshire, Durham, NH 03824, USA. Fax: q 1-603-862-4976. Ž . E-mail address: carl.bolster@unh.edu C.H. Bolster . 0169-7722r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-7722 01 00106-1