Published iii Environmental Science & Technology, July 1992, pp. 1410-1417, by the American Chemical Society Laboratory Investigations on the Role of Sediment Surface and Groundwater Chemistry in Transport of Bacteria through a Contaminated Sandy Aquifer Martha A. Scholl U.S. Geological Survey, Water Resources Division, Mail Stop 439, 345 Middlefield Road, Menlo Park, California 94025 Ronald W. Harvey* U.S. Geological Survey, Water Resources Division, USA, 325 N The effects of pH and sediment surface characteristics on sorption of indigenous groundwater bacteria were determined using contaminated and uncontaminated aquifer material from Cape Cod, MA. Over the pH range of the aquifer (5-7), the extent of bacterial sorption onto sediment in uncontaminated groundwater was strongly pH-dependent, but relatively pH-insensitive in contaminated groundwater from the site. Bacterial sorption was also affected by the presence of oxyhydroxide coatings (iron, aluminum, and manganese). Surface coating effects were most pronounced in uncontaminated groundwater (pH 6.4 at 10 ºC. Desorption of attached bacteria (up to 14% of the total number of labeled cells added) occurred in both field and laboratory experiments upon adjustment of groundwater to pH 8. The dependence of bacterial sorption upon environmental conditions suggests that bacterial immobilization could change substantially over relatively short distances in contaminated, sandy aquifers and that effects caused by changes in groundwater geochemistry can be significant. Introduction The transport of bacteria through porous media is a subject of current interest, having application in the fields 1410Environ. Sci. Technol., Vol. 26, No. 7, 1992 Broadway, Colorado Colorado, 80303-3328 of bioremediation and public health. Strategies for in-situ remediation of contaminated aquifers require information on the transport of pollutant-adapted or genetically engineered microbes. Where drinking water supplies are derived from groundwater, transport of pathogenic bacteria from sewage and solid waste disposal sites is also of great concern. In sandy aquifers, the degree of bacterial transport can be governed largely by sorptive interactions with stationary grain surfaces. An important aspect of modeling microbial transport in porous media is how best to account for sorption processes (1-8). The DLVO (Derjaguin-Landau and Verwey-Overbeek) theory of colloid stability (9) describes the interaction of electrostatic and van der Waals forces in flocculation of colloidal suspensions and is often used, in part, to describe bacterial sorption in aqueous systems (10-13). Colloid filtration theory has been invoked in models describing experimental observations of bacterial transport within a sandy aquifer and in sand columns (2, 4). Colloid filtration models quantify the physical mechanisms of particle contact with surfaces but use empirical coefficients to describe the effects of aqueous chemistry (14). Tobiason and O'Melia (15) modified the colloid filtration model to account for electrical double-layer (EDL) interaction forces; however,