Visualization and modeling of the colonization dynamics of a bioluminescent bacterium in variably saturated, translucent quartz sand M.L. Rockhold a, * , R.R. Yarwood b , M.R. Niemet c , P.J. Bottomley d , F.J. Brockman e , J.S. Selker f a Pacific Northwest National Laboratory, P.O. Box 999/MS K9-36, Richland, WA 99352, United States b Department of Crop and Soil Sciences, Oregon State University, Corvallis, OR 97331, United States c CH2M Hill, Corvallis, OR 97330, United States d Department of Microbiology, Oregon State University, Corvallis, OR 97331, United States e Pacific Northwest National Laboratory, P.O. Box 999 / MS P7-50, Richland, WA 99352, United States f Department of Biological and Ecological Engineering, Oregon State University, Corvallis, OR 97331, United States Received 22 June 2005; received in revised form 23 February 2006; accepted 5 May 2006 Available online 28 September 2006 Abstract An experimental and numerical investigation was conducted to study the colonization dynamics of a bioluminescent bacterium, Pseu- domonas fluorescens HK44, during growth in a porous medium under steady, variably saturated flow conditions. Experiments were con- ducted in a thin-slab light transmission chamber filled with uniform, translucent quartz sand. Steady, variably saturated flow conditions were established using drip emitters mounted on the top of the chamber, with glucose applied through a central dripper located directly above an inoculated region of the chamber. Periodic pulses of salicylate and a dye tracer were applied to induce bioluminescence of the bacterium to monitor colony expansion and to track changes in the hydraulic and transport properties of the sand. Changes in the appar- ent water saturation of the sand were quantified by monitoring light transmission through the chamber with a CCD camera. The col- onized region expanded laterally by about 15 cm, and upward against the flow by 7–8 cm during the 6-day experiment while apparent saturations in the colonized region decreased by 7–9% and the capillary fringe dropped by 5 cm. The observed data were reproduced approximately using a numerical model that accounted for the processes of water flow, solute and bacterial transport, cell growth and accumulation, glucose and oxygen consumption, and gas diffusion and exchange. The results of this study illustrate some of the com- plexities associated with coupled flow, reactive transport, and biological processes in variably saturated porous media, such as localized desaturation, capillary fringe lowering effects, and upstream movement of bacterial colonization, that may not readily observable using other experimental techniques. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Unsaturated porous media; Reactive transport; Bacteria; Bioluminescence 1. Introduction Sorption, geochemical reactions, and biodegradation are responsible for immobilizing or otherwise rendering innocuous many contaminants in subsurface environments. Natural attenuation, or intrinsic bioremediation, is there- fore being considered as a possible alternative to physical removal by pump-and-treat or other engineered systems for the cleanup of some hazardous waste disposal sites [36]. Natural attenuation is of particular interest because of the significant cost savings and reduced risk to workers that could be realized at some sites if it can be shown that the natural system provides adequate protection of ground- water resources. However, improved understanding of cou- pled flow, reactive transport, and biological process interactions may be required to fully evaluate the efficacy of natural attenuation. 0309-1708/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.advwatres.2006.05.026 * Corresponding author. E-mail address: mark.rockhold@pnl.gov (M.L. Rockhold). www.elsevier.com/locate/advwatres Advances in Water Resources 30 (2007) 1593–1607