The mobility of waterborne pathogens through porous media is important for maintaining safe drinking water supplies.  Although significant literature exists on the transport of live microbes, few investigations have been performed on the transport of bacterial endospores.  Understanding the mechanisms (e.g. straining and attachment) that influence the depth distribution of spores is important for predicting general attenuation mechanisms in porous media.  Sand column experiments were employed to evaluate the tendency for attachment and/or straining in unsaturated transport of waterborne bacterial spores.  Sporulated B. cereus was used as an analog for B. anthracis (anthrax).  The porous medium employed was silica sand with a known grain size distribution.  Sand preparation consisted of acid washing to remove all possible oxides, which would influence attachment, from the silica sand.  The sand was carefully packed in the column to retain a uniform grain-size distribution.  Columns were saturated and a “bottom-up” flow was established until steady state flow was reached and inflow electrical conductivity matched outflow electrical conductivity.  Liquid samples were collected with a fraction collector under cover to prevent microbial contamination.  Breakthrough curves showing concentration of colloids in the effluent as a function of pore volume were generated.  After the transport experiment, the columns were dissected according to length intervals, and sand from each depth interval was washed in a two-step washing procedure to distinguish the strained fraction from the attached fraction.  The depth distributions of spores retained by these two mechanisms for two cases:  deionized water, and artificial groundwater, are presented.