ROLE OF PHYSICAL HETEROGENEITY IN THE INTERPRETATION OF SMALL-SCALE LABORATORY AND FIELD OBSERVATIONS OF BACTERIA, MICROBIAL-SIZED MICROSPHERE, AND BROMIDE TRANSPORT THROUGH AQUIFER SEDIMENTS

The effect of physical variability upon the relative transport behavio r of microbial-sized microspheres, indigenous bacteria, and bromide wa s examined in field and flow-through column studies for a layered, but relatively well sorted, sandy glaciofluvial aquifer. These investigat ions involved repacked, sieved, and undisturbed aquifer sediments. In the field, peak abundance of labeled bacteria traveling laterally with groundwater flow 6 m downgradient from point of injection was coincid ent with the retarded peak of carboxylated microspheres (retardation f actor, RF = 1.7) at the 8.8 m depth, but preceded the bromide peak and the retarded microsphere peak (RF = 1.5) at the 9.0 m depth. At the 9 .5 m depth, the bacterial peak was coincident with both the bromide an d the microsphere peaks. Although sorption appeared to be a predominan t mechanism responsible for immobilization of microbial-sized microsph eres in the aquifer, straining appeared to be primarily responsible fo r their removal in 0.6-m-long columns of repacked, unsieved aquifer se diments. The manner in which the columns were packed also affected opt imal size for microsphere transport, which in one experiment was near the size of the small (approximately 2 mum) groundwater protozoa (flag ellates). These data suggest that variability in aquifer sediment stru cture can be important in interpretation of both small-scale field and laboratory experiments examining microbial transport behavior.