USE OF MICROBIAL SURROGATES FOR INJECTION AND RECOVERY TESTS IN FRACTURED-ROCK AND GRANULAR AQUIFERS

Concern over deteriorating ground-water quality has stimulated multidisciplinary research focusing on fate and transport of pathogens. We performed controlled field and laboratory experiments to assess effects of various physicochemical factors (trace contaminants, subsurface materials, and microbial properties) on subsurface microbial movement. Safety and regulatory issues typically require use of microbial surrogates such as microspheres; non-pathogenic bacteria and protozoa; and/or bacteriophage. We present two case studies: a granular aquifer in OH and a fractured-granite aquifer in NH. Results applied to predicted subsurface transport behavior of pathogens assumed physicochemical factors affect pathogens and surrogates similarly.

For a bank filtration, we assessed Cryptosporidium oocyst travel through glacial outwash aquifer sediments near the Great Miami River, OH by performing flow through column experiments. Columns were filled with riverbank sediments and river water or groundwater from near the study site was used as carrier fluids. Substantial numbers (1-5%) of small (~3 µm), light (buoyant density- 1.06 g/cc) Cryptosporidium oocysts were transported through sediments. Their transport preceded the conservative tracer and microspheres, and was similar to live flagellates under comparable conditions- suggesting buoyant density and cell size control oocyst transport in subsurface granular systems.

To assess transport in fractured granite, a 33 m forced-gradient field study was performed at a USGS groundwater study site (Mirror Lake, NH). We found substantial differences in transport patterns for nonmotile and motile rods, cocci and inert microspheres. Maximum breakthrough of a Staphylococcus isolate preceded the conservative tracer and non motile bacterial populations. Bacterial breakthroughs preceded those of 1.0 µm carboxylated microspheres; the nonmotile Pseudomonas and microspheres had similar breakthrough patterns- suggesting reversible interactions with fracture surfaces.

Cumulative results suggest that both biotic and abiotic mechanisms determine microbial subsurface transport behavior. Finally, surrogates are potentially useful in looking at controls of pathogen transport in carefully controlled laboratory columns and in aquifers.