Cell density and non-equilibrium sorption effects on bacterial dispersal in groundwater microcosms.

The relative importance of dispersion, physical straining, non-equilibrium sorption, and cell density on the dispersal of bacteria was examined in saturated, flow-dynamic sand columns. The bacterial breakthrough as a result of different size distributions of sand particles was followed by measuring the effluent concentration of 3H-adenosine-labelled cells of a Bacillus sp. and an Enterobacter sp. strain suspended in groundwater. The breakthrough curves were compared with theoretical curves predicted from an advective-dispersioe equilibrium sorption model (ADS), an ADS model with a first order sink term for irreversible cell reactions, a two-site model (equilibrium and nonequilibrium sorption sites), and a filtration model. Bacterial sand: water isotherms were linear in the experimental concentration range but had positive intercepts. The partition coefficients ranged from 15 to 0.4 for the Bacillus sp., and 120 to 0.4 for a Pseudomonas sp., and decreased with increasing particle size of the dominant fraction. In a kinetic study, the partition coefficient for the Enterobacter sp. in the smaller particle sand was 63 after one hour, but had decreased to 9 after 19 hours. Bacteria were detected in the effluent after one pore volume, which was earlier than predicted by the sand : water partition coefficients, and displayed an apparent nonequilibrium breakthrough. Dispersion effects and physical straining appeared to be insignificant in the experiments, but tailing of the elution part of the curves indicated slow reversible sorption, and nonequilibrium sorption may have been the main determinant of dispersal retardation. The reversible non-equilibrium sorption invalidated some of the assumptions behind all models except, possibly, the two-site model. Consequently, the models described the large particle sand data better where sorption was of less importance for the dispersal. The dispersal retardation was also affected by the bacterial cell density, both in the pore water and on the sand, suggesting that population characteristics may be an important factor for the bacterial distribution between the water and sand habitats. The retardation factor decreased from 13.7 to 7.8 when the cell density in the loading solution was increased from 3× 10(8) to 1.2 × 10(9) cells ml(-1). Presaturation of the sand with bacteria had a similar effect.