RHODAMINE WT AND BACILLUS-SUBTILIS TRANSPORT THROUGH AN ALLUVIAL GRAVEL AQUIFER

A field study was conducted to characterize the transport of rhodamine WT dye and endospores of the bacterium Bacillus subtilis through pref erential flowpaths in an alluvial gravel aquifer using natural gradien t tracer experiments. Preliminary tracer experiments were conducted wi th rhodamine WT to determine preferential flowpaths using a resin bag method, because of spatial heterogeneity at the study site. Rhodamine WT, Ci, and B. subtilis endospores were then injected 0.1 to 1.6 m bel ow the water table, and downgradient ground water was monitored. Rhoda mine WT behaved like Cl in the field and was used as a nonreactive tra cer in this study. The method of time moments (MTM) and a curve fittin g method (CFM) using the AT123D model were applied to estimate transpo rt parameters of rhodamine WT and B. subtilis endospores along prefere ntial flowpaths within the aquifer, assuming each individual flowpath to be uniform. The results from CFM fit the observed data better than did MTM. Ground water velocities through preferential flowpaths ranged from 30 to 85 m/day with a median of 63 m/day, and longitudinal dispe rsivities ranged from 0.71 to 5.24 m with a median of 2.70 m. These va lues are within the range reported in the literature for similar hydro geological conditions. B. subtilis endospores exhibited slightly faste r velocities (median retardation factor 0.86) and lower longitudinal d ispersivities (0.23 to 2.4 m, median 0.79 m) compared to rhodamine WT. When multiple concentration peaks were observed in the rhodamine WT b reakthrough curves (BTCs), the bacteria always arrived with the first rhodamine WT peak, regardless of whether the first peak was small or l arge. For those wells with a single peak for both the dye and the bact eria, the bacteria tended to coincide with the front portion of the dy e BTC. This suggests that convection is relatively more important for B. subtilis endospores than for the dye, probably due to pore size exc lusion of the spores. For this reason, the use of non-reactive solute tracers to reflect the movement of microorganisms may not be appropria te. The total removal rates of B. subtilis varied from 2.4 to 9.36 day (-1) (median 3.36 day(-1)), probably resulting from filtration, sedime ntation, and irreversible adsorption, as die-off was not observed in t he field during the 45 days of the study. The longevity of B. subtilis endospores in ground water makes it a good choice to use as a bacteri al tracer in the study of transport processes in the absence of die-of f.