Rhodamine WT and Bacillus subtilis Transport through an Alluvial Gravel Aquifer

Abstract

AbstractA field study was conducted to characterize the transport of rhodamine WT dye and endospores of the bacterium Bacillus subtilis through preferential flowpaths in an alluvial gravel aquifer using natural gradient tracer experiments. Preliminary tracer experiments were conducted with rhodamine WT to determine preferential flowpaths using a resin bag method, because of spatial heterogeneity at the study site. Rhodamine WT, Cl, and B. subtilis endospores were then injected 0.1 to 1.6 m below the water table, and downgradient ground water was monitored.Rhodamine WT behaved like Cl in the field and was used as a nonreactive tracer in this study. The method of time moments (MTM) and a curve fitting method (CFM) using the AT123D model were applied to estimate transport parameters of rhodamine WT and B. subtilis endospores along preferential 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 dispersivities ranged from 0.71 to 5.24 m with a median of 2.70 m. These values are within the range reported in the literature for similar hydrogeological conditions. B. subtilis endospores exhibited slightly faster velocities (median retardation factor 0.86) and lower longitudinal dispersivities (0.23 to 2.4 m, median 0.79 m) compared to rhodamine WT. When multiple concentration peaks were observed in the rhodamine WT breakthrough curves (BTCs), the bacteria always arrived with the first rhodamine WT peak, regardless of whether the first peak was small or large. For those wells with a single peak for both the dye and the bacteria, the bacteria tended to coincide with the front portion of the dye BTC. This suggests that convection is relatively more important for B. subtilis endospores than for the dye, probably due to pore size exclusion of the spores. For this reason, the use of non‐reactive solute tracers to reflect the movement of microorganisms may not be appropriate. 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, sedimentation, and irreversible adsorption, as die‐off was not observed in the 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 bacterial tracer in the study of transport processes in the absence of die‐off.