Transport of Escherichia coli in 25 m quartz sand columns

摘要

To help improve the prediction of bacteria travel distances in aquifers laboratory experiments were conducted to measure the distant dependent sticking efficiencies of two low attaching Escherichia coli strains (UCFL-94 and UCFL-131). The experimental set up consisted of a 25 m long helical column with a diameter of 3.2 cm packed with 99.1% pure-quartz sand saturated with a solution of magnesium sulfate and calcium chloride. Bacteria mass breakthrough at sampling distances ranging from 6 to 25.65 m were observed to quantify bacteria attachment over total transport distances {aL) and sticking efficiencies at large intra-column segments (a,) (>5 m). Fractions of cells retained (F,) in a column segment as a function of a, were fitted with a power-law distribution from which the minimum sticking efficiency defined as the sticking efficiency of 0.001% bacteria fraction of the total input mass retained that results in a 5 log removal were extrapolated. Low values of aL in the order 10"4 and 10"3 were obtained for UCFL-94 and UCFL-131 respectively, while arvalues ranged between 10' 5 to 10~3 for UCFL-94 and 10~5 to 10~4 for UCFL-131. In addition, both aL and a, reduced with increasing transport distance, and high coefficients of determination (0.99) were obtained for power-law distributions ofa, for the two strains. Minimum sticking efficiencies extrapolated were 10~7 and 10~8 for UCFL-94 and UCFL-131, respectively. Fractions of cells exiting the column were 0.19 and 0.87 for UCFL-94 and UCL-131, respectively. We concluded that environmentally realistic sticking efficiency values in the order of 10~4 and 10~3 and much lower sticking efficiencies in the order 10"5 are measurable in the laboratory, Also power-law distributions in sticking efficiencies commonly observed for limited intra-column distances (<2 m) are applicable at large transport distances(>6 m) in columns packed with quartz grains. High fractions of bacteria populations may possess the so-called minimum sticking efficiency, thus expressing their ability to be transported over distances longer than what might be predicted using measured sticking efficiencies from experiments with both short (<1 m) and long columns (>25 m). Also variable values of sticking efficiencies within and among the strains show heterogeneities possibly due to variations in cell surface characteristics of the strains. The low sticking efficiency values measured express the importance of the long columns used in the experiments and the lower values of extrapolated minimum sticking efficiencies makes the method a valuable tool in delineating protection areas in real-world scenarios.