Transport and Retention of Fecal Bacteria at Sewage-Polluted Fractured Rock Sites
1994
Malard, Florian | Reygrobellet, Jean-Louis | Soulié, Michel
In 1991 to 1993, a bacteriological investigation was made of the numbers of fecal bacteria present in groundwater of a limestone site located in the Lez Basin, Southern France. Sampling was carried out in closely spaced 60-m deep wells located on either side of a sewage-polluted river to determine the spatio-temporal distribution of total coliforms, fecal coliforms, and fecal streptococci in groundwater. High differences in fecal bacteria concentrations observed between wells were largely attributable to two types of infiltration mechanisms controlling the groundwater recharge. A quick infiltration process promoted channelized flow of bacterial contaminants through large vertical fractures connected to enlarged bedding joints. In such conductive channels, the reduction of fecal bacteria concentrations resulted only from the flushing and dilution ability of groundwater. However, in the less permeable parts of the site, clogging of the small-sized fissures of the unsaturated zone by sludge particles might reduce the seepage rates and remove bacterial contaminants from percolating water. This straining effect was considered as a temporary retention process because fecal bacteria were eluted by subsequent rainwater percolation. Results of another bacteriological survey carried out on a thinly fissured gneiss aquifer contaminated by percolating sewage from numerous septic tanks also demonstrated that a similar accumulation and elution cycle had to be considered to explain the temporal pattern of well contamination.
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