TRANSPORT AND RETENTION OF FECAL BACTERIA AT SEWAGE-POLLUTED FRACTURED ROCK SITES

In 1991 to 1993, a bacteriological investigation was made of the numbe rs of fecal bacteria present in groundwater of a limestone site locate d in the Let Basin, Southern France. Sampling was carried out in close ly 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 differe nces in fecal bacteria concentrations observed between wells were larg ely attributable to two types of infiltration mechanisms controlling t he groundwater recharge. A quick infiltration process promoted channel ized how of bacterial contaminants through large vertical fractures co nnected to enlarged bedding joints. In such conductive channels, the r eduction of fecal bacteria concentrations resulted Only from the flush ing and dilution ability of groundwater. However, in the less permeabl e parts of the site, clogging of the small-sized fissures of the unsat urated zone by sludge particles might reduce the seepage rates and rem ove bacterial contaminants from percolating water. This straining effe ct was considered as a temporary retention process because fecal bacte ria were eluted by subsequent rainwater percolation. Results of anothe r bacteriological survey carried out on a thinly fissured gneiss aquif er contaminated by;percolating sewage from numerous septic tanks also demonstrated that a similar accumulation and elution cycle had to be c onsidered to explain the temporal pattern of well contamination.