Transport of free and particulate-associated bacteria in karst

Karst aquifers, because of their unique hydrogeologic characteristics, are extremely susceptible to contamination by pathogens. Here we present the re sults of an investigation of contamination of a karst aquifer by fecal indi cator bacteria. Two wells intercepting zones with contrasting effective hyd raulic conductivities, as determined by pump test, were monitored both duri ng the dry season and in response to a rain event. Samples were also collec ted from the adjacent ephemeral surface stream, which is known to be impact ed by an upstream wastewater treatment plant after rainfall. Whole water an d suspended sediment samples were analyzed for fecal coliforms and enteroco cci. During the dry season, pumping over a 2-day period resulted in increas es in concentrations of fecal coliforms to greater than 10,000 CFU/100 mi i n the high-conductivity well; enterococci and total suspended solids also i ncreased, to a lesser degree. Toward the end of the pumping period, as much as 50% of the fecal coliforms were associated with suspended sediment. Irr igation of an up-gradient pine plantation with primary-treated wastewater i s the probable source of the bacterial contamination. Sampling after a rain event revealed the strong influence of water quality of the adjacent Terri eu Creek on the ground water. Bacterial concentrations in the wells showed a rapid response to increased concentrations in the surface water, with fec al coliform concentrations in ground water ultimately reaching 60,000 CFU/1 00 mi. Up to 100% of the bacteria in the ground water was associated with s uspended sediment at Various times. The results of this investigation are e vidence of the strong influence of surface water on ground water in karst t errain, including that of irrigation water. The large proportion of bacteri a associated with particulates in the ground water has important implicatio ns for public health, as bacteria associated with particulates may be more persistent and more difficult to inactivate. The high bacterial concentrati ons found in both wells, despite the difference in hydraulic conductivity, demonstrates the difficulty of predicting vulnerability of individual wells to bacterial contamination in karst. The extreme temporal variability in b acterial concentrations underscores the importance of event-based monitorin g of the bacterial quality of public water supplies in karst. (C) 2000 Else vier Science B.V. All rights reserved.