Physico-chemical and biological dynamics of a sewage-polluted limestone aquifer

The spatio-temporal pattern of groundwater contamination at a 500 m2 fractured limestone site polluted with sewage effluent was investigated over two years. In the context of a previously defined conceptual groundwater flow model, a multidisciplinary approach involving the collection of physico-chemical, bacteriological and biological (macro- and meiofauna) data was developed. Results showed that much of the sewage-polluted water infiltrated rapidly through some large subvertical fractures at the site and circulated with little or no contaminant attenuation through the solution-enlarged parts of a bedding joint. In such preferential contaminant paths, groundwater had low physico-chemical and bacteriological “stability”, highest average contaminant concentrations, and a high epigean organism component. Observation of the spatial distribution of organisms a short period after flooding also suggested that contaminants circulating through these conductive fractures entered the adjacent small-sized fissure matrix of the saturated zone. In this small aperture region of the site that was recharged by slow infiltrations of sewage-polluted surface water, groundwater had a greater physico-chemical and bacteriological “stability”, lowest average contaminant concentrations, and a high hypogean organism component. This lower sensitivity to surface pollution was partly due to attenuation processes such as retention of fecal indicator bacteria or nitrification of effluent ammonium which occurred in slow-moving water parts of the unsaturated zone.