PHYSICOCHEMICAL AND BIOLOGICAL DYNAMICS OF A SEWAGE-POLLUTED LIMESTONE AQUIFER

The spatio-temporal pattern of groundwater contamination at a 500 m(2) fractured limestone site polluted with sewage effluent was investigat ed over two years. In the context of a previously defined conceptual g roundwater flow model, a multidisciplinary approach involving the coll ection 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 frac tures at the site and circulated with little or no contaminant attenua tion through the solution-enlarged parts of a bedding joint. In such p referential contaminant paths, groundwater had low physico-chemical an d bacteriological ''stability'', highest average contaminant concentra tions, and a high epigean organism component. Observation of the spati al distribution of organisms a short period after flooding also sugges ted that contaminants circulating through these conductive fractures e ntered the adjacent small-sized fissure matrix of the saturated zone. In this small aperture region of the site that was recharged by slow i nfiltrations of sewage-polluted surface water, groundwater had a great er physico-chemical and bacteriological ''stability'', lowest average contaminant concentrations, and a high hypogean organism component. Th is lower sensitivity to surface pollution was partly due to attenuatio n processes such as retention of fecal indicator bacteria or nitrifica tion of effluent ammonium which occurred in slow-moving water parts of the unsaturated zone.