The Covington aquifer is used as a major source of drinking water for east St. John the Baptist Parish, Louisiana. A recent subsurface geologic study in Herbert and Hanson (1991) suggested that the Covington aquifer might communicate hydrologically with two hazardous-waste-injection reservoirs, thus putting the freshwater Covington aquifer in danger of contamination. A case history shows how seismic-reflection and electric well-log data were used to determine the possibility of hydrologic communication between the Covington aquifer and the hazardous-waste-injection reservoirs.Cross-sections, net-sand isopachs, and structure maps are constructed for three hazardous-waste-injection reservoirs and the Covington aquifer. Four previously undetected normal faults bisect the three injection reservoirs and the Covington aquifer. The presence of these faults significantly increases the possibility of communication by acting as conduits and allowing vertical migration of fluids along the fault planes, or by causing juxtaposition of sand against sand and allowing leakage to occur across the fault planes. In addition, the upper injection reservoir is part of a fluvial system that occasionally scours down into the Covington aquifer, depositing sand from the injection reservoir interval directly on the Covington aquifer sand. Because of this stratigraphic contact, the injection reservoir is possibly in hydrologic communication with the Covington aquifer.The injection of liquid-hazardous-waste into these reservoirs could jeopardize not only the water quality of the Covington aquifer, but possibly the quality of other shallow freshwater aquifers, should vertical migration occur along the fault planes. Without the use of seismic data, the risk of contaminating underground sources of drinking water would not have been properly assessed.