Groundwater resources in southern Tangipahoa Parish and adjacent areas were studied to determine their potential for development as an alternative to the Mississippi River as a water-supply source for Jefferson Parish. Eight major aquifers consisting of thick sand units that underlie the study area are, in descending order: (1) shallow, (2) upper Ponchatoula, (3) lower Ponchatoula, (4) Abita, (5) Covington, (6) Tchefuncta, (7) Hammond, and (8) Amite. A fault zone, referred to as the Baton Rouge fault, crosses southern Tangipahoa Parish. Analyses of geophysical logs indicated that the deep aquifers south of the fault zone had been displaced from 350 to 400 feet, and that the deeper aquifers were not in hydraulic connection with the flow system north of the fault. The groundwater resources of southeastern Louisiana are immense and the quality of groundwater in Tangipahoa Parish is suitable for most uses. The quality of water in these aquifers generally meets the U.S. Environmental Protection Agency's standards for public supply. The hydrologic system underlying Tangipahoa Parish and adjacent areas in 1990 supplied about 19 Mgal/d of water that was suitable for public supply. However, substantial increases in pumping from the aquifer system would result in renewed water-level declines throughout the hydrologic system until a new equilibrium is established. A test we11 in southern Tangipahoa Parish, penetrated all eight aquifers. Total thickness of freshwater sand beds penetrated by the 3003-ft test hole was more than 1900 ft. Resistivity values from an electric log of the test typically averaged 200 ohm-meters, which indicates that the water has low dissolved-solids and chloride concentrations. An analysis of the Abita aquifer at Ruddock in St. John the Baptist Parish, for two of three hypothetical well fields, indicated that for a hypothetical we11 field with a pumping rate of 112 Mgal/d, the freshwater/saltwater interface could arrive at the outer perimeter we11 in 10 to 14 years. The 1990 location of the interface in the Abita aquifer is 1.9 mi from the southernmost part of the potential location of the 112 Mgal/d well field.