ABSTRACT: This project to evaluate the ground-water age and chemistry relations in aquifer systems in Lake, Porter, and La Porte Counties is, in part, an extension of earlier water quality studies in northwestern Indiana (Hasenmueller and others, 1994; Branam and others, 1997). These projects include baseline surveys of the distribution of tritium, deuterium, and 18O, and major and trace elements in bedrock and unconsolidated postglacial and glacial aquifers, and examine the relation of the water chemistry to the types of the geologic materials incorporated in the aquifers. Aquifers in unconsolidated materials within the southern Lake Michigan rim region lie within sediment packages defined as depositional sequences and are composed of the Quaternary Valparaiso, Wheeler, Lake Border, and Lake Michigan terrain sequences (oldest to youngest). Bedrock units that immediately underlie these deposits include the Wabash Formation (Silurian), Muscatatuck Group (Devonian), the Antrim Shale (Devonian), and the Ellsworth Shale (Devonian and Mississippian). Water samples from the shallower units generally can be characterized as calcium-magnesium-bicarbonate-type. Water samples from both the deeper unconsolidated units and bedrock units are generally characterized as either sodium-bicarbonate or sodium-chloride-type water. Sodium-bicarbonate-chloride water is the type for the majority of water samples from the bedrock units in the northern part of the project area. Water samples from bedrock wells, located in the southwestern part of the project area, have a sodium-calcium-magnesium-bicarbonate chemistry. The concentration of tritium was measured and used in a qualitative manner to date the ground water in the aquifers. A tritium activity greater than 2.0 tritium units (TU) was assumed to indicate post-1952 recharge. Tritium values of <0.8 to 2.0 TU were reported for 40 bedrock- and unconsolidated-aquifer samples. Tritium values of >2.0 to 28.2 TU were reported for the waters from 40 samples and are indicative of varying degrees of post-1952 ground-water recharge. That recharge is notable in samples from the Valparaiso sequence in areas where the clay cap is thin or absent, or the Valparaiso fan complex is present. Most water from the unconsolidated materials appears to be of postglacial origin with delta ¹⁸O values ranging from -9.30 to -6.66 parts per thousand and delta ²H values ranging from -61.84 to -51.83 parts per thousand. The delta ¹⁸O and delta ²H values are somewhat more negative (lighter) than the current mean average annual values calculated for this area. In contrast, water from bedrock units appears to be partly glacial in origin, having delta ¹⁸O values ranging from -19.98 to -8.22 parts per thousand and delta ²H values ranging from -148.15 to -57.47 parts per thousand. Less negative (heavier) delta ¹⁸O values are associated with the Wabash Formation in southern Lake County and the Antrim Shale in northern Porter County. These values suggest a mixing and dilution of the bedrock waters with younger water, or perhaps an initial dilution of melting glacial water with late glacial age (warmer) midwestern precipitation. Waters having the more negative (lighter) delta ¹⁸O values, ranging from -19.98 to -12.89 parts per thousand, and delta ²H values, ranging from -148.15 to -90.04 parts per thousand, presumably contain much older water that has not been recharged since Pleistocene time. Nontritiated, more highly mineralized (>750 mg/L total dissolved solids) ground water and isotopically lighter ground water characterize confined aquifer systems and suggest a longer residence time. The sensitivity of such aquifers to contamination may be less than that of aquifers that contain tritiated, less mineralized water. Less expensive monitoring techniques can be employed in some cases where the aquifer vulnerability assessment has been based on specific geologic materials in combination with deep water that has not been recharged since Pleistocene time. A Geographic Information System (GIS) format for presentation of the project data was chosen to allow better visualization of the relationship of the ground-water characteristics to the geologic and physiographic units in the three-county area. Database tables of the aquifer properties, geologic data, water chemistry, isotopes, and well location information were compiled. The data set is presented as both ArcView map views and layouts.