Hydrologic-hydrochemical investigations were conducted to determine the long-term fate of hazardous chemical waste disposed in the Texas Gulf Coast Tertiary formations by deep-well injection. The study focused on the hydrostatic section of the Frio Formation because it is the host of a very large volume of injected waste and because large databases of formation pressures and water chemistry are available. Three hydrologic regimes exist within the Frio Formation: a shallow fresh to moderately saline water section in the upper 3,000 to 4,000 ft (914 to 1,219 m): an underlying 4,000- to 5,000-ft-thick (1,219 to 1,524-m) section with moderate to high salinities: and a deeper overpressured section with moderate to high salinities. The upper two sections are normally pressured and reflect either fresh-water or brine hydrostatic pressure gradients. Geopressured conditions are encountered as shallow as 6,000 ft (1,829 m). The complexity of the hydrologic environment is enhanced due to extensive depressurization in the 4,000- to 8,000-ft-depth (1,219 to 2,438-m) interval, which presumably results from the estimated production of over 10 billion barrels (208 x 10 to the power of 6 cu m) of oil equivalent and associated brines from the Frio in the past 50 yrs. Because of the higher fluid density and general depressurization in the brine hydrostatic section, upward migration of these brines to shallow fresh ground waters should not occur. Depressured oil and gas fields, however, may become sinks for the injected chemical wastes. (Copyright (c) Springer Verlag 1990.)