Two extensive plumes (combined area > 1000 km2) have been delineated within the Ogallala aquifer in the Southern High Plains, TX, USA. Salinity varies within the plumes spatially and increases with depth; Cl ranges from 50 to >500 mg 1-1. Variable-density flow modeling using SUTRA has identified three broad regions of upward cross-formational flow from the underlying evaporite units. The upward discharge within the modeled plume area is in the range of 10-4-10-5 m3 day-1, and the TDS concentrations are typically >3000 mg 1-1. Regions of increased salinity, identified within the Whitehorse Group (evaporite unit) underlying the Ogallala aquifer, are controlled by the structure and thickness variations relative to the recharge areas. Distinct flow paths, on the order of tens of km to >100 km in length, and varying flow velocities indicate that the salinization of the Ogallala aquifer has been a slow, ongoing process and may represent circulation of waters recharged during Pleistocene or earlier times. On-going pumping has had negligible impact on the salinity distribution in the Ogallala aquifer, although simulations indicate that the velocity distribution in the underlying units may have been affected to depths of 150 m after 30 years of pumping. Because the distribution of saline ground water in this region of the Ogallala aquifer is heterogeneous, careful areal and vertical characterization is warranted prior to any well-field development. (C) 2000 Elsevier Science B.V.Two extensive plumes (combined area >1000 km2) have been delineated within the Ogallala aquifer in the Southern High Plains, TX, USA. Salinity varies within the plumes spatially and increases with depth; Cl ranges from 50 to >500 mg l-1. Variable-density flow modeling using SUTRA has identified three broad regions of upward cross-formational flow from the underlying evaporite units. The upward discharge within the modeled plume area is in the range of 10-4-10-5 m3 day-1, and the TDS concentrations are typically >3000 mg l-1. Regions of increased salinity, identified within the Whitehorse Group (evaporite unit) underlying the Ogallala aquifer, are controlled by the structure and thickness variations relative to the recharge areas. Distinct flow paths, on the order of tens of km to >100 km in length, and varying flow velocities indicate that the salinization of the Ogallala aquifer has been a slow, ongoing process and may represent circulation of waters recharged during Pleistocene or earlier times. On-going pumping has had negligible impact on the salinity distribution in the Ogallala aquifer, although simulations indicate that the velocity distribution in the underlying units may have been affected to depths of 150 m after 30 years of pumping. Because the distribution of saline ground water in this region of the Ogallala aquifer is heterogeneous, careful areal and vertical characterization is warranted prior to any well-field development.