Regional ground-water mixing and the origin of saline fluids: midcontinent, United States.

Ground waters in three adjacent regional flow systems in the midcontinent exhibit extreme chemical and isotopic variations that delineate large-scale fluid flow and mixing processes and two distinct mechanisms for the generation of saline fluids. Systematic spatial variations of major ion concentrations, H, O, and Sr isotopic compositions, and ground-water migration pathways indicate that each flow system contains water of markedly different origin. Mixing of the three separate ground waters exerts a fundamental control on ground-water composition. The three ground waters are: (i) dilute meteoric water recharged in southern Missouri; (ii) saline Na-Ca-Cl water in southeastern Kansas of far-traveled meteoric origin that acquired its salinity by halite dissolution; and (iii) Na-Ca-Cl brines in north-central Oklahoma that may have originated as Paleozoic seawater.