The Edwards-Trinity (Plateau) aquifer system is composed of several hydrochemically distinct Cretaceous-age siliciclastic and carbonate intervals. Abundances of and relationships between major ionic constituents demonstrate geographic and stratigraphic hydrochemical variability. The generally semi-confined Antlers (Trinity) siliciclastic aquifer is overall more saline than is the generally unconfined overlying karst-influenced Fredericksburg Division (Edwards) carbonate aquifer.  The operational parameter LOG[(Ca+Mg)/(Na²)] shows significantly lower values in Antlers than in Edwards water and reflect relatively greater abundance of Na⁺ compared with Ca²⁺ and Mg²⁺.  Corresponding values of Na/Cl are generally greater than 1 (molar or equivalent units) and demonstrate relative concentrations of Na⁺ that exceed expected values based on dissolution of halite (1.0) or mixing with residual seawater (0.86). Corresponding values of (Ca+Mg)/(HCO₃+SO₄) that reflect the dissolution products from calcite, dolomite, and gypsum are less than 1 (equivalent units) and demonstrate relative depletion of Ca²⁺ and Mg²⁺.  These data are best explained by cation exchange whereby Na⁺ has been exchanged on clay surfaces for Ca²⁺ and Mg²⁺, thus enriching sodium and depleting calcium and magnesium in groundwater. The greater abundance of clay in the Antlers explains the relative prominence of exchange in its water compared to that from the Edwards.