Lake-water Mg:Ca responds to endogenesis of carbonate minerals, providing a valuable indicator of paleosalinity when water-column cation ratios are preserved in calcareous lake sediments. Typically Mg:Ca and total dissolved solids (TDS) correlate positively over a broad range of ionic compositions where calcium carbonate precipitation occurs. However, in groundwater-fed lakes where inflow solutes are dominated by HCO3–, Ca2+, and Mg2+ ions, and concentration of conservative solutes is limited by outflow, CaCO3 formation and depletion of major source-water ions results in a negative correlation between Mg:Ca and TDS at low lake salinity. This relationship is promoted by high pCO2 of inflowing groundwater, a common characteristic of groundwater-fed lakes such as our field example, a groundwater flow-through lake in western Montana, United States. Equilibrium modeling of our field example shows that evaporative evolution is expected to reverse the slope of the Mg:Ca/TDS relationship at moderately higher lake concentration. Generally, the TDS at the point of Mg:Ca/TDS reversal will depend on the initial concentration of less-reactive ions, and so on the source lithology for groundwater solutes.