Interactions between human activities and natural processes shape specific conductance and ion composition of United...

Specific conductance (SC) and major ions are important water quality constituents and key indicators of freshwater salinization. Being relatively conservative and less influenced by biological activity, they integrate hydrologic, climatic, and anthropogenic influences, allowing us to track and understand lake responses to global change. However, the patterns and drivers of macroscale (regional to continental) variation of these water chemistry parameters for populations of lakes are relatively unknown. We examined the spatial variations in SC (N=9,784 lakes) and major anion and cation composition and concentrations (N=1,218 lakes) across the conterminous United States, and quantified their relationships with a wide range of natural and human factors. High SC lakes were mainly located in the Plains, Desert Southwest, and Florida. While calcium and bicarbonate were the dominant ions in 46% of the study lakes, chloride was dominant in 14% of lakes, and magnesium and sulfate were the dominant cation and anion, respectively, in 8% of lakes. Among the multiple natural drivers we tested, the primary controls of SC and ion composition were soil, hydrology, and climate. However, their influences differed substantially across ecoregions and among lakes due to interactive effects among them and with human disturbances. Moreover, chloride exhibited strong anthropogenic signals, with concentrations predominantly related to road density and urban development. Our study builds upon previous local and regional work by documenting nationwide variations in ion concentrations and composition. While natural processes shape baseline ionic assemblage, human activities are the primary context modulating these effects and can cause considerable deviations in ion concentrations and composition, contributing to freshwater salinization and impacting ionic interactions and lake biota.