Natural and human drivers of salinity and major ion composition in United States lakes

Salinity and major ion composition are important for understanding and predicting lake water quality and responses to global changes. However, little is known about salinity and major ionic composition for populations of lakes at the continental scale, nor the corresponding relationships with natural and human factors operating at multiple spatial scales. To fill these knowledge gaps, we examined the spatial patterns in salinity using specific conductance as a proxy (N=9,785 lakes) and major ion concentrations (N=1,218 lakes) across the conterminous United States. We then quantified relationships between a wide range of multi-scaled natural and human factors and both salinity and ion composition. Most lakes had relatively low salinity (median=206μS/cm), although 4% were classified as saline (>1,500μS/cm) and mostly were located in the Plains, Desert Southwest, and Southeast regions. Calcium and bicarbonate were the dominant or most common ions in 61% of US lakes, with the remaining lakes dominated by magnesium or sodium and sulfate or chloride ions. Lake salinity was strongly related to natural factors (e.g., lake elevation, soil, and hydrology) and influenced by human factors including agriculture and atmospheric deposition. Major ion composition was associated with similar natural factors, but was also strongly affected by road density, urban development, agricultural activities, and atmospheric deposition. This macroscale understanding of salinity and major ions and their complex relationships to natural and human characteristics around lakes is needed to assess, predict, and manage lake impairments from human alterations of ion chemistry.