INCREASING SALINIZATION OF GROUNDWATER IN THE UNITED STATES (Invited Presentation)

A 34-year study of groundwater-quality trends in the United States identified significant increasing concentrations of Na, Cl, and dissolved solids (DS) more frequently than any of the 28 other constituents analyzed for trends. This study by the U.S. Geological Survey (USGS) evaluated decadal scale changes in groundwater quality in principal aquifers used for domestic supplies based on samples from 82 networks of 20-30 wells each. Most of those networks have been sampled three times at 10-year intervals between 1988 and 2022. Data on concentrations of 28 constituents have been analyzed for step changes from decade to decade or trends across all 3 decades. Nearly half of the networks had statistically significant increases in Na and Cl concentrations. Those increases are related to road-deicing salt application rates in urban areas of the northeastern US and upper Midwest regions or to agricultural irrigation, high evaporation rates, and saline soils in arid to semiarid regions of the western US. Concurrent with increasing salinity, corrosivity indices such as the Cl/SO₄ ratio are increasing in the same regions. Furthermore, increasing Na and Cl concentrations may cause cascading problems further affecting drinking-water safety. Increases in Na and Cl concentrations have been reported by USGS to correlate with increasing Ra in a low-pH aquifer in New Jersey, where road salt application rates were correlated with the rate of increases of Ra, Na, and Cl. Excess Na can mobilize Ra into solution by reverse cation exchange, while Cl can enhance Ra solubility by forming RaCl⁺ and RaCl₂ complexes.