Hidden threats: Groundwater salinity is on the rise

U.S. groundwater is witnessing a disturbing trend – increasing salinity. This is the conclusion of a study by the U.S. Geological Survey (USGS) that spanned three decades. 

The research highlights the impact of rising groundwater salinity on infrastructure, ecosystems, and human health.

Decades of monitoring 

For over 30 years, the USGS has been tracking groundwater quality in wells across the country. The comprehensive analysis examined up to 500 chemical constituents including major ions, metals, pesticides, volatile organic compounds, fertilizers, and radionuclides. 

Among these, Na and Cl ions and dissolved solids – which are indicators of salinity – had significantly increased.

Focus of the study 

The research is an ongoing effort under the National Water Quality Network, which evolved from the National Water Quality Assessment Project initiated in 1992. 

“The original goal was to evaluate the status of water quality in the nation, including groundwater, surface water, and ecological health,” said Bruce Lindsey, a hydrologist with USGS.

Over the years, the experts have zeroed in on certain constituents with potential long-term adverse effects.

Types of wells

The researchers classified the wells into three categories: domestic, urban, and agricultural. Domestic wells, representing medium depth aquifers and drinking water, are unregulated private wells. 

By contrast, urban and agricultural wells are shallower, ranging between 30 to 50 feet. “The purpose of (sampling) those were to understand the status and trends in the very shallowest water levels,” explained Lindsey. The shallow wells acted as “sort of a sentinel of what might be moving deeper into the aquifer, so to speak.”

Constituents of concern

After identifying 82 networks with 20 to 30 wells each, the team pinpointed 28 constituents of concern. These results are accessible on the USGS’s interactive groundwater map, detailing decadal shifts.

“If we look at all 28 constituents across all 82 networks, dissolved solids, chloride, and sodium had statistically significant increases more frequently than any other constituents that we have on our list,” said Lindsey. “If you look at the map, you’ll see patterns right away that jump out.”

Problem areas

Two primary regions stood out in terms of rising salinity. The Northeastern and Upper Midwest areas, especially around urban centers with cold climates, were impacted due to substantial road salt application. 

Conversely, arid zones, notably in the southwest, faced natural soil salinity, which was exacerbated by irrigation practices.

“When irrigating agriculture in arid regions, you get a lot of evaporation,” explained Lindsey. “So if the salinity of the irrigation water is relatively low, but a large percentage of it evaporates, [salinity levels] can become high.”

Study implications 

The repercussions of increased salinity are numerous. Elevated chloride concentrations in groundwater, which feed into many streams, can disturb aquatic ecosystems. 

Lindsey warned that these imbalances can take decades to materialize and an equal time to reverse, even if salinity sources are managed better.

On the infrastructure front, increased salinity means heightened corrosivity. This can result in lead and other metals leaching from household plumbing components. 

New concerns

The study also revealed a unique concern tied to salinity. In a southern New Jersey sandy aquifer, high salinity combined with low pH water mobilized radium, a radioactive element dangerous to humans. 

“It goes back to road salt,” explained Lindsey. “Road salt is increasing, causing sodium and chloride to increase, which is causing radium to increase.”

Lindsey noted that there seems to be increased awareness of the environmental effects of road salt, with trucks spreading less salt or municipalities switching to a lower-concentration brine. He hopes that future research will highlight other cascading impacts of increasing salinity in groundwater. 

“The fact that there may be streams that are not able to sustain aquatic life, or that your pipes might start corroding, or this other more rare issue where there’s radium, shows there are other negative aspects (to rising groundwater salinity).”

The research was presented at the 2023 meeting of the Geological Society of America.

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