Scientists from the U.S. Geological Survey have been monitoring groundwater quality in wells across the country for more than three decades, looking for harmful chemicals or residual substances that may cause harm to ecosystems or humans. In all, they have measured up to 500 chemical constituents, including major ions, metals, pesticides, volatile organic compounds, fertilizers, and radionuclides.
Of these constituents, there have been significant increases of Na and Cl ions and dissolved solids—all related to salinity.
The researchers sampled wells within three different network types: domestic areas, urban areas, and agricultural areas. Domestic wells, or private wells that are not regulated by the U.S. Environmental Protection Agency or a local municipality, represented medium depth aquifers and drinking water. Urban and agricultural wells were shallower, usually around 30 feet to 50 feet deep.
“The purpose of [sampling] those were to understand the status and trends in the very shallowest water levels,” said Bruce Lindsey, a hydrologist with USGS. The shallow wells acted as “sort of a sentinel of what might be moving deeper into the aquifer, so to speak.”
The USGS team identified 82 networks, each with 20 to 30 wells, and identified 28 constituents to track that had levels of concern. Water was sampled every 10 years to track changes in chemical concentrations. These constituents and sampling results can be seen on the USGS’s interactive groundwater map, which shows decadal changes.
“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,” Lindsey said. “If you look at the map, you’ll see patterns right away that jump out.”
One of these spots is the Northeast and Upper Midwest regions, “particularly around urban areas where there’s cold weather and a lot of road salt,” Lindsey said. “We obtained data on road salt application and found correlations between these increases in chloride and sodium and dissolved solids with the road salt application rates.”
But another region also had elevated levels of Cl, Na, and dissolved solids — the arid regions of the country, especially in the Southwest. These regions naturally have high salinity in the soil to begin with, but irrigation complicates the issue.
“When irrigating agriculture in arid regions, you get a lot of evaporation,” Lindsey explained. “So, if the salinity of the irrigation water is relatively low, but a large percentage of it evaporates, [salinity levels] can become high.”
These rising levels of Na, Cl, and dissolved solids can cause multiple problems, starting with the environment. Many streams are fed by groundwater, and higher concentrations of chloride in the water can knock out the natural balance that aquatic life is used to.
“[A rising level] is something that can take 20, 30, 40 years to develop…which means that it can also take that long to recover if management of the sources of salinity changes,” Lindsey said.
Click here to see the USGS’s interactive groundwater map, which shows decadal changes.