Predicting Groundwater Quality in Unmonitored Areas

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Groundwater provides nearly one-half of the Nation’s drinking water, and sustains the steady flow of streams and rivers and the ecological systems that depend on that flow.  Unless we drill a well, how can we know the quality of the groundwater below? Learn about how the USGS is using sophisticated techniques to predict groundwater quality and view national maps of groundwater quality.

Although groundwater is a safe, reliable source of drinking water for millions of people nationwide, high concentrations of some chemical constituents can pose potential human-health concerns. Some of these contaminants come from the rocks and sediments of the aquifers themselves, and others are chemicals that we use in agriculture, industry, and day-to-day life. When groundwater supplies are contaminated, millions of dollars can be required for treatment so that the supplies can be usable. Contaminants in groundwater can also affect the health of our streams and valuable coastal waters. By knowing where contaminants occur in groundwater, what factors control contaminant concentrations, and what kinds of changes in groundwater quality might be expected in the future, we can ensure the availability and quality of this vital natural resource in the future.

At the USGS, researchers are using information from gathered from thousands of wells across the nation to determine how myriad factors, from groundwater age to pesticide use, affect which chemical constituents are dissolved in groundwater and at what concentrations.  The constituents include trace elements, such as arsenic and manganese; radionuclides, such as radon and  uranium; nutrients, such as nitrate and phosphorus; and manmade chemicals, such as the herbicide atrazine.  At elevated concentrations, some of these constituents in drinking water can pose a threat to human health. Others are simply a nuisance, causing drinking water to have an unpleasant smell, taste, or appearance; cause skin or tooth discoloration; or cause corrosion or sedimentation of plumbing.

 

National Maps of Groundwater Quality

By combining data on the concentrations of chemical constituents measured in thousands of wells, locations of contaminant sources, and factors that affect how a constituent behaves in groundwater, scientists can use sophisticated statistical approaches to predict where a constituent is likely to occur in groundwater and at what concentration. These maps can be used by water resource managers to (1) anticipate water quality in unsampled areas or depth zones, (2) design targeted monitoring programs, (3) inform groundwater protection strategies, and (4) evaluate the sustainability of groundwater sources of drinking water.

View regional and national maps for the following chemical constituents and contaminants:

 

Mapping Groundwater Quality in 3-D

Just as the characteristics of an aquifer can vary greatly with depth, so can the quality of the groundwater. Domestic wells, such as those used by homeowners, typically pump groundwater from relatively shallow depths.  Public-supply wells that serve many thousands of people typically pump groundwater from much greater depths—sometimes hundreds of feet or more. 

Three-dimensional, or 3-D, depictions of groundwater quality are being developed for four Principal Aquifers: the Glacial Aquifer system, which extends across the northernmost one-third of the United States; the Central Valley aquifer, in California; the Mississippi Embayment aquifer in south-central U.S.; and the Northern Atlantic Coastal Plain aquifer, in the southeast. These depictions focus on groundwater quality at depths tapped by domestic-supply wells and public-supply wells. The information gained also will be used to assess the vulnerability of streams to contaminants derived from groundwater in these areas. 

 

NAWQA GW 2

Scientist are using models to predict concentrations of nitrate in groundwater of California's Central Valley. Concentrations are predicted at the typical depth of a domestic (private) well (55 meters deep; left) and typical depth of a public-supply well (122 meters deep, right).  (From Ransom and others, 2017)

NAWQA GW 1

Statistical models are used to create 3-D maps of estimated concentrations of nitrate in California's Central Valley. The views shown here are from the southeast (top) and the southwest (bottom). The vertical scale is depth in meters. (From Ransom and others, 2017)