Safe, dependable and lasting supplies of water underpin the country's food production, its energy independence, and the health of people and ecosystems. At any given moment some parts of the country are short. Pumping groundwater on a large scale has reduced the amount held underground and the amount flowing out into rivers and lakes. In some regions, particularly arid and semiarid ones, supply does not meet demand, and severe drought adds to the strain. If drought persists, the authors warn, shortages could harm people and threaten the flows that ecosystems need to stay healthy.
As part of the national census of water resources, the U.S. Geological Survey carried out a national assessment of brackish groundwater, to update what is known about it as a source that could add to or replace fresh water. The aims were to gather the available data into one database of the country's brackish groundwater and to publish a summary of where it is, its physical and chemical properties and how it is used. Congress authorised the work in the Omnibus Public Land Management Act of 2009. The previous national compilation dated from the mid-1960s, and far more hydrological and chemical data have been collected since.
What makes water brackish
All natural water carries dissolved solids, and enough of them makes surface water or groundwater brackish. Broadly, brackish groundwater holds more dissolved solids than fresh water and less than seawater, though many different schemes have been used to put numbers on the categories. Dissolved solids, also called total dissolved solids, measure everything organic and inorganic dissolved in a sample: minerals, metals, salts and so on.
The U.S. Environmental Protection Agency's secondary maximum contaminant level, a voluntary standard that applies only to public water systems, recommends no more than 500 milligrams per litre (mg/L) of dissolved solids, though many supplies exceed it. Water above 1,000 mg/L is generally regarded as unsuitable for drinking: it can taste bitter, salty or metallic, smell bad or even be toxic. Water with more dissolved solids still has many uses other than drinking.
The assessment set these categories:
- Fresh: less than 1,000 mg/L.
- Brackish: 1,000 to 10,000 mg/L.
- Highly saline: more than 10,000 mg/L.
Seawater, for comparison, holds about 35,000 mg/L. "Saline groundwater" usually means anything at 1,000 mg/L or above, so it takes in both the brackish range and saltier water.
Where it is
The assessment looked at depths of less than 3,000 feet below the land surface, because few data were available deeper down. Chemistry data for about 380,000 groundwater samples were gathered from 33 sources and summarised on a coarse three-dimensional grid. Brackish groundwater turned up somewhere within 3,000 feet of the surface in every state except New Hampshire and Rhode Island.
Most of the known brackish groundwater is in the Western Midcontinent region, with other large occurrences in the Coastal Plains, the Eastern Midcontinent and the Southwestern Basins. The Atlantic coast states have the most observations, but their groundwater is mostly fresh, with little brackish water except along the shore.
Deeper water tends to carry more dissolved solids and to be brackish more often. Across the country, about 70 percent of sampled wells between 1,500 and 3,000 feet deep produced brackish or highly saline water, against fewer than 20 percent of wells between 0 and 50 feet. The median dissolved-solids concentration was 334 mg/L in wells between 0 and 50 feet deep and 3,692 mg/L in wells between 1,500 and 3,000 feet.
What is in it
Salt content is not the only limit. Other substances in brackish groundwater can restrict its use, and the mix varies from place to place because it depends partly on local geology, hydrology and climate. Those differences matter: they bear on whether using the water is feasible, how it can be treated and what that costs. A particular substance may exceed the standard for a given use, and knowing which ones are present shows whether the water needs special treatment beyond removing salt. Arsenic, for example, can exceed the EPA's drinking-water standard of 10 micrograms per litre.
Scaling is another limit. Minerals come out of solution and build up on surfaces or membranes that store, carry or filter water, and that can restrict flow — inside pipes, for instance, narrowing the supply lines. Corrosion, which is less common with brackish groundwater, wears away metal surfaces. Water often has to be treated to remove what causes scaling or what exceeds health limits.
Reverse osmosis is the most common desalination treatment in the United States. It pushes water under pressure through a thin, semipermeable barrier that holds back dissolved substances. If the source water is not first treated to reduce its tendency to scale, reverse osmosis systems can fail, so knowing which minerals are present, and in what amounts, allows the costs and benefits of the water and a given treatment to be weighed.
One measure is the Langelier saturation index, which the water-supply industry uses to judge whether water will corrode or scale, based on how saturated it is with calcite. A negative value means the water is unlikely to deposit calcite and may be corrosive; a positive value means it is likely to deposit calcite in distribution or treatment systems. Of 14,380 samples with at least 1,000 mg/L of dissolved solids, only 4 percent had an index above 1 (a higher risk of scaling) and only 2 percent had one below −1 (a higher risk of corrosion). Minerals that could form scale include calcite, barite and chalcedony, depending on where the water comes from.
How much wells can draw
To judge brackish groundwater as a resource, one needs to know how well the aquifers holding it store and pass water. That depends on physical properties of the sediments such as hydraulic conductivity, storage coefficient, specific yield and porosity. These are poorly known for most brackish zones, so the assessment used well-yield data, available for about 16 percent of wells, as a measure of how much usable water wells producing brackish water can supply.
Compaction and cementation of deeper sediments could hold back development in some areas, but reported yields suggest that both shallow and deep wells could produce brackish groundwater fast enough for many uses. In the regions with the most brackish groundwater, many sampled wells report yields of at least 10 gallons per minute. In the Southwestern Basins a large share can deliver at least 100 or even 1,000 gallons per minute. These are probably minimums, because they come from reported pumping rates rather than the most a well could produce. A high yield, though, is no guarantee that a well can keep producing, or produce without harm.
Share of brackish groundwater samples from wells reporting yields of at least 10, 100 and 1,000 gallons per minute, in the Coastal Plains, Eastern Midcontinent, Southwestern Basins and Western Midcontinent regions. USGS chart.
Judging whether brackish groundwater can be developed sustainably also needs more about the wider water system: where it is recharged and where it discharges, and how brackish groundwater connects with fresh groundwater and surface water. With that, one can assess whether pumping could:
- push more mineralised water into fresh zones, or the reverse;
- lower groundwater levels substantially and make the land sink;
- set off knock-on effects on streams and other surface water as the system adjusts to the new withdrawals.
Can it stand in for fresh water?
Water providers are turning to brackish groundwater to supplement or replace fresh water, for drinking and for other uses: cooling power stations, irrigation, aquaculture, and oil and gas work such as drilling, enhanced recovery and hydraulic fracturing. What has held it back is a lack of basic knowledge about where it is, how easy it is to reach, what it yields, what it contains, how it is used, what treatment it needs and what developing it does to the environment.
Knowing more about where brackish groundwater is and what it is like will help develop it efficiently and sustainably. Finding new sources, especially where fresh water is scarce, could strengthen the country's water security. In places, using brackish water could ease the pressure on fresh water used for drinking, business and recreation, and developing and treating it to different standards for different uses could help regions under strain make their fresh water go further.
The assessment gives an up-to-date basic picture of where brackish aquifers are and what they are like, and a basis for choosing where future research could help develop them. The next step, the authors write, is detailed information on individual aquifers — what is needed to judge whether they can be developed sustainably, including how pumping them would affect the water around them.
The assessment's data, compiled from more than 30 national, regional, state and local sources, can be downloaded from the USGS brackish groundwater site.
Sources
- Jennifer S. Stanton and Kevin F. Dennehy, Brackish Groundwater and its Potential to Augment Freshwater Supplies, USGS Fact Sheet 2017–3054, July 2017. https://doi.org/10.3133/fs20173054
- The full assessment: Stanton and others, Brackish Groundwater in the United States, USGS Professional Paper 1833, 2017. https://dx.doi.org/10.3133/pp1833
- Data: https://water.usgs.gov/ogw/gwrp/brackishgw/
- The fact sheet's three maps are left out because their base map is Esri's copyrighted imagery; the well-yield chart is USGS's.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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