Brackish Groundwater in the Southwest: A Salty Resource That Could Stretch Water Supplies

Irrigation, one possible use for brackish groundwater. Jeff Vanuga, USDA Natural Resources Conservation Service.

Why look at salty water

Secure, sustainable water underpins food, energy and the health of people and ecosystems. But heavy pumping of fresh groundwater has stressed aquifers in places — lowering the water stored in them and cutting the flow to rivers and springs. In parts of the southwestern United States, supply can't meet demand without such harm, and drought makes it worse.

As part of the national census of water resources, the U.S. Geological Survey completed a national brackish groundwater assessment: a database of what is known about brackish groundwater across the country, and a report on where it is, what it's like and how it's used. This summary covers the Southwest Basins region and its five largest principal aquifers.

What "brackish" means

Brackish water carries moderate to high amounts of dissolved solids — minerals, metals and salts. The main ones are calcium, magnesium, sodium and potassium (positively charged), sulfate, chloride and bicarbonate (negatively charged), and silica (neutral).

Water too salty to drink can still serve many other purposes.

Where it's found

Groundwater quality here varies sideways and with depth, following geology, minerals, structure, drainage and development.

• Four of the aquifers — Basin and Range basin-fill, California Coastal Basin, Central Valley and Rio Grande — are loose to partly consolidated stream and lake sediments, with volcanic rock in places. The Basin and Range carbonate-rock aquifers are marine limestone, dolomite, sandstone, shale and volcanic rock. Brackish water also occurs outside all five.
• Salts are usually lower where water recharges the ground, as along mountain fronts, and higher where it discharges, often mid-basin.
• Brackish water sits mainly in two zones:
• • shallow, where evaporation concentrates minerals, especially at the end of closed basins;
  • deep, from long flow paths, soluble salts (gypsum, anhydrite, halite), inflow from neighboring rock, or saline water trapped in marine sediments.
• Other sources: stream leakage, thermal springs, oil-field brines and seawater intrusion on the California coast.

Aquifer by aquifer:

• Central Valley: brackish in the upper, unconfined part; saline in marine sediments at depth.
• Rio Grande: shallow brackish water in closed basins, and where deep basin water, made brackish by long flow paths, rises.
• Basin and Range basin-fill: shallow brackish water in closed basins near playas, above confining layers, and where deep water rises; below a few thousand feet, confined saline water poorly connected to shallower layers.
• Basin and Range carbonate-rock: brackish where there is geothermal activity and groundwater discharge.
• California Coastal Basin: shallow brackish water from evapotranspiration, saline water applied at the surface (oil-field brines, irrigation water) and seawater intrusion; deeper brackish water in marine sediments.

Gypsum: groundwater that dissolves evaporite minerals like this becomes more brackish. David Anning, USGS.

How much there is

The assessment used dissolved-solids data from about 34,000 wells sampled by federal, state and local agencies, fed into a 3-D model: 4 layers of cells 6.2 by 6.2 miles, at 0–50, 50–500, 500–1,500 and 1,500–3,000 feet below land surface.

• The grid covers about 216,000 cubic miles underground, but wells sample only about 14 percent of it (30,300 cubic miles); the other 86 percent is unknown.
• Brackish conditions above 3,000 feet add up to about 9,300 cubic miles — about 31 percent of the volume the wells represent.
• Most of it lies between 50 and 1,500 feet deep.

Figure 3. Share of the 9,300 cubic miles of observed brackish groundwater in each depth interval. USGS.

Shallow water is cheaper to pump. Brackish water lies within 50 feet of the surface over large parts of the Central Valley and the Rio Grande system in south-central New Mexico, and within 50 to 500 feet over large parts of the California Coastal aquifers, the basin-fill aquifers of southwestern Arizona and western Utah, and the Rio Grande system in central New Mexico and west Texas.

Volumes are for grid cells with well data, down to 3,000 feet; "share" is of the volume those cells represent. "Other" includes wells outside, or not assignable to, the five aquifers — among them 6 in Southern Nevada volcanic rock and 31 in alluvial or glacial sand and gravel. USGS.

A resource worth weighing

About 22.5 million acre-feet of groundwater is pumped each year from the five aquifers for irrigation, public supply and industry. (An acre-foot is about 326,000 gallons — almost enough to fill a football field a foot deep.) Only about 0.6 million acre-feet (about 3 percent) of that is brackish or highly saline.

The potential is large. If just 1 percent of the brackish water in the well-sampled volume could be pumped and used, that would be about 93 cubic miles — 310 million acre-feet. There is probably more: the 86 percent of the region without wells may hold usable brackish water too, and the national study's predictive model shows a high probability of brackish groundwater across much of the Southwest.

Using it

Livestock watering. Jeff Vanuga, USDA Natural Resources Conservation Service.

• Treatment: water too salty to drink at first can sometimes be improved by reverse osmosis, or by blending with fresh water.
• Other uses: cooling at power plants, irrigation, livestock, aquaculture, and oil and gas drilling, enhanced recovery and hydraulic fracturing.

Oil and gas drilling and hydraulic fracturing can use brackish water. David Anning, USGS.

Cooling during power generation. Courtesy of the U.S. Nuclear Regulatory Commission.

Limits:

• Contaminants: arsenic, nitrate and uranium were the constituents most likely to exceed drinking-water standards in southwestern brackish water, matching other studies. The basin-fill aquifers had the largest share of samples over livestock limits, for arsenic, boron and fluoride; elsewhere, untreated brackish water was generally fine for livestock. The same three are a concern for irrigation.
• Scaling: mineral deposits can clog pipes, tanks and treatment systems.
• Yield: low elsewhere in the country, but high enough in the Southwest. About 80 percent of the wells yielded more than 100 gallons per minute, and almost 60 percent of brackish wells more than 1,000. Compaction and cementation could limit deep development in places, but shallow and deep wells alike produce enough for many uses.

Brackish well yields (median, gallons per minute):

Next steps

The assessment updated the basic picture of brackish aquifers in the Southwest and pointed to where research should go. Its data, compiled from more than 30 national, regional, state and local sources, can be downloaded from the USGS.

What's needed next: detailed water chemistry and aquifer data for specific areas of interest, to judge whether pumping brackish water is sustainable — including its effects on neighboring water supplies.

Preparing to sample a municipal well near Phoenix, Arizona, for one of the Southwest's many groundwater-monitoring programs. David Anning, USGS.

Sources

Based on Brackish Groundwater and Its Potential as a Resource in the Southwestern United States, USGS Fact Sheet 2018–3010, by David W. Anning, Kimberly R. Beisner, Angela P. Paul, Jennifer S. Stanton and Susan A. Thiros, U.S. Geological Survey, drawing on the national assessment, Brackish Groundwater in the United States (Professional Paper 1833); a work of the United States government in the public domain. The introduction, definitions, occurrence section, tables, chart and photographs are taken from the fact sheet's PDF. The fact sheet's text puts most brackish water between 50 and 1,500 feet, as its chart shows, while one caption says 50 and 1,000; this page follows the text. Two maps drawn on licensed commercial base maps are not reproduced.