What Is Brackish Water? Salinity, Sources and Safety

What Is Brackish Water?

Brackish water is water that's saltier than fresh water but not as salty as the ocean. The U.S. Geological Survey puts numbers on it: in its national brackish groundwater assessment, brackish groundwater has dissolved solids between 1,000 and 10,000 milligrams per liter (mg/L). At those levels, mg/L and parts per million (ppm) are close enough to read as the same number. Ocean water, for comparison, carries about 35,000 ppm of salt.

You'll find brackish water where rivers meet the sea, in coastal wells where seawater has crept into the aquifer, and in aquifers far from any coast. It isn't drinkable untreated, and the filters most homes already own won't change that. The salt is dissolved in the water, not floating in it.

The short version: in wells and water supplies, brackish water starts at 1,000 mg/L of dissolved solids, twice the EPA's 500 mg/L guideline for drinking water. Making it drinkable takes desalination, which for most homes and businesses means reverse osmosis matched to how salty the water actually is.

How Salty Is Brackish Water?

"Brackish" covers a wide band, and where your water sits inside it changes what you can do with it. The USGS sorts water into salinity classes on its saline water and salinity page. The EPA's secondary drinking water standards set a 500 mg/L guideline for total dissolved solids (TDS), and list a salty taste, deposits and staining among the effects people notice above it. Line the two up and you get a working map:

Two things in that table are easy to miss. First, the brackish range is a working definition, not a legal limit. The USGS uses 1,000 to 10,000 mg/L "for purposes of this study" of groundwater. Estuary scientists use a much wider band: NOAA's estuaries tutorial says the salinity of brackish water can range from 0.5 to 35 parts per thousand, roughly 500 to 35,000 ppm. For wells and water supplies, the groundwater range is the one that matters.

Second, the membrane column doesn't break where the definition does. When Crystal Quest® specifies a reverse osmosis system, we match the membrane class to the feed water's TDS: standard membranes up to roughly 2,000 ppm, brackish-water membranes from roughly 2,000 to 10,000 ppm, and seawater membranes above that. Those are approximate working ranges, not hard cutoffs. What they mean in practice is that water at the low end of the brackish band can often run on the same membrane class as ordinary well water, and water at the high end can't.

Where Brackish Water Comes From

Water usually turns brackish in one of three places, and knowing which one applies tells you a lot about your own supply.

Where rivers meet the sea

Estuaries are the textbook case. NOAA's estuary explainer describes brackish water as a mixture of fresh water draining from the land and salty seawater. That's the picture most people have in mind, though nobody is filling a glass from a salt marsh.

Coastal wells

The same mixing happens underground along the coast. The USGS explains in its saltwater intrusion overview that the boundary between fresh groundwater and seawater in a coastal aquifer is a diffuse zone where the two mix. Groundwater pumping can reduce freshwater flow toward the coast and draw saltwater toward the fresh part of the aquifer. The USGS says this has happened to some degree in many U.S. coastal aquifers.

If you're on a coastal well and the water has started to taste salty, our guide to saltwater intrusion in well water covers the warning signs, the chloride test and what to do next.

Inland aquifers

Brackish water isn't only a coastal problem. Citing 2010 studies, the USGS reports that more than 95 percent of U.S. desalination facilities are inland, and that most are designed to treat groundwater in the brackish range. A salty well hundreds of miles from the ocean may simply be drawing from a brackish aquifer.

Can You Drink or Use Brackish Water?

Not as it is. The USGS is blunt about it: humans can't drink saline water, and drinking water that's too salty is dangerous to health as well as unpleasant. Brackish groundwater starts at 1,000 mg/L, and the EPA lists a salty taste as the effect people notice when chloride or sulfate climbs above 250 mg/L.

Those EPA numbers are guidelines. The EPA doesn't enforce its secondary standards; it sets them to help water systems manage taste, color and odor. So a well that reads a little over 500 mg/L isn't automatically a problem. It is a reason to find out what's making up that number.

Sodium deserves its own look on your lab report. The University of Maryland Extension notes that some wells carry enough sodium that people on sodium-restricted diets could get most of their daily allowance from a few glasses of water. If anyone in your home is on one, share your results with their doctor.

Brackish water can still be the source of perfectly good tap water. The USGS reports that public drinking water suppliers are increasingly turning to brackish groundwater, and that for drinking water, it's treated through reverse osmosis or other desalination processes before it reaches anyone's tap.

Irrigation and growing

Plants have limits too. The USGS notes that slightly saline water is sometimes used much like fresh water, and that in Colorado, water with up to 2,500 ppm of salt is used to irrigate crops. Moderately and highly saline water has far fewer uses. If you grow commercially, our guide to greenhouse and hydroponic water treatment covers how growers read salinity as electrical conductivity and when reverse osmosis earns its place.

Plumbing and appliances

Salty water is hard on a building, too. The EPA groups chloride and TDS with the standards related to corrosion and staining, and lists TDS among the ones related to scale and deposits. In a home, that shows up on pipes, fixtures and anything that heats water.

How to Tell If Your Water Is Brackish

Taste is a clue, not a measurement. To know where you stand, you need a number.

1. Screen it with a TDS meter. A handheld meter reads conductivity and converts it to TDS using a factor, and our guide to TDS in water explains why that factor shifts for brackish water. Treat the reading as an estimate that tells you whether to test further.
2. If it reads near or above 1,000, get a lab test. Ask for total dissolved solids, chloride and sodium, which tell you how salty the water is. Add hardness, iron, manganese and silica, which decide the pretreatment any membrane will need. Our guide to testing well water covers how to take a proper sample.
3. Test again if something changes. On a coastal well, a rising chloride trend tells you more than any single reading.

How Brackish Water Is Treated

This is where a lot of people spend money on the wrong thing. The salt in brackish water is dissolved, so the usual household filters don't touch it:

• Carbon filters take out chlorine, taste and odor compounds, but not dissolved minerals. A carbon filter barely moves a TDS reading.
• Sediment filters catch sand, silt and rust. Dissolved salt passes straight through.
• Water softeners swap calcium and magnesium for sodium, so the total dissolved solids stay about the same. UMD Extension notes that homes using softeners can end up with higher sodium and chloride.

What does work is desalination. Reverse osmosis takes the lead here. The USGS desalination explainer describes it as forcing water through a semi-permeable membrane that holds the salt back while the water passes through. For a single faucet, UMD Extension also lists distillation. Just don't confuse distillation with boiling a pot of salty water. Boiling drives the water off and leaves the salt behind in the pot; distillation captures that steam and condenses it back into water.

What changes as the water gets saltier

Reverse osmosis on brackish water isn't just a bigger version of an under-sink filter. Three things change:

• The membrane. Above roughly 2,000 ppm, we'd move from a standard membrane to a brackish-water membrane, and above roughly 10,000 ppm to a seawater membrane. Put a standard membrane on water that's too salty for it and its rejection rate falls, as our guide to RO rejection rate explains.
• The pressure. Saltier water pushes back harder against the membrane, so it takes more pressure to drive fresh water through. Our explainer on how reverse osmosis works puts brackish water at roughly 150 to 600 PSI. That's pump territory, well past ordinary household pressure.
• What goes to drain. Every RO system sends part of its feed water away as concentrate, carrying the salt the membrane rejected. On a well, that's water your pump had to lift. For a farm or plant, where the concentrate goes becomes part of the design. Our guide to reverse osmosis waste water covers the basics.

Pretreatment matters more as the stakes rise. Chlorine damages RO membranes, hardness scales them, and iron and silica foul them. What sits in front of the membrane decides how long it lasts.

Sizing it for a home, a farm or a business

For a household at the low end of the range, the usual first step is treating the water you drink and cook with at one tap, which is the setup our saltwater intrusion guide walks through. Higher up the range, send your lab report to a specialist before you choose any system. When salt causes trouble at every fixture, whole-house reverse osmosis comes into the picture, with the pretreatment, storage and drain water that come with it. Our look at whole house reverse osmosis pros and cons covers who actually needs one.

Farms, greenhouses, bottlers and other businesses usually need far more water. Crystal Quest Viper brackish water RO systems are offered in capacities from 1,500 to 180,000 gallons per day, and we've built brackish RO plants for disaster relief, including a 50,000 gallon-per-day system. Our commercial reverse osmosis sizing guide shows how to work out the capacity you need, and our commercial filtration team can review your water analysis with you.

Whatever the scale, the order stays the same. Get the lab report first, then match the membrane, the pretreatment and the capacity to it.