Fresh, Mineral or Salt Water? Why Water Type Matters When Drilling

If you want to find fresh water underground, you need to know that not all groundwater is drinkable. The U.S. Geological Survey classifies water with less than 1,000 parts per million (ppm) of dissolved salts as fresh, 1,000 to 3,000 ppm as slightly saline, 3,000 to 10,000 ppm as moderately saline and 10,000 to 35,000 ppm as highly saline; seawater is about 35,000 ppm. Brackish groundwater occurs in almost every state, and it generally becomes more common with depth, so the type of water you hit matters as much as finding water at all.

This guide explains why water type varies, how to estimate your risk before drilling and what your options are if a well comes in mineralized or salty.

Fresh, mineral and salt water: what the terms mean

All groundwater carries some dissolved minerals picked up from the rock and sediment it moves through. The total is measured as total dissolved solids (TDS), usually in milligrams per liter (mg/L), which is roughly equivalent to ppm for most groundwater.

"Mineral water" in everyday drilling talk usually means water that is technically fresh or slightly saline but high in specific constituents such as iron, manganese, calcium, sulfate or hardness. These affect taste, staining and scaling even when TDS is moderate. The only way to know what you have is a laboratory test.

Why water type changes from place to place

Several conditions make groundwater brackish or mineralized:

• Rock type. Water moving through evaporite deposits, marine shales or certain limestones can pick up salts and sulfates.
• Residence time. Water that has been underground longer has more time to dissolve minerals.
• Depth. Deeper water is often older and saltier.
• Coastal settings. Pumping near coasts can draw seawater into freshwater aquifers.
• Arid climates. Evaporation concentrates salts in soils and shallow groundwater.

Depth and the odds: where to find fresh water underground

The USGS National Brackish Groundwater Assessment, published in 2017, compiled roughly 380,000 groundwater samples. Its findings are useful for any landowner planning a well:

• Brackish groundwater was observed within 3,000 feet of the surface in every state except New Hampshire and Rhode Island.
• The median dissolved-solids concentration was 334 mg/L for wells 0 to 50 feet deep, compared with 3,692 mg/L for wells 1,500 to 3,000 feet deep.
• About 70 percent of sampled wells between 1,500 and 3,000 feet produced brackish or highly saline water, compared with less than 20 percent of wells 0 to 50 feet deep.
• The largest volumes of brackish groundwater are in the Western Midcontinent, with other significant areas in the Coastal Plains, Eastern Midcontinent and Southwestern Basins.

The practical takeaway: drilling deeper is not always the fix for a weak well. In some areas, deeper formations trade quantity for quality. Nearby well logs and any water-quality data from neighbors are the best guide to which formation in your area produces fresh water. Our article How Deep Is Groundwater? explains regional depth patterns.

Estimating water type before you drill

1. Ask neighbors and drillers. Local drillers usually know which formations come in salty or high in iron or sulfur.
2. Check state records. Some well logs and state reports include water-quality notes or test results.
3. Review USGS and state studies. Aquifer reports often map dissolved solids by formation and depth.
4. Consider geophysics. Electrical resistivity responds to salinity: the U.S. EPA notes that more dissolved solids in pore water lower resistivity. That makes resistivity useful for mapping fresh and saline zones, with one important caution: clay also lowers resistivity, so a low reading is not automatically salt water.
5. Plan to sample during drilling. Ask your driller about testing water from each producing zone so the well can be completed in the best one.

What groundwater detectors claim about water type

Several units in our groundwater detectors collection list water-type identification among their features. For example, the BR Systems BR 950 is marketed for fresh, mineral and salt water, and GER Detect states that the Fresh Result 2 Systems can differentiate fresh, saline and highly saline sources using a resistivity-based system. These are manufacturer statements. Because clay, moisture and mineralogy also influence electrical readings, treat any on-screen water-type indication as a preliminary signal and confirm with a certified laboratory test. Our guide to how groundwater detectors work covers the underlying measurements.

If your well comes in salty or mineralized

• Test thoroughly first. The EPA recommends that private well owners test at least annually for total coliform bacteria, nitrates, total dissolved solids and pH, using a state-certified laboratory.
• Talk to your driller about zones. Sometimes a well can be sealed off from a salty zone and completed in a fresher one.
• Consider treatment. According to the USGS, reverse osmosis is the most common desalination method in the U.S. Water softeners, iron filters and other systems address specific constituents. Scaling from minerals can damage pipes and treatment equipment, so the full chemistry matters.
• Match water to use. The USGS notes that brackish groundwater can be used for some purposes, such as certain irrigation, livestock or industrial uses, directly or after treatment. A county extension office can advise on crop and livestock tolerance.

Keeping a fresh well fresh

Finding fresh water is only half the job. Water quality can change over the life of a well, so a few habits help protect it:

• Avoid over-pumping. The USGS explains that pumping faster than an aquifer recharges lowers the water table around the well, forming a cone of depression. In coastal areas and in aquifers layered over saltier water, heavy pumping can pull poorer-quality water toward the well.
• Protect the wellhead. Keep the casing above ground with a sealed cap, slope soil away from it and keep chemicals, fuel and livestock waste well away.
• Test on a schedule. Annual testing creates a baseline. A sudden rise in TDS, chloride or nitrate is an early warning that something has changed.
• Test after events. The EPA recommends testing after flooding, nearby construction, well repairs or any change in taste, color or odor.
• Keep records. Store your well log, pump records and test results together, and pass them on if you sell the property.

Fresh water for livestock and irrigation

Households are not the only users who care about salinity. Livestock may drink less, or perform poorly, when water is too mineralized, and salty irrigation water can build up salts in soil over time and reduce crop yields. Tolerance varies by species, crop, soil and climate, so there is no single safe number. If a well is intended mainly for stock or irrigation, ask your county extension office which water-quality parameters to test for and how to interpret them for your operation. In some cases a slightly saline well that is unsuitable for drinking can still serve these uses, which can change the economics of a project.

Frequently asked questions

How do I know if groundwater is fresh before drilling?

You cannot know for certain until you test water from the well. Nearby well records, local drillers and USGS or state aquifer studies give the best advance indication.

Is deeper groundwater always cleaner?

Not necessarily. Deeper water may be better protected from surface contamination, but USGS data show it is more likely to be brackish or saline.

Can a groundwater detector tell fresh water from salt water?

Some manufacturers state this feature. Resistivity does respond to salinity, but clay can mimic it. Confirm with a laboratory test.

What counts as brackish water?

The USGS defines brackish groundwater as containing 1,000 to 10,000 mg/L of dissolved solids.

Can salty well water be made drinkable?

Often yes, with treatment such as reverse osmosis, but cost and maintenance rise with salinity. Get a full water analysis before choosing equipment.

Sources

• >USGS Water Science School: Saline Water and Salinity
• >USGS Fact Sheet 2017-3054: National Brackish Groundwater Assessment
• >US EPA: Protect Your Home's Water (Private Wells)
• >US EPA Environmental Geophysics: Electrical Methods