Saltwater Intrusion

Saltwater intrusion is the failure mode of every coastal aquifer in the United States, and it doesn't fit the usual contaminant frame on this site. There's no industrial source. There's no geologic source you can point to. The salt isn't being added to the water — the freshwater that used to be there is being removed, and seawater is taking its place. The diagnosis is the same chemistry every time: chloride, sodium, sulfate, and bromide rising in coastal wells. The cause is essentially never the well itself; the cause is regional pumping pressure dropping the freshwater head far enough that the seawater wedge can advance.

Once the wedge has advanced, retreating it is the work of decades. The economics of "we'll pump less and let the aquifer recharge" don't really work in regions whose entire municipal supply depends on the same aquifer. The practical result for affected communities is supply transition: deeper wells, a switch to municipal sources, brackish-water RO desalination plants, or land-use buyouts.

The mechanism

In a coastal aquifer at hydrologic equilibrium, freshwater (less dense) sits over saltwater (more dense) along an interface called the freshwater-saltwater interface or, classically, the Ghyben-Herzberg lens. The position of the interface depends on the freshwater hydraulic head: at sea level, every 1 foot of freshwater head above sea level corresponds to roughly 40 feet of freshwater extending below sea level before reaching the salt interface. Lose 1 foot of freshwater head, lose 40 feet of freshwater column.

This is why coastal aquifer pumping is so disproportionate in its consequences. A regional water-table drawdown of 5-10 feet (very common in coastal cities over a few decades) translates to a freshwater-saltwater interface that has risen 200-400 feet — and likely advanced inland substantially as well, depending on geology.

Where it's happening

Active or documented saltwater intrusion in US coastal aquifers includes:

• Long Island, NY (Northern Atlantic Coastal Plain) — Brooklyn-Queens since the 1930s; ongoing in parts of Nassau and Suffolk counties.
• Atlantic and Pacific coasts of Florida (Floridan) — Tampa Bay region (city well retirement programs), Brunswick GA, Hilton Head SC, Florida Panhandle.
• Southeastern Virginia (Hampton Roads, Eastern Shore) — managed aquifer recharge program responding to chloride increases.
• NJ shore — Cape May, parts of Atlantic and Ocean counties; mandatory summer pumping restrictions.
• Lower Eastern Shore Maryland — chloride elevation in Worcester and Somerset counties.
• San Francisco Bay margins and Salinas Valley, CA — Salinas Valley has had documented intrusion since the 1930s; Pajaro Valley similar.
• Coastal Georgia and the Carolinas — multiple confined aquifers showing chloride increases.
• Hawaii — basal lens aquifers underlying Honolulu and other islands; the original Ghyben-Herzberg framework was developed in part for Hawaiian conditions.

Diagnostic chemistry

The diagnostic markers, in approximate order of usefulness:

• Chloride — the workhorse indicator. Background freshwater chloride is typically <25 mg/L; values >100 mg/L suggest seawater influence. EPA secondary standard is 250 mg/L (taste threshold).
• Sodium — rises with chloride in roughly the seawater Cl:Na ratio (about 1.8:1 by mass). Disambiguates from road-salt sources (which also raise chloride but with very different ratios).
• Sulfate — also elevated in seawater (~2,700 mg/L). Combined chloride+sulfate elevation is a strong saltwater signature.
• Bromide — present in seawater at distinctive ratios; useful for source disambiguation in research settings.
• Specific conductance — overall mineralization indicator. Cheap and continuous monitoring possible.

The most useful single piece of information isn't any single test value, but the trend: testing chloride annually for 5-10 years and watching whether it's rising, stable, or falling. A well at 80 mg/L chloride that's been at 80 for two decades is in a different situation than one that's risen from 40 to 80 over the same period.

Treatment and supply transition

Treatment of saltwater-intruded water at residential scale is uncommon because the cost-effective answer is usually supply transition (a deeper well, a municipal connection). But for the cases where treatment is the answer:

• Reverse osmosis — the only practical residential option for moderately brackish water (chloride to roughly 1,500-2,000 mg/L). Brackish-water RO requires more pressure and produces more reject water than freshwater RO. Cost: $5,000-15,000 installed plus high ongoing operating cost.
• Distillation — works but slow and energy-intensive; impractical at household scale for primary supply.
• Blending with freshwater — only works if you have a clean source available.

The right answer in most cases is to monitor chloride trends, plan for transition before the well becomes unusable, and join the local groundwater management or coastal aquifer protection effort if there is one.