SALTWATER INTRUSION

Saltwater intrusion in coastal aquifers: causes, warning signs and how it is managed

How pumping, drainage and sea-level rise draw seawater into coastal aquifers, where it is happening in the U.S., how to detect it, and how utilities and regulators push it back.

By Argos Tellus · Updated October 2, 2026

KEY TAKEAWAYS

In coastal aquifers, fresh groundwater normally flows toward the sea and holds back denser seawater. Pumping that lowers fresh water levels lets saltwater move inland or upward — saltwater intrusion.
USGS identifies groundwater pumping as the primary cause of saltwater intrusion along the Atlantic coast; drainage canals that lowered the water table caused intrusion in parts of southeastern Florida.
Large withdrawals from the Floridan aquifer system have caused water-level declines and saltwater intrusion in parts of South Carolina, Georgia and Florida (USGS).
Sea-level rise, coastal land subsidence and growing demand will make intrusion worse, according to USGS.
Once a well field salts up, the fix is slow and costly — so the practical tools are monitoring chloride and water-level trends, managing pumping, and building barriers or alternative supplies early.

How saltwater intrusion works

Near the coast, fresh groundwater sits on top of and seaward-flowing against denser saltwater. The boundary between them — the freshwater–saltwater interface — stays roughly in place as long as enough fresh water flows toward the sea. A rough rule from the Ghyben–Herzberg relation is that for every foot fresh water stands above sea level, the interface lies about 40 feet below sea level.

When pumping lowers fresh water levels, that balance shifts. Saltwater can move in three ways: laterally inland from the ocean, upward from deeper saline zones beneath a pumping well (upconing), and downward from tidal rivers, canals or coastal waters. Because of the roughly 40-to-1 ratio, small drops in fresh water levels can allow large movements of the interface.

Where it is happening in the U.S.

Atlantic coast — USGS identifies pumping as the primary cause of intrusion, including in the Floridan aquifer system of South Carolina, Georgia and Florida.
Southeastern Florida — drainage canals lowered the water table and allowed seawater to move inland in the shallow Biscayne aquifer.
California — pumping in coastal basins such as the Salinas Valley and parts of Southern California has drawn seawater inland; California’s groundwater law (see SGMA) lists seawater intrusion as one of six undesirable results basins must avoid.
Gulf Coast — a study of groundwater levels in coastal wells found landward hydraulic gradients — conditions that favor intrusion — along a substantial share of the Gulf and East coasts.

Sea-level rise and subsidence make it worse

Rising seas push the saltwater boundary inland and flood low-lying areas with saltwater that can seep downward. Coastal land subsidence — often itself caused by groundwater pumping, as in Houston — has the same effect. USGS notes that sea-level rise, coastal subsidence and increasing water demand together will exacerbate the threat of seawater intrusion.

Warning signs and monitoring

Rising chloride or specific conductance in production and monitoring wells — the most direct indicator.
Fresh water levels falling toward or below sea level near the coast, or reversed (landward) gradients.
Increasing salinity in deeper parts of a well or during heavy pumping, a sign of upconing.
Long records matter: trends in chloride and water level per well, not single samples, show whether the interface is moving.

How saltwater intrusion is managed

Pumping management — reducing or relocating withdrawals near the coast, and spacing and scheduling wells to limit drawdown (see cone of depression and well interference).
Injection barriers — injecting fresh or treated water along the coast to build a hydraulic barrier, used in Southern California.
Managed aquifer recharge — adding water to raise fresh water levels.
Alternative supplies — surface water, reclaimed water or desalination to reduce reliance on threatened wells.
Salinity control structures on canals to keep tidal saltwater from moving inland.

Frequently asked questions

What causes saltwater intrusion?

Mainly groundwater pumping that lowers fresh water levels in coastal aquifers, letting denser seawater move inland or upward. Drainage canals, sea-level rise and coastal subsidence also contribute.

How do you detect saltwater intrusion?

By monitoring chloride or specific conductance and water levels in wells over time. Rising chloride, especially during heavy pumping, and fresh water levels near or below sea level are warning signs.

Can saltwater intrusion be reversed?

Partly and slowly. Reducing pumping, recharging the aquifer or building injection barriers can push the interface back, but restoring a salted aquifer can take years to decades.

What is the Ghyben–Herzberg relation?

An approximation that, because seawater is slightly denser than fresh water, the freshwater–saltwater interface lies about 40 feet below sea level for every foot the fresh water table stands above sea level.

HOW ARGOS HELPS

Phreatic — groundwater availability

Water-level forecasts, drawdown and interference-aware pump scheduling to keep levels above critical thresholds.

Data Vault — environmental data

Chloride and water-level histories per well, with anomaly detection to flag rising salinity early.

SOURCES

Figures are taken from the primary sources above and dated as of the update shown. Concentrations are expressed in mg/L. Spot an error? Email support@argostellus.com.

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