Coastal Lowlands (Texas-Louisiana Gulf)

The Coastal Lowlands Aquifer System is the multi-layered groundwater stack underneath the Gulf coastal plain from East Texas through Louisiana, Mississippi, Alabama, and into the Florida Panhandle. It is the aquifer underneath America's densest concentration of refineries, petrochemical plants, and oil-and-gas infrastructure — Houston-Galveston, Beaumont-Port Arthur, Lake Charles, Baton Rouge, New Orleans-Metairie. It is also the aquifer that has been most aggressively pumped to support that industrial concentration, and the consequences are visible from space.

The Houston-Galveston subsidence story is the defining hydrogeological event of this aquifer. Cumulative subsidence in parts of Harris and Galveston counties exceeds 10 feet since the 1920s — driven by groundwater withdrawal for industry and municipal supply. Subsidence at this scale changes flood patterns (areas that used to drain now don't), damages infrastructure, and is essentially permanent. The Harris-Galveston Subsidence District, established in 1975, has progressively shifted regional water supply from groundwater to surface water; subsidence rates have slowed but not stopped.

What it is, geologically

The aquifer consists of a thick wedge of Tertiary and Quaternary sands, gravels, silts, and clays deposited as the ancestral Mississippi and other Gulf rivers built the coastal plain over millions of years. The wedge thickens toward the coast — from a few hundred feet at the inland boundary to over 10,000 feet at the modern shoreline. Major named units include the Chicot, Evangeline, and Jasper aquifers in Texas-Louisiana, with the Catahoula confining unit separating them from underlying brackish water.

For private well owners in the Texas-Louisiana corridor, the relevant question is which named unit your well taps. Most domestic wells draw from the shallow Chicot or upper Evangeline; deeper municipal and industrial wells reach the Jasper and confining-zone systems. The shallow units are more vulnerable to surface contamination; the deeper units have salinity and structural-water-quality issues of their own.

Subsidence and saltwater intrusion

The two failure modes of this aquifer are coupled. Pumping drops the freshwater head; the aquifer compacts (subsidence); the freshwater-saltwater interface advances inland (intrusion). Both effects are documented across the corridor:

• Houston-Galveston — over 10 feet cumulative subsidence; documented saltwater intrusion in coastal Galveston County wells.
• Lake Charles, LA — multi-foot subsidence; Sabine Pass area wells affected by saltwater.
• New Orleans — combined natural land subsidence and groundwater-driven subsidence; one of the major drivers of flood vulnerability.
• Baton Rouge — long-term water-level decline; the city's "Baton Rouge Fault" region has documented saltwater encroachment.
• Mobile Bay region — chloride elevation in coastal Alabama wells.

If you're on a private well within roughly 20 miles of the Gulf coast, see saltwater intrusion for the chloride-trend monitoring approach.

Industrial contamination plumes

The petrochemical concentration along the Texas-Louisiana corridor has produced a corresponding density of groundwater contamination plumes. Specific concerns:

• Petroleum hydrocarbons (BTEX) — refinery and pipeline leaks. The Texas Commission on Environmental Quality maintains an extensive LUST (leaking underground storage tank) inventory; many active and legacy plumes affect downgradient wells. See VOCs.
• Chlorinated solvents — chemical manufacturing and degreasing operations across the corridor; multiple Superfund sites with documented groundwater plumes.
• "Cancer Alley" between Baton Rouge and New Orleans — over 150 industrial facilities concentrated along an 85-mile stretch of the Mississippi River; documented elevated cancer rates in adjacent communities; multiple ongoing groundwater investigations.
• Produced-water releases — historical oil and gas operations released large volumes of saline produced water at the surface; some legacy releases are still affecting shallow groundwater.
• PFAS — multiple military bases and industrial sites along the corridor with documented AFFF and industrial PFAS contamination. See PFAS.

Other water quality

• Iron and manganese — pervasive in the reducing-conditions aquifer; nearly every Texas-Louisiana home has treatment. See iron and manganese.
• Hydrogen sulfide — the rotten-egg smell that comes with reducing conditions; companion problem to iron.
• Bacteria — shallow rural wells in the corridor are vulnerable. See bacteria.
• Radium — naturally elevated in some Tertiary sand sub-regions, particularly parts of southeastern Texas. See radium.
• Hardness — moderate to hard, particularly in the deeper units.

PFAS risk modeling

The USGS 2024 national PFAS-in-groundwater predictive model estimates a 20.6% mean probability of detecting any PFAS compound at typical private-well drinking-water depth across the Coastal Lowlands (Texas-Louisiana Gulf). Roughly 8.5% of the aquifer's area falls in the model's >50% probability band — land where a freshly drilled domestic well is predicted more likely than not to show some detectable PFAS. That is above the national CONUS aquifer median of 19%, and places the Coastal Lowlands (Texas-Louisiana Gulf) in the upper half of CONUS Principal Aquifers.

This is a 1 km-resolution statistical prediction, not a measurement of any specific well. What PFAS is and how it gets into groundwater → · How to test a private well for PFAS → · Full state-and-aquifer breakdown with methodology →