Mississippi Alluvial

The Mississippi Alluvial aquifer is the shallow groundwater system underneath the Mississippi River's modern flood plain — a band of Quaternary sediments hundreds of feet thick stretching from southern Missouri to the Louisiana coast. It supports one of the most concentrated irrigated-agriculture operations in the world: the Arkansas Grand Prairie's rice fields, the Mississippi Delta's cotton and soybeans, Louisiana's sugarcane. About 11 trillion gallons per year of groundwater is pumped from this aquifer for irrigation — the most water-per-acre of any major US aquifer.

For private well owners, the alluvial water is a different problem than the irrigation story. The aquifer's reducing chemistry — low oxygen, high organic matter, slightly acidic conditions — mobilizes naturally occurring arsenic in the sediments. The Mississippi Delta has some of the highest documented arsenic exposures in US private wells, and the resulting drinking-water exposures correlate with the elevated rates of bladder, lung, and cardiovascular disease that have been documented across Delta counties for decades.

What it is, geologically

The aquifer consists of Quaternary alluvial sands and gravels deposited by the modern Mississippi River and its tributaries, overlying older Tertiary sediments. Saturated thickness ranges from a few feet at the basin's edges to over 200 feet in the central Delta. The water table is shallow — typically 10-50 feet below ground surface in non-pumped areas, deeper in active irrigation zones.

The aquifer is highly transmissive (sand and gravel) and unconfined to semi-confined. Recharge happens through direct precipitation infiltration and from leakage from the Mississippi River itself.

The mining problem

Despite high transmissivity and reasonable recharge, the Mississippi Alluvial is being pumped faster than it recharges in major irrigation regions. Arkansas's Grand Prairie — the rice belt — has experienced cumulative water-table declines of 50-100 feet since intensive irrigation began in the 1940s. The 2018 USGS Mississippi Alluvial Plain assessment estimated a long-term storage decline averaging several million acre-feet per year.

The state response has been the Bayou Meto and Grand Prairie surface-water projects — federally funded efforts to deliver Mississippi River water to irrigators in lieu of groundwater. Implementation has been slow and politically contested.

Reducing chemistry and arsenic

The mineralogical basis for arsenic in this aquifer is different from the volcanic-source arsenic of the western US. In the Mississippi Alluvial, arsenic is bound to iron oxides in the alluvial sediments. Under reducing conditions — low dissolved oxygen, high organic matter, conditions that prevail in much of the shallow alluvial aquifer — those iron oxides dissolve, releasing the bound arsenic into solution.

The geographic pattern is well-documented:

• Mississippi Delta (Bolivar, Sunflower, Coahoma, Washington counties): ~25-30% of private wells exceed the 10 μg/L MCL.
• Eastern Arkansas rice country: 10-15% above MCL in some sub-regions.
• Louisiana parishes along the river: variable, with hot zones in Tensas, Madison, East Carroll.

This is the same chemistry that produced the catastrophic Bangladesh arsenic crisis. The US scale is smaller and the population density lower, but the per-well concentrations are comparable.

Health disparities

The Mississippi Delta has been a documented health-disparity zone since the 1960s. The Delta's cancer mortality rates exceed national averages substantially — bladder, kidney, lung, and certain leukemias notably. While many factors contribute (poverty, healthcare access, occupational exposures, smoking patterns, dietary), the chronic arsenic exposure from private wells is a meaningful and addressable component.

Studies linking Delta arsenic exposures to specific health outcomes:

• Argos et al. (2014) — multi-state US cohort showed elevated all-cause and cancer mortality at exposures down to 5-10 μg/L.
• Mississippi State Department of Health surveys — documented arsenic exposures and corresponding skin-lesion rates in the Delta in the 2000s.
• The Strong Heart Study (American Indian populations on alluvial aquifers in OK and the Dakotas) has linked arsenic to cardiovascular outcomes.

Water quality, beyond arsenic

• Bacteria — shallow water table plus septic-heavy rural areas plus livestock operations equals high bacterial vulnerability. Annual testing is essential. See bacteria.
• Nitrate — surprisingly variable. Some Delta sub-regions show low nitrate despite intensive agriculture (the reducing chemistry that mobilizes arsenic also denitrifies); other regions show classic ag-runoff elevation. See nitrates.
• Iron and manganese — the same reducing conditions that release arsenic also release iron and manganese. Most Delta wells have nuisance-level iron. See iron and manganese.
• Pesticides — atrazine, glyphosate, and other agricultural chemicals are detected episodically; rarely at concentrations of acute concern but worth knowing.

PFAS risk modeling

The USGS 2024 national PFAS-in-groundwater predictive model estimates a 19.0% mean probability of detecting any PFAS compound at typical private-well drinking-water depth across the Mississippi Alluvial. Roughly 5.7% 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 close to the national CONUS aquifer median of 19%, and places the Mississippi Alluvial 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 →