Geohydrology and water quality of the Mississippi River alluvial aquifer, northeastern Louisiana

The Mississippi River alluvial aquifer in northeastern Louisiana is in the alluvium of the Mississippi, Arkansas, and Ouachita Rivers and underlies approximately 5,000 square miles (13,000 square kilometers) in the Mississippi River valley. The aquifer, of Pleistocene age, is a southeastward-thickening wedge of sand and gravel that ranges in thickness from about 20 to 135 feet (6 to 41 metres). These sediments are essentially flat lying with an irregular base. Fine sand, silt, and clay of Holocene age overlie and generally confine the aquifer. These fine-grained deposits range in thickness from 0 to 100 feet (0 to 30 metres) but generally do not exceed 40 feet (12 metres).

Water levels in the Mississippi River alluvial aquifer generally are less than 30 feet (9 metres) below land surface. Annual water-level fluctuations have a maximum range of about 20 feet (6 metres) near the Mississippi River and a minimum range of only a few feet in  interstream areas. Seasonal declines occur locally, especially in Morehouse Parish, because of large withdrawals for irrigation.

The Mississippi River alluvial aquifer can yield water in sufficient quantity for most uses throughout the area. Aquifer tests indicate that the transmissivity ranges from 13,000 to 45,000 feet squared per day (1,200 to 4,200 metres squared per day). The hydraulic conductivity ranges from 130 to 530 feet per day (40 to 160 metres per day). Storage coefficients range from 0.001 to 0.05. Large diameter wells yield as much as 7,000 gallons per minute (440 litres per second) near Tallulah in Madison Parish.

The quality of ground water in the Mississippi River alluvial aquifer varies areally and with depth, but generally the water is hard to very hard and high in iron concentration, Where fresh, the water typically is a calcium bicarbonate type generally ranging in hardness from 200 to 600 mg/1 (milligrams per litre) and in iron concentration from 0.00 to 30 mg/1. However, in southern Richland Parish and local areas in adjacent parishes, a hardness of less than 100 mg/1 and an iron concentration of less than 0.30 mg/1 are common.

Fresh water occurs to or below the base of the alluvial aquifer in most of the area. The chloride concentration typically is less than 50 mg/1. However, chloride concentrations are as high as 4,000 mg/1 in parts of the area. At some localities, salty water (chloride concentrations greater than 250 mg/1) occurs in the aquifer at depths less than 30 feet (9 metres). The largest occurrence of salty water is in Franklin Parish near and parallel to the subsurface contact between clays of the Jackson Group and sands of the Cockfield Formation of the Claiborne Group. Thus, salty water moving from the Cockfield into the alluvium may be the source of some of the high-chloride water. Smaller bodies of salty water occur in the aquifer along the Jackson-Cockfield contact in Madison and Caldwell Parishes. Probably some contamination from pits formerly used for disposal of salty water occurs in northern Franklin and southern Richland Parishes, Other small occurrences of salty water in Concordia, Morehouse, Ouachita, and Tensas Parishes may be related to contamination by salt-water disposal pits and wells, leaky oil-well casings, and oil-well drilling mishaps.