The Red River alluvial aquifer of Louisiana underlies approximately 2,000 square miles in the Red River Valley of Louisiana. The aquifer is Pleistocene in age and consists of clay, silt, sand, and gravel deposited by the Red River. Sand and gravel constitute the lower two-thirds of the deposit, the most productive part of the aquifer. The aquifer ranges from 40 to 150 feet in thickness and reaches its maximum thickness in Avoyelles and Catahoula Parishes. Fine sand, silt, and clay of Holocene age overlie and generally confine the aquifer. These fine-grained deposits generally do not exceed 50 feet in thickness.

The aquifer is recharged by downward seepage of rainfall in the valley, by lateral movement of water from adjacent Pleistocene and Tertiary formations, and by upward movement of water from underlying formations of Tertiary age. When large quantities of seepage occur locally within short periods of time, a temporary reduction in mineralization takes place in the upper part of the aquifer. Lateral recharge from Pleistocene and Tertiary formations generally upgrades water in the alluvial aquifer near the edge of the valley. Recharge from underlying aquifers may upgrade or downgrade the quality of water in the lower part of the alluvial aquifer.

The Red River and its major tributaries recharge the Red River alluvial aquifer in local zones near the river during high stream stages, but noticeable water-quality changes occur only following periods of prolonged high stages.

Ground water in the alluvium generally moves toward the streams and down the valley. Water levels fluctuate seasonally and are generally less than 30 feet below land surface. Annual water-level fluctuations have a maximum range of about 30 feet near the Red River, and a minimum range of only a few feet in some interstream areas.

The Red River alluvial aquifer can yield 2,000 gallons per minute or more of water to individual wells where thick beds of coarse sand and gravel occur in the southern part of the study area. Aquifer tests indicate that the transmissivity ranges from 2,000 to 27,000 feet squared per day. The hydraulic conductivity ranges from 100 to 300 feet per day. Storage coefficients range from 6.0x10^-4 to 1.0x10^-3.

Water in the aquifer typically is of the calcium magnesium bicarbonate type. The water is hard to very hard, ranging in hardness from about 100 to 2,300 mg/L (milligrams per liter). However, hardness generally is in the 200- to 600-mg/L range. The ions that appear to be most variable in occurrence and sensitive to change with time are calcium, magnesium, iron, sulfate, and chloride. Iron concentrations range from less than 0.3 mg/L to as high as 49 mg/L but generally are 1 to 10 mg/L. In areas where rapid recharge occurs, shallow wells may have iron concentrations of less than 0.3 mg/L. Sulfate concentrations generally range from 0.5 to 50 mg/L but locally exceed 250 mg/L. The highest concentration detected was 1,900 mg/L. Chloride concentrations typically are less than 50 Mg/L. However, concentrations of chloride exceed 250 mg/L locally and are as high as 4,400 mg/L in a few areas. For this report, water with chloride concentrations greater than 250 mg/L is considered salty.

The base of freshwater in the Red River Valley generally coincides with the base of the alluvium at depths of 80 to 150 feet. However, locally, the aquifer contains salty water, and the base of freshwater occurs within the aquifer; in parts of Rapides, Caddo, and Natchitoches Parishes, the base of freshwater extends below the alluvium. The largest occurrence of salty water is in Natchitoches Parish, with other smaller bodies occurring in Red River, Caddo, and Bossier Parishes.

Water quality in the aquifer varies both areally and with depth; the quality also varies with time, depending on the quantity and chemical character of the recharge water.