SOURCE OF IRON-RICH WATERS IN FLOODPLAIN ALLUVIUM OF THE LITTLE TALLAPOOSA RIVER, PIEDMONT PHYSIOGRAPHIC PROVINCE, GEORGIA, USA

This study examines groundwater chemistry of a riparian wetland adjacent to the Little Tallapoosa River in the Piedmont Physiographic Province, Carroll County, Georgia. Results show a complex spatial distribution of groundwater composition and relatively little temporal variation. We sampled groundwater from 20+ piezometers installed in three types of regolith: 1) organic-rich, predominantly fine-grained, Holocene floodplain alluvium, 2) pedogenically altered (Pleistocene?) terrace alluvium, and 3) saprolite. We also analyzed bedrock waters from deep wells in and near the site. Saprolite wells include one beneath contemporary floodplain alluvium and one in an upland position adjacent to the floodplain. Regolith wells consist of 3.18 cm (1.25 in) PVC pipe completed at depths between 1.8 m (6.0 ft) and 4.8 m (16.0 ft). Samples were collected six times over a twelve-month period under conditions including drought and flood. Most waters are of a mixed-cation bicarbonate type; iron bicarbonate waters are prominent in the floodplain; sodium chloride waters are also present. Approximate TDS ranges from 30 to 300 mg/L; pH ranges from 4.5 to 6.1. Concentrations of redox sensitive species (O₂, NO₃^-, Fe²⁺, Mn²⁺) vary considerably within the dataset and field-measured dissolved oxygen ranges from below detection limit to greater than 2.0 mg/L. Results show that hydrochemistry of this floodplain environment is heterogeneous over very small lateral distances and depths. The major geochemical process differentiating waters appears to be iron reduction driven by oxidation of unevenly distributed organic matter in Holocene floodplain alluvium. Some heterogeneity may also be explained by diverse groundwater flowpaths into floodplain sediment.