MAJOR-ION AND STABLE-ISOTOPIC COMPOSITION OF GROUNDWATER AND SURFACE WATER FROM A GEORGIA PIEDMONT FLOODPLAIN

Hydrochemical and stable isotopic monitoring of the Little Tallapoosa River floodplain in the Georgia Piedmont, Carroll County, Georgia, reveal complex spatial distribution of groundwater and surface-water composition and relatively little temporal variation. We sampled groundwater from 7 piezometers installed in floodplain alluvium, one crystalline bedrock well, the Little Tallapoosa River, two small tributary streams, and ephemeral springs on the floodplain margin. Samples were collected on an approximately monthly basis with additional sampling immediately following major rainfall/runoff events. Waters are of a mixed cation-bicarbonate type; groundwaters from clay-rich alluvium tend toward a sodium-bicarbonate type. Groundwaters, tributary waters, spring waters and cold-season river waters plot close to the deuterium/oxygen global meteoric water line (MWL). Groundwaters cluster strongly and are statistically indistinguishable based on stable isotope composition. Warm-season river waters deviate from the MWL along an apparent evaporation trend. Waters from different sedimentary facies, springs, tributary streams and the river are distinguishable based on major ion composition, particularly chloride, nitrate and sodium concentrations. River water composition is most variable, probably reflecting seasonal evaporation/dilution. Results suggest that hydrochemistry of this floodplain environment is dominated by groundwater discharge and that infiltration of river water during floods is minimal. Furthermore, river water occupying floodplain depressions after floods is flushed quickly from the system by groundwater discharge. Thus, floodplain groundwaters and surface-waters are predominantly of local origin and river-borne threats to floodplain ecological health will likely take the form of sediment-bound rather than dissolved contaminants.