Groundwater dynamics in a floodplain on the South Carolina coastal plain: Implications for methylmercury transport

Knowledge of flow paths through floodplain wetlands has important implications for understanding the formation and maintenance of floodplain peat deposits and for contaminant discharge to creeks. Here numerical models based on streams in the South Carolina coastal plain were developed to explore groundwater flow paths and rates under several scenarios. The floodplain scenarios were based on McTier creek in the headwaters of the South Fork of the Edisto River and differed in stratigraphy, permeabilities of the strata, and the water level in the adjacent creek. The study suggests that, under low flow conditions, the majority of groundwater is discharged into the creek through a channel lag layer, if present. The study also suggests that, during flood stage, groundwater discharge focuses at the margin of the floodplain but also flows upward through organic-rich sediments and discharges at the submerged surface of the floodplain. During flood stage, upward flow through the levee deposit layer is increased as the contrast in permeabilities between the layers decreases. This upward flow through the levee deposit layer may be a source of methylmercury to the adjacent McTier Creek.