Groundwater Discharge to a River Delta Driven by the Mississippi River Stage

Groundwater discharge is recognized as an important vector for freshwater and biogeochemical constituents to coastal systems. In many coastal environments, prolific aquifer sources drive groundwater through permeable sediments into bays and estuaries. Scientists have taken advantage of these permeable sites to understand the structure and magnitude of impact of the subterranean estuary. One coastal system in which flow patterns are less easily defined are river deltas, where thick, heterogeneous sediment deposits and anisotropic flow lines make discharge more difficult to quantify. While the Mississippi River is a significant freshwater, sediment, and organic matter source to the Gulf of Mexico, its importance as a driver for groundwater discharge has been debated for over a decade. We studied groundwater flow into the Mississippi River delta around Barataria Basin, Louisiana, to understand the forces driving groundwater discharge and the magnitude of groundwater contributions to the estuary. By applying the geochemical tracer, Rn-222, we take advantage of a conservative, naturally-occurring tracer typically present at activities 2-3 orders of magnitude greater in groundwater than surface water. We first identified groundwater input points to surface water bayous using Rn-222 as a tracer, and applied a mass balance of Rn-222 sources and sinks to quantify groundwater flow. Mississippi River stage is one important driver of groundwater flow within the alluvial aquifer adjacent to the river channel. As stage increased during the spring flood season, the hydraulic head of the aquifer increased, thus driving groundwater into bayous draining riverine swamps. While small, local groundwater inputs were found in other regions of the estuarine basin, the main groundwater inputs occurred within the swamps and bayous closest to the river channel itself. The average groundwater seepage rate into these swamps was about 2 cm/day and equivalent to about 130 million m^3/day. As the frequency and duration of large river floods increase along the Mississippi River, implications for this groundwater discharge are that it will be a persistent source of freshening to this coastal environment basin and is likely a vector for biogeochemical constituents to the Gulf of Mexico.