Groundwater dynamics and surface water–groundwater interactions in a prograding delta island, Louisiana, USA

Abstract

Summary Deltas in coastal environments are assumed to function as chemical “buffers”, filtering nutrient-rich terrestrial runoff through the island structures and aquatic ecosystems as it travels to the sea, but the magnitude of this effect cannot be accurately quantified without understanding the physical relationships between the surface water and groundwater. The groundwater hydrology of young, prograding delta systems and its relationship to surrounding surface water dynamics are poorly understood. This study developed a new conceptual model of the hydrology of a prograding delta island groundwater system. The study was based on field data collected at Pintail Island, a 2 km 2 island within the Wax Lake Delta in Louisiana. Hydraulic properties and processes were quantified at multiple depths and locations spanning the island elevation gradient. Groundwater and surface water levels were monitored. A weather station recorded precipitation, air, and wind conditions. The groundwater within Pintail Island was both spatially and temporally dynamic throughout the study period of 9-September-2013 to 4-February-2014. The aquifer within the distal limbs of the island responded to surface water dynamics as a connected, saturated unconfined aquifer would, and its groundwater was controlled by the surrounding surface water fluctuations of semi-diurnal winds and tides. The aquifer within the older, higher elevation island apex was a lower-permeability system with subaerial fine sediments overlying deeper, sandier sediments. In contrast to the more bayward zone of the island, this more interior zone was controlled by storm recharge, low-permeability sediments, and low head gradients, but little affected by diurnal surface water fluctuations. Groundwater flow was directed outward from the interior of the island apex and the levees toward the delta channels and the central island lagoon, but storms and high tides temporarily reversed flow directions at some locations and times, likely with significant biogeochemical consequences. This empirically-based conceptual model of the heterogeneous and dynamic hydrogeology of a young, prograding delta island provides an essential foundation for further study of prograding coastal delta island hydrology, ecology, and nutrient exchange.