Collaborative Research: Are buried paleochannels effective reactors for water and solute transport in the deltaic subterranean estuary?

COLLABORATIVE RESEARCH:

ARE BURIED PALEOCHANNELS EFFECTIVE REACTORS FOR WATER AND

SOLUTE TRANSPORT IN THE DELTAIC SUBTERRANEAN ESTUARY?

Alexander Kolker, Louisiana Universities Marine Consortium

Karen Johannesson, Tulane University

Jaye Cable, University of North Carolina-Chapel Hill

Rivers deltas are one of the largest stores of minerals and organic-rich sediments on earth. Their low relief and close proximity to the ocean also make them one of the best systems to study how coastal ecosystems will respond to climate change and sea-level rise in the coming decades. A wealth of previous studies focused on the role deltas play in cycling carbon, nutrients, metals and other elements. However, very little of this research has investigated the subsurface connections between the main river and adjacent bays and wetlands. Hydrological models of coastal bays associated with river deltas suggest more freshwater is entering these systems than expected. This study will investigate whether this missing component is groundwater discharge through buried ancient channels in these deltas. Modern deltas consist of bays and bayous, sedimentary deposits, and marshes formed and abandoned as the river changed course throughout the Holocene. Over time, sandy-bottom bayous are buried and may leave only a trace of their former existence at the surface. These buried channels commonly retain a hydraulic connection to the main river, thus acting as a conduit for discharge to adjacent bays during the annual spring/summer high river stage. Within the Mississippi River Delta (MRD) system, this hydraulic connection may be enhanced by 20 to 40 ft as a consequence of flood control levees which may produce a river stage (i.e. head) as much as 15 to 22 ft above the adjacent bay water levels. The central hypothesis of this research is that buried ancient channels in deltas act as a vast network of subterranean estuaries, which play a critical role in the transport of groundwater, nutrients, and some metals to deltaic bays and ultimately the ocean. The research addresses several major questions: Is deltaic submarine groundwater discharge sufficient to satisfy current deltaic hydrologic and biogeochemical budgets? How effective are paleochannel networks in the delivery of water and elemental mass fluxes to the coastal ocean? These questions will be addressed using geophysical surveys that can produce images of the structure of the delta sediments and their salinity, as determined through sonars and electrical conductivity. These studies will be conducted in concert with studies of geochemical tracers (e.g. salt, radioactive and stable isotopes) and detailed a study of hydraulic gradients between the river and a paleochannel system (e.g. piezometers, pressure loggers, flow nets). Using the assembled understanding of hydrogeologic flow patterns and rates, we will estimate the biogeochemical mass fluxes associated with seasonal groundwater flow through this paleochannel/subterranean estuary network (e.g. N, C, P, Si, Fe). Results should improve the understanding of the role that the delta?s geology plays in its hydrology and chemistry and how fluxes of water, nutrients and metals vary over space and time. This work will then add to the understanding of how deltas and other coastal ecosystems function, particularly in light of sea-level rise predictions for the next 50 years.

Rivers have long been recognized a playing an important role global chemical cycles. Despite this recognition, relatively little is know about how these chemical cycles function below the sea floor, and the implications this has for the chemistry of the coastal zone. The proposed study will examine these processes in the Mississippi River Delta. This delta sits at the mouth of the largest river in North America, and is the entry point to one of the most important economic pathways in the United States. The Mississippi River Delta also has a long history of scientific research, government water quality monitoring, and stakeholder involvement. This provides the team with excellent baseline information and opportunities to link findings to the needs of society. The work will contribute to the education of three graduate students and several undergraduate students. These students will study at leading research universities and have the opportunity to work at a marine laboratory.