Groundwater Discharge and Associated Nitrogen Transport in a Northern Gulf Coast Estuary

James Connors
University of South Alabama, Mobile, Alabama 36688

Weeks Bay is a small estuary located on the northern Gulf Coast that lies at the confluence of two rivers (Fish River to the north and Magnolia River to the east). The bay drains approximately 520 square kilometers of predominantly agricultural land in Baldwin County, Alabama. A two-year hydrogeological investigation focused on detecting and quantifying groundwater discharges into the bay, measuring the amount of dissolved inorganic nitrogen delivered to the bay via groundwater, and delineating nitrogen fate and transport along shallow influent groundwater flow paths. The work involved the installation and quarterly monitoring of offshore piezometers and onshore monitoring wells, and found that groundwater contributes, on average, about four percent of the bay's overall river input. Groundwater discharges into the bay from a shallow (watertable) aquifer that is in direct hydraulic contact with Weeks Bay. The bay is separated hydraulically from a deeper, confined aquifer by a local dense clay confining unit. This confining unit stratigraphically ties into extremely low-permeability (fine-grained) sediments that have been deposited along the bay bottom. Nitrogen concentrations in groundwater of the water-table aquifer displayed a great degree of seasonal variability but showed a trend of nitrate reduction along groundwater flow paths as they passed through anoxic shoreline sediments. Peak nitrate concentrations detected occasionally in bay waters could not be correlated to groundwater influx, but there is evidence that longer-term nitrogen concentrations in the water column are similar to those detected in submarine groundwater discharges.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012