Evaluating Groundwater Discharge onto the Continental Shelf in a River Dominated Coastal Environment

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The transport of groundwater into coastal zones may be a significant process in the geochemical, nutrient, and carbon budgets of many marine nearshore environments. This project will address the manner in which we evaluate interactions between groundwater and surface water on river-dominated coastal margins. According to Johannes (1980), groundwater discharge should occur anywhere that an aquifer is hydraulically connected to the sea through permeable rock or bottom sediments and where the hydraulic head is above sea level. Such conditions are met in most coastal areas. Recent studies have suggested that groundwater may play an important role in transporting water and bioactive elements to coastal waters along both typical continental shelves and river-dominated ocean margins. Relationships between groundwater, the substrate through which it flows, and the receiving surface waters are of significant environmental concern since the magnitude of groundwater discharge is not yet assessed along most of the world’s coastlines. The challenge is how to quantitatively assess the extent of groundwater flow.

Groundwater flow will be assessed using a multiple tracer approach. We will employ ²²²Rn/²²⁶Ra, ⁴He/³He/tritium, and ²²³Ra/²²⁴Ra to examine groundwater flow onto the continental shelf adjacent to the Mississippi River. ²²²Rn and ⁴He/³He/³H will be used as groundwater tracers, ²²³Ra/²²⁴Ra will be used to assess resuspension events and mixing, and ³H will provide groundwater age information. Groundwater samples will be collected from land-based wells near the Louisiana coast, deep porewater samples collected via Kasten cores, and water column samples from the lower Mississippi and adjacent continental shelf will be collected while aboard the R.V. Pelican throughout the project. In order to evaluate the connection between groundwater inputs and surface water tracer inventories, we will construct a balance of all possible inputs and outputs of these natural tracers. While these tracers are naturally enriched in groundwater relative to surface waters, they are present throughout the environment and therefore other sources must be considered. Once sources and sinks have been established, the advective component can be accessed by application of an advection-diffusion model after evaluating the lateral and vertical tracer concentrations in groundwaters and porewaters throughout the study area. This field-based research will be used to develop and constrain a numerical hydrologic model for the study area.

We will also compare the relationship between groundwater contributions and river stages, with high flow conditions occurring in the spring and lower flow conditions occurring in the fall. With the use of several geochemical and hydrogeological methods we will accurately quantify subsurface flow onto the continental shelf of the coast of Louisiana. In order to assess this relationship, we sampled groundwater wells on a quarterly basis and sampled the water column a total of six times with three cruises during the fall of 2003 and three cruises in the spring of 2004. This study will not only provide valuable information about groundwater discharge on the Louisiana continental shelf, but it will also serve as a model for environmental assessments of coastal areas throughout the world as well as other bodies of water where groundwater discharge influences water dynamics.

Groundwater Methods Water Column Methods Preliminary Results

Acknowledgements and Pictures