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Nutrient Processing at the Land-Ocean Interface: Assessing Groundwater Transformations through Reactive Transport Modeling

PI: Christof Meile (Department of Marine Sciences, The University of Georgia, Athens, GA, USA)

Support: Georgia Sea Grant College Program, R/HAB-23

Timeframe: 3/1/06 - 1/31/10

Project Objectives:
1) To develop a multi-dimensional reactive transport groundwater model.
2) To determine the fate of anthropogenic nutrient loading at the land-ocean interface.
3) To identify conditions for in situ mitigation of high nutrient loadings.
4) To provide the public with visual representations of coastal subsurface elemental cycling


    In order to assess nutrient removal during passage through the subsurface, we developed and applied a multi-dimensional reactive transport groundwater model. The model combines the main transformation processes with a physical description of density-driven fluid flow.

  • We investigated the changes in nitrogen (N) and phosphorus (P) concentrations during the groundwater transition from the upland to the coastal ocean under a variety of idealized environmental settings. Model simulations showed that the prevalent flow characteristics and redox conditions in the freshwater-seawater mixing zone determine the extent of nutrient removal and the input of nitrogen and phosphorus to coastal waters (Spiteri et al. 2008a).
  • We applied the model at two well-studied coastal aquifers. Simulations at Waquoit Bay (MA) suggest that removal of NO3- through denitrification is inhibited by the limited overlap between two distinct freshwater plumes, as well as by the refractory nature of terrestrial dissolved organic carbon (Spiteri et al. 2008). Model application to Moses Hammock (GA) helped explain intriguing oscillations in redox conditions observed over spring-neap tidal cycles (Porubsky et al. 2010).
  • Building on the model validation in the field, the fate of nutrients originating from dysfunctional septic tanks in the coastal zone was studied. Model simulations showed that they can constitute a significant source of N, particularly so in coastal settings. There, a significant amount of organic matter breakdown can take place via sulfate reduction. The resulting build up of sulfide inhibits denitrification, the main pathway of nitrate removal from an aquifer. Because N is limiting in many coastal waters, this highlights the need for proper maintenance and inspection of septic tanks to limit coastal eutrophication (Meile et al. 2010).



Where Groundwater Meets The Ocean: Nutrient cycling in the subsurface at the land-ocean interface.  http://octopus.marsci.uga.edu/gw

Porubsky, W., Joye, S., Moore, W.S., Tuncay, K., Meile, C. (in revision): Field measurements and modeling of groundwater flow and biogeochemistry at Moses Hammock, a backbarrier island on the Georgia coast.

Porubsky, W., Joye, S., Moore, W.S., Tuncay, K., Meile, C. (submitted): Hammock groundwater biogeochemistry and flow: Field measurements and modeling.

Meile, C., Porubsky, W.P., Walker R. and Payne, K. (2010): Controls on groundwater nutrient mitigation: Natural attenuation of nitrogen loading from septic effluents. Water Research 44(5): 1399-1408.

Porubsky, W.P. and Meile, C. (2009): Controls on groundwater nutrient mitigation: Natural attenuation of nitrogen loading from septic effluents. Proceedings of the Georgia Water Resources Conference, April 27-29, 2009 in Athens GA.

Spiteri, C., Slomp, C., Charette, M.A., Tuncay, K. and Meile, C. (2008): Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): field data and reactive transport modeling. Geochimica et Cosmochimica Acta 72(14): 3398-3412.


Hagens, M. and Meile, C. Biogeochemical dynamics in salt marsh environments: the role of intertidal hotspots. Goldschmidt Conference Knoxville TN, June 2010.

Meile, C. Nutrient dynamics in the shallow coastal subsurface. Modeling microbial processes and biogeochemical transformations in porous media. Department of Marine Sciences UGA, Athens GA, April 6, 2009.

Meile, C., Porubsky, W.P. and Joye, S. Biogeochemistry and groundwater flows. LTER Duplin Interactive Studies Workshop, Athens GA, January 5, 2009

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This page was updated November 22, 2010