Groundwater as a nutrient and carbon source to coastal systems
Serious declines in coastal water quality and ecosystem health have resulted from population growth and agricultural, commercial, and industrial activities in coastal watersheds and from increased loading of anthropogenic wastes (organics and nutrients) originating at localized (e.g., sewage, industrial effluent) and diffuse (e.g., agricultural run-off) sources. Ultimately, when excessive amounts of anthropogenic materials arrive in coastal waters, they lead to eutrophication, which can be loosely defined as the increase in labile organic matter supply to an ecosystem. Visible signs of eutrophication, including increased frequency of harmful algal blooms, water column hypoxia/anoxia, fish kills, and reduced water quality, are apparent in coastal environments across the globe. Understanding the causes of eutrophication and documenting an ecosystem's response to eutrophication are two key research challenges facing coastal biogeochemists today.
Predicting the response of coastal ecosystems to land use change and eutrophication requires robust models that include all relevant sources of nutrients and organic materials. Groundwater is an important, but poorly understood, source of nutrients and organic materials to coastal waters. Along the coast of Georgia, groundwater inputs are thought to be important, but the magnitude of this input term has not been quantified. This lack of information makes it impossible to predict how the quality (chemical composition) or quantity (freshwater flux) of this “source term” will respond to increased development pressures in coastal regions.
We are examining groundwater geochemistry and sediment biogeochemistry in coastal ecosystems in Georgia (Dover Bluff along the Satilla River; at several sites on Sapelo Island in the LTER domain; on coastal hammocks within the LTER domain; and at Cabretta Beach, on Sapelo Island) and South Carolina (the Okatee estuary near Bluffton, SC). We are collaborating with Dr. Christof Meile (UGA) and Drs. Billy Moore (U South Carolina, geochemical tracers) and Alicia Wilson (U South Carolina, hydrology). This work represents an integrated effort to quantify the impact of coastal development and land use change on ecological, chemical, and physical processes through the study of both groundwater and surface water components. We are quantifying the flux and chemical signature of shallow coastal groundwater and the data will be used to contrast the chemical composition of groundwater and surface waters. The impact of groundwater on biological processes is also being evaluated in the marsh and in estuarine tidal creeks.
The Okatee River Estuary
The Okatee River estuary (32.34º N, 80.89º W), SC, is a tributary of the Colleton River, and out flow from the system makes its way eventually to Port Royal Sound. The Okatee headwaters lie adjacent to the Sun City housing development and recreational complex. Further development in the region is expected to quadruple (from ca. 5,000 individuals to ca. 20,000 individuals) the watershed population in the coming years when Sun City development is complete and occupied (Fred Holland, SC Dept. Natural Resources, Marine Resources Research Institute, pers. comm.). As a result, the septic inputs to the surficial groundwater, and to Okatee system, are likely to increase. The Okatee is surrounded by an extensive marsh of Spartina alterniflora but Salicornia and Juncus are found in saltpans and near the upland boundary, respectively.
We established a series of monitoring sites - T1, 278, and GD – along the salinity gradient in the Okatee Estuary. Salinity in the Okatee is highly dependent on freshwater discharge. Site T1 in the upper reaches of the Okatee can have salinities ranging from 0 to 20 ppt. The GD site further downstream in the Okatee is less influenced by freshwater discharge, with salinities typically near seawater levels. The PB site on Malind Creek, a small tidal creek feeding the Okatee, typically has salinities comparable to the T1 site. In January 2003, an additional five sites between the 278 and GD sites were sampled to evaluate spatial variability in porewater biogeochemistry. Two transects of groundwater monitoring wells, installed by Dr. Carolyn Ruppel as part of a collaborative NOAA Sea Grant project, were sampled bimonthly to evaluate spatial and temporal trends in groundwater biogeochemistry.
The Satilla River Estuary
The Satilla River (bottom right panel on the figure above) is a coastal plain blackwater river that drains a 9143 km2 watershed. The average flow rate is 65 m3 sec-1, and the narrow floodplain is bordered largely by cypress swamps and bottomland forests.
The Satilla River carries a high dissolved organic load (25 mg L-1) and a low sediment load, and has a low pH (ca. 6). The Satilla River has received minimal human impact along its floodplain and within its watershed, although development pressure is expected to increase in the coming years. Marshes within the tidally influenced portion of the estuary are exposed daily, implying that significant groundwater-derived inputs could be expected, particularly at low tide. Dover Bluff marsh (DB, 30.99º N, 81.50º W) lies on Umbrella Creek along the brackish intertidal portion of the Satilla River estuary, GA.
The Dover Bluff residential community (~50 homes) lies immediately adjacent to a Spartina alterniflora salt marsh and tidal creek (Umbrella Creek). The homes employ septic systems to process household waste. A network of upland groundwater monitoring wells and marsh wells and piezometers was installed along two transects extending from the upland across the marsh to the tidal creek by Dr. Carolyn Ruppel at the Georgia Institute of Technology as part of a collaborative NOAA Sea Grant project. Monitoring well depth varies from 3 to 5 m. Bundled marsh piezometers are installed at three depths (0.5, 1.0 and 2.0 m) providing access to groundwater within the marsh.
This remote sensing component relies on the use of thermal IR imaging to visually determine where cold groundwater discharges into warm surface waters. This technique is most successful in summer when temperature differences between warm surface and colder groundwater are at their maximum (Portnoy et al. 1998). Numerous sub-marsh groundwater flows were documented in the Okatee (note the cooler areas in images B, C, D, and E above). Additionally, discrete plumes entering a tidal creek to the N of the N well transect. [Joye et al. unpublished data; Porubsky et al. in revision].
To study the factors controlling microbial metabolism in coastal sediments impacted by groundwater flow, we collect sediment cores from marsh and creekbank locations. The figures below show Nat Weston (PhD 2005) collecting cores from Dover Bluff (Satilla River) and discussing his work with Dover Bluff residents.
To collect groundwater for geochemical sampling, we install groundwater monitoring wells by hand augering down to a sampling layer we are interested in. The two photos below show PhD student Charles Schutte hand augering a well into the sandy creek bank sediments of Moses Hammock and a well-transect along a coastal hammock being installed by the LTER team.
Sapelo Island (middle right panel on the figure above and below) is a pristine barrier island that lies within the domain of the Georgia Coastal Ecosystems LTER project (see the GCE web site for detailed information about the project). The pristine marshes on Sapelo Island are usually saline but can be influenced by freshwater discharge from the Altamaha River. Our primary study sites on Sapelo are Moses Hammock, which is located at the upper reaches of the Duplin River within the Sapelo Island National Estuarine Research Reserve, and Cabretta Island, on the southern end of the island (see map below). At Moses Hammock, existing well fields (installed by LTER investigator Dr. Carolyn Ruppel of the USGS) border a small marsh area to northwest. Additional wells were installed to the south end of the hammock where marsh is more extensive and permanent plots are located. The site contains a transition from upland to salt marsh to tidal creek, and the marsh is dominated by Spartina alterniflora. The Cabretta site lies between Nanny Goat Beach and Dean Creek (figure below) and contrasts with the Moses Hammock in that the well transect spans the marsh-upland-beach transition. The location of the transect was chosen because it (1) allows meaningful 2-D approximation of a 3-D system, (2) is similar in size to other, developed barrier islands, (3) is easily accessible from the UGA Marine Institute Lab, and (4) falls within a designated study area of the GCE-LTER (GCE6, Dean Creek).
Joye's groundwater related research strives to
- Document with and between site spatial and temporal trends in groundwater biogeochemical signatures.
- Document rates and pathways of microbially-mediated transformations of groundwater-derived C, N and P that occur in sediments as groundwater transits the upland-marsh-tidal creek ecotone.
- Determine the importance of groundwater as a source for labile nutrients, organic matter, and trace gases to coastal waters.
Specific questions regarding groundwater dynamics
- How does groundwater biogeochemistry vary between developed and pristine sites?
- What is the impact of tidal pumping of coastal aquifers on biogeochemical processes, like nitrification and denitrification, within those aquifers?
- Is groundwater a source of labile nutrients and perhaps organic matter to coastal waters?
- How do coastal sediment microbes alter groundwater-derived C, N, and P?
Rosalynn Lee (PhD 2007), Charles Schutte (PhD student), Nat Weston (PhD 2005), Bill Porubsky (PhD 2008), Christelle Hyacinthe (Post Doc 2007-2010)
Moore, W. S., J. Krest, G. Taylor, E. Roggenstein, S. B. Joye, and R. Y. Lee, 2002. Thermal evidence of water exchange through a coastal aquifer: Implications for nutrient fluxes. Geophysical Research Letters, 10: 1029/2002GL014923, 31 July 2002
Moore, W. S., J. Blanton, and S. B. Joye, 2006. Estimates of flushing times, submarine groundwater discharge, and nutrient fluxes to Okatee Estuary, South Carolina. Journal of Geophysical Research, Vol. 111, No. C9, C09006, 10.1029/2005JC003041.
Porubsky, W.P., S.B. Joye, W.S. Moore, K. Tuncay, and C. Meile, 2010. Hammock groundwater flow and biogeochemistry: Field measurements, laboratory assays and predictive modeling. Biogeochemistry, DOI: 10.1007/s10533-010-9484-8.
Wilson, A., W.S. Moore, S.B. Joye, J. Anderson and C. Schutte, 2010. Storm-driven groundwater flow in a salt marsh. Water Resources Research, doi:10.1029/2010WR009496.
Georgia/NOAA Sea Grant
South Carolina Sea Grant
National Science Foundation LTER program
National Science Foundation EAR/Hydrologic Sciences