NS31A-01 INVITED
What Controls Submarine Groundwater Discharge?
Numerous processes have been implicated in controlling submarine groundwater discharge (SGD) to coastal zones since Ghyben, Herzberg and Dupuit developed models of fresh water discharge from coastal aquifers at the turn of the 19th century. Multiple empirical and modeling techniques have also been applied to these environments to measure the flow. By the mid-1950's, Cooper had demonstrated that dispersion across the fresh water-salt water boundary required salt water entrained into fresh water flow be balanced by recharge of salt water across the sediment-water interface seaward of the outflow face. Percolation of water into the beach face from wind and tidal wave run up and changes in pressure at the sediment-water interface with fluctuating tides have now been recognized, and observed, as processes driving seawater into the sediments. Within the past few years, variations in water table levels and the 1:40 amplification from density difference in fresh water and seawater have been implicated to pump salt water seasonally across the sediment- water interface. Salt water driven by waves, tides and seasonal water table fluctuations is now recognized as a component of SGD when it flows back to overlying surface waters. None of these processes are sufficiently large to provide measured volumes of SGD in Indian River Lagoon, Florida, however, because minimal tides and waves exist, flat topography and transmissive aquifers minimize fluctuations of the water table, and little water is entrained across the salt water-fresh water boundary. Nonetheless, the saline fraction of SGD represents more than 99% of the volume of total SGD in the Indian River Lagoon. This volume of saline SGD can be driven by the abundance of burrowing organisms in the lagoon, which pump sufficient amounts of water through the sediment- water interface. These bioirrigating organisms are ubiquitous at all water depths in sandy sediment and thus may provide one of the major sources of SGD world wide. Because bioirrigated water is well oxygenated and passes through sedimentary pore spaces, its influence may be quite large on fluxes of diagenetic reactive components, including organic matter, nutrients, and redox sensitive metals. While fresh meteoric groundwater may be confined to the shoreline in most cases and delivers new material from continents to the ocean, seawater circulating through sediments as part of SGD is apparently a much greater fraction of the total water flux and hence has the potential to significantly impact sediment diagenetic processes and subsequent export of nutrients and other solutes from the sediment to the water column.
NS31A-02
Fresh Versus Marine Submarine Groundwater Discharge: How 222Rn Might Help Distinguish These Two Sources
Pore water distributions of 222Rn (t1/2 = 3.83 d), obtained during two sampling trips 9-12 May 2005 and 6-8 May 2006, are used to determine spatial and temporal variations of fluid discharge from a seepage face located along the mainland shoreline of Indian River Lagoon, Florida. Porewater samples were collected from a 30 m transect of multi-level piezometers and analyzed for 222Rn via liquid scintillation counting; the mean of triplicate measurements was used to represent the porewater 222Rn activities. Sediment samples were collected from five vibracores (0, 10, 17.5, 20, and 30 m offshore) and emanation rates of 222Rn (sediment supported) were determined using a standard cryogenic extraction technique. A conceptual 222Rn transport model and subsequent numerical model were developed based on the vertical distribution of dissolved and sediment-supported 222Rn and applicable processes occurring along the seepage face (e.g. advection, diffusion, and nonlocal exchange). The model was solved inversely with the addition of two Monte Carlo (MC) simulations to increase the statistical reliability of three parameters: fresh groundwater seepage velocity (v), irrigation intensity (α0), and irrigation attenuation (α1). The first MC simulation ensures that the Nelder-Mead minimization algorithm converges on a global minimum of the merit function and that the parameters estimates are consistent within this global minimum. The second MC simulation provides 90% confidence intervals on the parameter estimates using the measured 222Rn activity variance. Fresh groundwater seepage velocities obtained from the model decrease linearly with distance from the shoreline; seepage velocities range between 0.6 and 42.2 cm d-1. Based on this linear relationship, the terminus of the fresh groundwater seepage is approximately 25 m offshore and total fresh groundwater discharge for the May-2005 and May-2006 sampling trips are 1.16 and 1.45 m3 d-1 m-1 of shoreline, respectively. We hypothesize that the 25% increase in specific discharge between May-2005 and May- 2006 reflects higher recharge via precipitation to the Surficial aquifer during the highly active 2005 Atlantic hurricane season. Irrigation rates generally decrease offshore for both sampling periods; irrigation rates range between 4.9 and 85.7 cm d-1. Physical and biological mechanisms reasonable for the observed irrigation likely include density-driven convection, wave pumping, and bio-irrigation. The inclusion of both advective and nonlocal exchange processes in the model permits the separation of submarine groundwater discharge into fresh submarine groundwater discharge (seepage velocities) and (re)circulated lagoon water (as irrigation).
NS31A-03
Flow paths of submarine groundwater discharge (SGD) and its relation to Fe diagenesis: A case study from Indian River Lagoon, Florida.
In the subterranean estuary, flow paths for submarine groundwater discharge (SGD) depend on two sources of water - freshwater and recirculated seawater. The lengths of freshwater flow paths increase as discharge points move offshore across the outflow face. Recirculated seawater flow paths can have different lengths depending on mechanisms driving the flow, with the longest flow paths resulting from diffusive entrainment of saltwater at the freshwater – saltwater boundary. As a consequence, pore water at the freshwater-saltwater boundary should be more reduced than pore water close to the sediment-water interface. In the subterranean estuary of Indian River Lagoon, Florida, porewater collected from a 30 m long shore perpendicular transect and at 250 m offshore have concentrations of Fe and Mn that trace the redox conditions. Porewater Cl concentrations, seepage meter results and 222Rn modeling indicate the outflow face extends 25 m offshore. Dissolved Fe and Mn concentrations are low at the shoreline (1.05 and 0.28 µM, respectively) and at 250 m offshore (0.48 µM and 0.56 µM, respectively) but their concentrations are orders of magnitude greater at the distal end of the seepage face at 22.5 m (261 and 2.9 µM, respectively). Cores from 0, 10, 17.5, 20, 22.5, 30, and 250 m offshore have sediment that varies from orange-yellow to black. The black sediment occurs at the top of the core and generally thickens from a feather edge about 5 cm thick at the shoreline to about 60 cm thick 30 m from shore. The dissolved Fe concentrations are greatest in the orange sediments but the dissolved sulfide concentrations are greatest in the black sediments. Dissolved Mn maxima occur at shallower depths than the Fe maxima. The distributions of Fe, Mn and sulfide concentrations suggest the orange sediment may be caused by Fe-Mn oxyhydroxides coatings and the black sediment may be caused by Fe-sulfide precipitates. The observed elevated concentrations of dissolved Fe and Mn at the distal end of the outflow face may result from longer flow paths, allowing a greater amount of time for reduction of Fe oxides. A diagenetic model that accounts for dissolved Fe pathway from the source of Fe-Mn oxyhydroxides to the sink of Fe-sulfides is currently being developed. This model will be used to quantify Fe production and uptake within the sediments and comparisons with estimated flow paths of SGD.
NS31A-04
Imaging the Stratification and Tidal Dynamics of a Saltwater-Freshwater Interface Using Continuous Shallow Electrical Resistivity Techniques in the Karst Coastal Biscayne Aquifer, South Florida.
Shallow ground resistivity was used to image the saline-freshwater mixing zone in the subsurface due to saltwater intrusion at a site less than 2 kilometers from the coastline of Biscayne Bay, an estuary in South Florida. Resistivity arrays have several advantages when applied to coastal groundwater environments. Experiments can be set up to measure several depths by varying the electrode spacing. Additionally Resistivity techniques can be used to resolve multiple changes in lithology and compositions making them ideal for saltwater intrusion studies. Miami-Dade county and adjoining the Biscayne Bay are underlain by the karst Biscayne aquifer system where the freshwater aquifer is in direct contact with the saline bay system. Saltwater intrusion has been a continuous problem since anthropogenic alteration of the hydrological system occurred to manage flood control and promote agriculture. To better understand the nature and dynamics of the saltwater intrusion in the near coastal shallow subsurface, a Supersting 28-electrode resistivity unit, configured in a Wenner array, was used to produce a 2D image of the freshwater/brackish water/saltwater stratification, and was deployed in a continuous data collection mode to observe the movement of the saltwater mixing zone through a tidal cycle. Results show at a depth of 9m below the surface there is a 2m meter thick freshwater layer above the brackish/saltwater mixing zone. In addition, the continuous imaging of the mixing zone over a tidal cycle shows a fluctuation landward and seaward of the mixing zone as the tide floods and ebbs. This technique provides an efficient tool for detection and characterization of the saltwater/freshwater mixing zone in shallow coastal aquifers and lends new insight to the stratification and thicknesses of the fresh and saline layers. In addition the detection of cyclical tidal movement and an estimate of its magnitude will aid in the understanding of the mixing zone geochemistry and hydrological dynamics such as submarine groundwater discharge.
NS31A-05
Using buoy data to identify and assess Nor'easters in the New York metropolitan region.
New York coastal regions are frequently exposed to high water level, winds and large waves generated by a subset of mid latitude cyclones, referred to as Nor'easters. These systems are primarily responsible for erosion of the barrier beaches, and for the general westward transport of sediment throughout the littoral system that extends from Montauk Point to New York Harbor. Previous studies identifying Nor'easters have either used relatively coarsely resolved meteorological conditions to provide a general regional climatology of Nor'easters, or have used localized water level or beach erosion data to identify and characterize the storms. The use of meteorological conditions to identify the Nor'easters allows a clear, independent assessment of impacts on the coastal environment, but impacts and overall intensity of these storms can exhibit substantial spatial variability that may not be capture by the coarse resolution approach using meteorological data. A technique was developed to use atmospheric pressure and temperature data from National Buoy Data Center buoys in the New York City area to identify Nor'easters and characterize a climatology. In addition, a three level intensity scale was developed using pressure time series data. The technique was applied to data from several offshore buoys in the New York City region. Our study identified 202 Nor'easters during 16 year period from 1992 through 2006. The impact of these storms on the coastal environment was assessed by compositing "storm" values of regional water level data, precipitation gauges measurements, and beach surveys along the south shore of Long Island. This technique has the advantage of using meteorological data to identify the storms and is localized enough that can be used to complement storm surge and beach erosion data.