H24A-01 INVITED 16:00h
Comparing Advective and Diffusive Transport in Marine Sediments
In this contribution we contrast diffusive and advective solute transport in marine sediments. The microbial decomposition of organic matter in aquatic deposits is controlled by the transport processes that carry degradable substances and metabolites into and out of the bed. In most marine deposits this transport is caused mainly by molecular diffusion and bioturbation. In sandy sediments that dominate the shallow shelf, the permeability of the bed permits the flow of water through the upper layers of the sediment that is driven by the interaction of sediment topography and boundary layer currents. In-situ measurements show that these advective pore water flows can pump several hundred liters sea water per m2 and day through the upper layer of a permeable sand bed and thus, can cause exchange of solutes that exceeds that driven by diffusion by several orders of magnitude. The fundamentally different transport characteristics of advective pore water flows produce distinct differences between the biogeochemical zonation of permeable and cohesive beds. The role of the filtering shallow shelf sands in the marine cycles of matter still is largely unknown despite the fact that permeable shelf sediments may intercept or remove land-derived nutrients along the land-to-sea continuum, thereby regulating coastal primary production.
H24A-02 16:15h
Utilising Physical, Chemical, And Stable Isotope Techniques To Delineate The Flows Within A Coastal Wetlands System
The coastal wetlands system under study comprises a series of small lakes and is very unique in the sense that the lakes within the system display different hydrochemistry and stable isotopic composition although they are connected by channels and form as a cluster of inter-connected lakes. The complex flow systems and the transient nature of the interactions between surface water and groundwater present in the wetlands system were delineated using both chemical and stable isotope data to supplement existing classical hydraulic data. The spatial and temporal variations of chemical and isotopic composition of the individual water bodies within the system were measured for an annual cycle, to provide a unique data set for the analysis. A purely hydraulic analysis of the region surrounding the wetlands would indicate that the wetlands are flow-through bodies, however the chemical and isotope information indicates the lakes almost invariably act as discharge points for the surface water flows and the north south regional groundwater flow. Large volumes of groundwater flow were found within an observed northeast-southwest trending paleochannel within the wetlands system, and in this case, the chemical and isotopic evidence are complimentary with the hydraulic study. The isotope and chloride results from the surface water bodies allowed for the accurate determination of the composition of the major creeks in the system, and a simple portioning model indicated that groundwater is the predominant source for the inflowing creeks. Similarly, the deuterium versus Oxygen-18 and deuterium versus chloride relationships observed in the system portray two distinct evaporation trends, one through the hypersaline lakes and the other through less saline lakes which indicates that the isotopic composition of the water bodies are affected to a great extent by high dissolved salts content. The superposition of these data sets provided a unique vision of the flow system and clearly shows that certain lakes are significantly different from a compositional standpoint to other lakes although a bathymetric survey conducted on the wetlands system indicated otherwise.
H24A-03 16:30h
An Evaluation of Groundwater Contributions along the Continental Shelf of Louisiana, USA
Recent studies have suggested that groundwater may play an important role in transporting water and bioactive elements to coastal waters along continental shelves, including 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 goal of this study is to assess the groundwater contribution to the coastal waters of Louisiana, a river-dominated continental shelf environment. We have employed multiple geochemical tracers (222Rn/226Ra, 4He/3He/3H, and short-lived radium isotopes) and will evaluate the groundwater discharge using a mass balance approach. Fieldwork was conducted over the last eighteen months and was designed to assess groundwater contributions under high flow (spring) and low flow (fall) Mississippi River conditions. Land-based groundwater samples were collected from monitoring wells sampled on a quarterly basis and continental shelf samples were collected during six 4-day cruises (fall 2003 and spring 2004). Groundwater samples were analyzed by liquid scintillation techniques with total 222Rn values ranging from 270 to 3077 dpm l-1. Water column samples were collected throughout the study area during each cruise and variability of unsupported 222Rn was evident between the fall and spring with higher bottom water samples found in the spring. Water column inventories during the initial cruise ranged between 0 and 29381 dpm m-2. The majority of the stations sampled on the shelf had significantly higher 222Rn inventories than that supplied simply by diffusion alone. Interestingly, stations beyond the 30m coutour tended to have the greatest unsupported water column inventories. This indicates an additional source of 222Rn to these waters, including horizontal transport or an advective flux. Excess 222Rn values indicate variability between high and low river flow conditions as well as variability within each flow condition.
H24A-04 16:45h
Use of Continuous Resistivity Profiling to Detect Low-Salinity Ground Water Beneath the Upper Neuse River Estuary, North Carolina
The Neuse River Estuary (NC) has recently experienced fish kills associated with low dissolved oxygen events and blooms of toxic dinoflagellates, along with other problems linked to eutrophication. As part of a larger project to constrain nutrient budgets, a field investigation was initiated in April 2004 to study occurrence and discharge of fresh and brackish ground water and nutrients beneath the estuary itself. A continuous resistivity profiling (CRP) system was used to map the depth of the freshwater-saltwater interface (FSI) in sub-estuarine ground water. A total of 154 km of lines surveyed yielded 108 km of high-quality data after processing. Typical depth penetration of the CRP system was 20 to 27 m below the sediment surface. Patterns observed in the data included downstream and offshore deepening of the FSI in sub-estuarine ground water, as well as offshore plumes of low-salinity water beneath shoals and in buried paleochannels. In transects near the head of the NW-SE trending upper estuary, the resistivity-defined FSI ($>$ 25 ohm-m) was 11-18 m below the sediment surface. Shore-parallel tracks collected less than 800 m from shore in $<$ 3 m of water indicated that the FSI along the northeast shore and most of the southwest shore of the estuary was $>$ 10 m below the sediment surface, with isolated zones where the FSI was at or near the sediment surface, and longer stretches with the FSI $>$ 24 m deep. An exception to this was an area of apparent discharge along approximately 6 km of 9-m-high bluffs in the Riverdale area of the southwestern shore. Offshore data collected parallel to the estuary axis between Cherry Point and Thurman showed no significant low-salinity ground water in the eastern half of the upper estuary, except for a few plumes extending offshore from the south shore. One of these plumes originated at a discharge area adjacent to Cherry Point Marine Corps Air Station and extended at least halfway across the estuary, with the depth of the FSI increasing with distance from shore. Available seismic data indicate that the plume may lie in a buried paleochannel. Elevated surface water concentrations of radon are also consistent with discharge in this area. A second plume was observed extending beneath a shoal offshore from Cherry Point, with the FSI at a depth of about 8 m out to the edge of the shoal.
http://soundwaves.usgs.gov/2004/06/fieldwork.html
H24A-05 INVITED 17:00h
Predictions of the Fresh Water Component of Submarine Groundwater Discharge: Model-Based Approaches
The discharge of the fresh water component of groundwater to the near-shore marine environment is a difficult process to quantify because of numerous processes that interact to influence the rate of fluid exchange across the seabed interface. Examples are presented that illustrate some of the factors that must be considered when comparing model-based predictions of the volumetric fresh water discharge to estimates derived from either direct measurement or by inference from geochemical tracers. Two sites will be considered; one a near-shore experiment within Apalachee Bay in the northeastern Gulf of Mexico, and the other a regional-scale study of submarine groundwater discharge on the continental margin off the Mississippi River delta.
H24A-06 17:15h
Submarine groundwater discharge to Salt Pond (MA) estimated from continuous 222Rn measurements
Submarine groundwater discharge (SGD) is the dominant means of freshwater delivery to many coastal embayments on the glaciated coast of Cape Cod (MA). This discharge is the focus of considerable research, in part because the common practice of domestic wastewater treatment via septic systems has led to elevated levels of nutrients in groundwater. However, SGD is frequently diffuse, rendering it difficult to quantify. SGD was therefore estimated by a variety of means in Salt Pond, a weakly stratified estuary 8.2 ha in area with a maximum depth of 9 m that is located at the northern (inland) end of Nauset Marsh (MA). One approach relied on continuous measurements of radon, which offers an excellent means of quantifying SGD delivery to coastal waters because radon is: 1) strongly enriched in groundwater relative to surface water; 2) non-reactive; 3) continuously supplied by long-lived parent isotopes; and 4) provides an integrated signal from a wide area. Continuous measurements of radon, salinity, temperature and water depth were carried out for four days in June, 2004 in a narrow channel that connects Salt Pond to the adjacent Nauset Marsh. The radon-based SGD estimate was derived using a mass balance approach based on radon outflow from the pond, corrected for inputs from the adjacent Nauset Marsh and losses due to gas exchange and decay. In addition, the radon content of groundwater was determined from piezometer samples. This approach yielded a pond-integrated upper limit of discharge of roughly 5 cm/d, a figure that is very similar to a multi-year average flow estimate from a previously-developed hydrologic model. SGD estimates using seepage meters deployed in shallow waters were more than a factor of two higher than the radon and model estimates, which may indicate that discharge occurred only in the shallow regions of the pond and not through the deeper, fine-grained sediments. Further research is needed in order to determine the reasons for the differing estimates.
H24A-07 17:30h
The Impact of Surface/Ground Water Interactions on Wetland Hydrology
The crucial role of surface water/ground water interactions in water resources and hydrologic applications has been taking center stage recently. The interaction of ground water occurs with all types of surface waters (e.g., streams, lakes, wetlands and reservoirs) and pollutants in either surface or ground water get mixed and the quality of both sources is affected by each other. Wetlands are land areas which are frequently transitional between uplands and flooded systems. In the last two decades, the beneficial aspects of treatment wetlands have been studied. Yet, investigating surface/ground water interactions within wetlands has only recently become a critical issue, especially in order to understand the effect of wetland hydrology on water quality. For this purpose, a comprehensive wetland model is being developed that incorporates these surface/ground water interactions. The effect of wetlands on storm water runoff is investigated by routing the overland flow through the wetland area, collecting the runoff within the stream and transporting it to the receiving water using diffusion wave routing techniques. An implicit finite difference numerical scheme is used to solve the diffusion wave formulation. The wetland model includes Thornthwaite evapotranspiration and Green-Ampt infiltration models in the water budget, in addition to rainfall and groundwater recharge/discharge terms. The exchange of water between the wetland and subsurface is represented by Darcy's Law. In addition, the newest version of the well-known EPA Stormwater Management Model (SWMM5) is incorporated into this wetland model to simulate the runoff quantity and quality flowing into the wetland area from upstream urban areas. A preliminary application of the model to the Duke University West Campus and the Duke University constructed wetland area in the Sandy Creek watershed is presented for a one-year continuous simulation. The obtained velocity profiles are used to investigate the effect of surface/ground water interactions on the concentration distribution in the wetland areas.
H24A-08 17:45h
An Integrated Surface Water and Groundwater Model of Fluid Flow and Thermal, Salinity, Sediment and Reactive Biogeochemical Transport
This paper presents the development and application of an integrated surface water and groundwater model to simulate hydrodynamics and thermal, salinity, sediment and reactive biogeochemical transport. The hydrodynamic module for tidal waters solves three-dimensional Navier-Stokes equations with or without the hydrostatic assumptions. The Richards equation is used to simulate the subsurface flow in both vadose and saturated zones. The Boussinesq approximation is employed to deal with the buoyancy force due to temperature and salinity variations. The moving free surface is explicitly handled by solving the kinematic boundary condition equation using a node-repositioning algorithm. The transport module solves the energy equation for temperature distribution and a number of mass transport equations for the salinity, sediment, and solute fields. The Arbitrary Lagrangian-Eulerian (ALE) representation is adopted for all transport equations including momentum transport. The solution is obtained with finite element methods or a combination of finite element and Semi-Lagrangian (particle tracking) methods. The model is applied to Loxahatchee Estuaries for the investigation of its minimum flow requirements to maintain ecological balance.