Abstract Fresh ground water is widely distributed in subsurface sediments below the coastal bays of the Delmarva Peninsula (Delaware, Maryland, and Virginia). These conditions were revealed by nearly 300 km of streamer resistivity surveys, utilizing a towed multichannel cable system. Zones of high resistivity displayed by inversion modeling were confirmed by vibradrilling investigations to correspond to fresh ground water occurrences. Fresh water lenses extended from a few hundred meters up to 2 km from shore. Along the western margins of coastal bays in areas associated with fine‐grained surficial sediments, high‐resistivity layers were widespread and were especially pronounced near tidal creeks. Fresh ground water layers were less common along the eastern barrier‐bar margins of the bays, where sediments were typically sandy. Mid‐bay areas in Chincoteague Bay, Maryland, did not show evidence of fresh water. Indian River Bay, Delaware, showed complex subsurface salinity relationships, including an area with possible hypersaline brines. The new streamer resistivity system paired with vibradrilling in these investigations provides a powerful approach to recovering information required for extension of hydrologic modeling of shallow coastal aquifer systems into offshore areas.
Results of geophysical surveys in Indian River Bay, Delaware, indicate a complex pattern of salinity variation in subestuarine ground water. Fresh ground-water plumes up to about 20 meters thick extending hundreds of meters offshore are interspersed with saline ground water, with varying degrees of mixing along the salinity boundaries. It is possible that these features represent pathways for nutrient transport and interaction with estuarine surface water, but the geophysical data do not indicate rates of movement or nutrient sources and reactions. In the current study, samples of subestuarine ground water from temporary wells with short screens placed 3 to 22 meters below the sediment-water interface were analyzed chemically and isotopically to determine the origins, ages, transport pathways, and nutrient contents of the fresh and saline components. Apparent ground-water ages determined from chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), tritium (3H), and helium isotopes (3He and 4He) commonly were discordant, but nevertheless indicate that both fresh and saline ground waters ranged from a few years to at least 50 years in age. Tritium-helium (3H-3He) ages, tentatively judged to be most reliable, indicate that stratified offshore freshwater plumes originating in distant recharge areas on land were bounded by relatively young saline water that was recharged locally from the overlying estuary. Undenitrified and partially denitrified nitrate of agricultural or mixed origin was transported laterally beneath the estuary in oxic and suboxic fresh ground water. Ammonium produced by anaerobic degradation of organic matter in estuarine sediments was transported downward in suboxic saline ground water around the freshwater plumes. Many of the chemical and isotopic characteristics of the subestuarine ground waters are consistent with conservative mixing of the fresh (terrestrial) and saline (estuarine) endmember water types. These data indicate that freshwater plumes detected by geophysical surveys beneath Indian River Bay represent lateral continuations of the active surficial nitrate-contaminated freshwater flow systems originating on land, but they do not indicate directly the magnitude of fresh ground-water discharge or nutrient exchange with the estuary. There is evidence that some of the terrestrial ground-water nitrate is reduced before discharging directly beneath the estuary. Local estuarine sediment-derived ammonium in saline pore water may be a substantial benthic source of nitrogen in offshore areas of the estuary.
Abstract The small bays along the Atlantic coast of the Delmarva Peninsula (Delaware, Maryland, and Virginia) are a valuable natural resource, and an asset for commerce and recreation. These coastal bays also are vulnerable to eutrophication from the input of excess nutrients derived from agriculture and other human activities in the watersheds. Ground water discharge may be an appreciable source of fresh water and a transport pathway for nutrients entering the bays. This paper presents results from an investigation of the physical properties of the surficial aquifer and the processes associated with ground water flow beneath Indian River Bay, Delaware. A key aspect of the project was the deployment of a new technology, streaming horizontal resistivity, to map the subsurface distribution of fresh and saline ground water beneath the bay. The resistivity profiles showed complex patterns of ground water flow, modes of mixing, and submarine ground water discharge. Cores, gamma and electromagnetic‐induction logs, and in situ ground water samples collected during a coring operation in Indian River Bay verified the interpretation of the resistivity profiles. The shore‐parallel resistivity lines show subsurface zones of fresh ground water alternating with zones dominated by the flow of salt water from the estuary down into the aquifer. Advective flow produces plumes of fresh ground water 400 to 600 m wide and 20 m thick that may extend more than 1 km beneath the estuary. Zones of dispersive mixing between fresh and saline ground water develop on the upper, lower, and lateral boundaries of the plume. The plumes generally underlie small incised valleys that can be traced landward to streams draining the upland. The incised valleys are filled with 1 to 2 m of silt and peat that act as a semiconfining layer to restrict the downward flow of salt water from the estuary. Active circulation of both the fresh and saline ground water masses beneath the bay is inferred from the geophysical results and supported by geochemical data.
Stable oxygen and carbon isotope profiles from fossil scallop shells provide detailed paleoenvironmental information for the Pliocene and early Pleistocene of the Middle Atlantic Coastal Plain. Scallop specimens were collected from strata which represent at least five major marine transgressions. Minimum and maximum paleotemperatures were calculated from the {delta}{sup 18}O ranges recorded in each shell profile, after adjusting for changes in seawater {delta}{sup 18}O related to changes in global ice volume. Paleotemperature ranges from each stratigraphic unit were compared with modern conditions on the shelves of the Middle and South Atlantic Bight, and with paleotemperatures estimated by Hazel (1971b, 1988) from the ostracode faunas. The mollusk-isotope records indicate that the marine climate of the Atlantic Shelf was mild temperate during the deposition of the Sunken Meadow Member of the Yorktown Formation in the early Pliocene. The climate became warm temperate during the middle and late Pliocene transgressions which deposited the Rushmere, Morgarts Beach and Moore House Members of the Yorktown Formation and the Chowan River Formation. During the deposition of the James City Formation in the early Pleistocene, temperatures returned to a mild temperate climate similar to that of the modern Virginia Bight shelf. The character of the isotope profilesmore » indicates that hydrographic conditions were generally stable and similar to those of the modern Middle Atlantic Bight. The {delta}{sup 13}C profiles of most of the shells show trends suggestive of spring phytoplankton blooms and summer water-column stratification. Anomalies in several profiles are interpreted as reduced salinity events, probably related to river discharge, which most commonly occur in the spring. There is no convincing evidence in the shell profiles for upwelling.« less
With billions of dollars worth of property and infrastructure threatened by coastal erosion, there is a pressing societal need to identify and mitigate this hazard. Following a congressional mandate, the Federal Emergency Management Agency conducted a pilot study to examine historical shoreline change and to determine the cost effectiveness of mapping erosion hazards under the National Flood Insurance Program. The mapping phase of the study confirmed that long-term erosion is a widespread problem, but also revealed that further research to examine mapping techniques and methods of data analysis is necessary to improve our ability to accurately predict future shoreline change. In sediment-starved coastal regions (e.g., the mid-Atlantic seaboard), the local geologic framework can provide a critical but often ignored context for the interpretation of long-term erosion rates. A general geostatistical technique for quantifying the influence of nearshore geology in controlling long-term erosion is being developed using the northern Delmarva Peninsula as a case study. Some of the lowest erosion rates are found in areas where relict Pleistocene and older shorelines intersect the modern barrier-island system, providing resistance to shoreface erosion and, in some instances, a steady supply of beach-compatible material. In addition to the geostatistical analyses, a novel approach for determining the error associated with several methods of calculating erosion rates was tested in Delaware and New York. In these case studies, the best erosion forecasts were those derived from linear-regression rates calculated using 19th and 20th century shoreline positions, excluding shorelines surveyed after major storms.
A symposium on the topic of episodic sea-level change during the Quaternary was convened as part of the Southeastern Section meeting of the Geological Society of America on April 1-2, 1993, in Tallahassee, Florida. The symposium was organized for the purpose of examining and comparing some of the recent evidence for episodicity in sea-level change and in the geologic response to such changes. The sessions examined the evidence of Quaternary sea level history which has been gathered in recent years through geologic, paleontologic and geophysical studies in the southeastern United States region.
This paper reports on streaming resistivity (“DC resistivity”) surveys conducted in Maryland and<br>Virginia Atlantic coastal bays in the spring of 2001. Surveys in Assawoman, Isle of Wight, and<br>Chincoteague Bays, MD and VA, were used to study profiles of electrical resistivity of submarine strata<br>to delineate submarine freshwater discharge and submarine saltwater interfaces and salinity distributions<br>in submarine groundwater. The studies follow similar resistivity surveys in Rehoboth and Indian River<br>Bays in spring of 2000 (Krantz and others, 2000; Madsen and others, 2001; and Manheim and others,<br>2001).<br>The Delmarva Peninsula coastal studies are part of larger cooperative programs between the U.S.<br>Geological Survey, regional federal and state organizations, and academic institutions. They address the<br>problem of excess nutrient discharge into Delmarva coastal bays. Like the Delaware coastal bays,<br>Maryland and Virginia coastal bays receive excess nutrients due to human activities. The excess<br>nutrients enhance growth of phytoplankton and fouling macroalgae, which impedes boat operation, coats<br>beaches, and lays down organic–rich mats. This organic matter fosters anoxic conditions in the bottom<br>sediments. Growing stagnation alters the habitat for benthic organisms and reduces biological diversity.<br>Recent studies suggest that excessive organic growth inhibits natural mechanisms (like denitrification)<br>that help transform and remove nutrients from the bay systems.<br>Submarine discharge of nitrate-enriched ground waters was inferred from preliminary estimates<br>of land-based hydrologic flow-nets (Andres, 1987, 1992) in the Delaware coastal bays (Rehoboth and<br>Indian River), and modeled by Cerco and others, 1994. Subsequently, as a part of a large consortium<br>study (CISNET) led by the University of Delaware, T. McKenna of the Delaware Geological Survey<br>(2000) and coworkers performed overflights of Rehoboth and Indian River bays in the winter of 1999<br>(McKenna and others, 2001). Remote sensing (infrared temperature measurements) of surficial coastal<br>waters in the winter detected a number of areas where warmer water anomalies signified submarine<br>discharge in the near-coastal environment. A recent summary (Dillow and Greene, 1999) based on land data estimates that roughly 24%<br>(272,000 pounds) of the total nitrate loads from groundwater enters the Maryland bays through<br>submarine groundwater discharge (SGD). This nitrate flux is associated with about 13% of the<br>estimated 100 million gallons per day total water influx estimated to enter the Maryland coastal bays<br>through SGD. The Maryland SGD fraction is a smaller proportion than estimated for the Delaware<br>inland bays (up to 80%). Dillow and Greene (1999) point out that there is uncertainty about the<br>pathways of submarine groundwater discharge. Postulated pathways range from immediate sub-bay<br>coastal margin discharge (corresponding to the Ghyben-Herzberg model), to long-distance transport in<br>aquifers extending under the barrier bar (Assateague Island) and discharging into the Atlantic Ocean.
