D. Reide Corbett

Timing and magnitude of recent accelerated sea-level rise (North Carolina, United States)

We provide records of relative sea level since A.D. 1500 from two salt marshes in North Carolina to complement existing tide-gauge records and to determine when recent rates of accelerated sea-level rise commenced. Reconstructions were developed using foraminifera-based transfer functions and composite chronologies, which were validated against regional twentieth century tide-gauge records. The measured rate of relative sea-level rise in North Carolina during the twentieth century was 3.0–3.3 mm/a, consisting of a background rate of ~1 mm/a, plus an abrupt increase of 2.2 mm/a, which began between A.D. 1879 and 1915. This acceleration is broadly synchronous with other studies from the Atlantic coast. The magnitude of the acceleration at both sites is larger than at sites farther north along the U.S. and Canadian Atlantic coast and may be indicative of a latitudinal trend.

Rapid wetland expansion during European settlement and its implication for marsh survival under modern sediment delivery rates

Fluctuations in sea-level rise rates are thought to dominate the formation and evolution of coastal wetlands. Here we demonstrate a contrasting scenario in which land-use–related changes in sediment delivery rates drive the formation of expansive marshland, and vegetation feedbacks maintain their morphology despite recent sediment supply reduction. Stratigraphic analysis and radiocarbon dating in the Plum Island Estuary (Massachusetts, United States) suggest that salt marshes expanded rapidly during the eighteenth and nineteenth centuries due to increased rates of sediment delivery following deforestation associated with European settlement. Numerical modeling coupled with the stratigraphic observations suggests that existing marshland could survive, but not form under the low suspended sediment concentrations observed in the estuary today. These results suggest that many of the expansive marshes that characterize the modern North American coast are metastable relicts of high nineteenth century sediment delivery rates, and that recent observations of degradation may represent a slow return to pre-settlement marsh extent. In contrast to ecosystem management practices in which restoring pre-anthropogenic conditions is seen as a way to increase ecosystem services, our results suggest that widespread efforts to restore valuable coastal wetlands actually prevent some systems from returning to a natural state.

Radon tracing of groundwater input into Par Pond, Savannah River Site

Abstract The groundwater contribution into Par Pond, a former cooling reservoir for two nuclear reactors located on the Department of Energys Savannah River Site (South Carolina), was estimated using a standard hydrologic budget as well as one augmented by a natural tracer approach. We determined a geochemical budget for 222 Rn, normally found at much higher concentrations in groundwater than surface waters, to assist in constraining the hydrologic estimates. The radon budget accounted for all quantifiable surface sources and sinks including the flux across the sediment-water interface which was determined by application of an advection-diffusion model. All hydrologic parameters and radon concentrations were monitored seasonally from February 1994 to August 1995. Using the water balance approach alone, the average groundwater discharge entering the lake was estimated to have an upper limit of approximately 0.95 ± 0.13 m 3 s −1 . The groundwater contribution obtained using the combined hydrologic/ 222 Rn approach ranged from 0.17 to 0.76 m 3 s −1 with a best estimate of 0.35 ± 0.16 m 3 s −1 . Lake profiles show enhanced 222 Rn concentrations in some areas indicating that groundwater enters Par Pond mostly through a small region in the northern portion of the lake, probably via small seeps or springs. Estimates show that groundwater plays a significant role in the overall water budget of the lake, accounting for 10%–33% of the total estimated inflow from all measured sources. Our results show that supplementing a standard hydrological water balance with radon budget considerations helps to constrain estimated groundwater flow into surface reservoirs.

Radon Tracing of Submarine Groundwater Discharge in Coastal Environments

This chapter discusses the radon tracing of submarine groundwater discharge (SGD) in coastal environments. Direct discharge of groundwater into the coastal zone may be an important material flux pathway from land to sea in some areas. It has been largely ignored because of the difficulty in assessing its magnitude. While measurement problems persist, there is a growing recognition that groundwater flow into the sea is important. This chapter reviews an approach, using a simple one-dimensional model, to measure SGD via use of 222 Rn as a natural tracer. Radon has certain advantages over other potential geochemical tracers of groundwater discharge. Typically, it is greatly enriched in groundwater compared to seawater; it can be measured at very low concentrations, and is completely conservative. On the other hand, as a gas it is subject to losses at the air–sea interface which may limit its use in shallow water environments. The radon tracing is an excellent qualitative tool for identifying areas of spring or seepage inputs in most coastal environments. It is a good quantitative tool in shallow marine environments characterized by large amounts of SGD.

A multi-level pore-water sampler for permeable sediments

ABSTRACT The construction and operation of a multi-level piezometer (multisampler) designed to collect pore water from permeable sediments up to 230 cm below the sediment-water interface is described. Multisamplers are constructed from 1 inch schedule 80 PVC pipe. One-quarter-inch flexible PVC tubing leads from eight ports at variable depths to a 1 inch tee fitting at the top of the PVC pipe. Multisamplers are driven into the sediment using standard fence-post drivers. Water is pumped from the PVC tubing with a peristaltic pump. Field tests in Banana River Lagoon, Florida, demonstrate the utility of multisamplers. These tests include collection of multiple samples from the permeable sediments and reveal mixing between shallow pore water and overlying lagoon water.

FORAMINIFERA IN THE ALBEMARLE ESTUARINE SYSTEM, NORTH CAROLINA: DISTRIBUTION AND RECENT ENVIRONMENTAL CHANGE

This study investigated the surface and subsurface distributions of foraminifera (both live populations and dead assemblages) throughout the Albemarle Estuarine System (AES) to determine the utility of the modern foraminiferal assemblages as models for paleoenvironmental interpretations in this estuarine and barrier island system. Thirty-seven species were recognized in the dead assemblages from 49 stations; 19 species comprised the living populations. Cluster analysis of the dead assemblages defined five biofacies: the calcareous foraminiferal nearshore marine and inlet biofacies, and the dominantly agglutinated foraminiferal estuarine shoal, estuary, inner estuary, and marsh biofacies. Paleoenvironmental reconstruction of three cores from the central Albemarle basin, based on the distribution of dead surface foraminiferal assemblages, recognized the inner estuarine and estuarine biofacies. Radionuclide tracers ( 210 Pb and 137 Cs) provided the geochronologic framework for each core. The westernmost core was capped by the inner estuarine biofacies overlying the estuarine biofacies, indicating either accumulation of a seasonal ephemeral layer of sediment from a lower brackish, upstream environment or increased freshwater discharge since the 1990’s as a result of increased tropical storm and hurricane activity. The two easternmost cores indicated that, in the early 19 th century, Albemarle Sound populations included calcareous foraminiferal species. These taxa were adapted to the higher salinities that resulted from several inlets that were open adjacent to the AES prior to 1828. Taphonomic processes (test transport, test dissolution, mechanical test breakage) are active but, with the exception of test dissolution in a relatively restricted geographic area, they do not significantly alter surficial foraminiferal assemblages in the transition into subfossil assemblages. Thus, foraminiferal distributions are useful for characterizing modern estuarine environments and for interpreting paleoenvironmental changes in sediments deposited over the past few hundred years in coastal North Carolina.

Evaluation of sediment dynamics in coastal systems via short-lived radioisotopes

The Neuse and Pamlico River Estuaries are shallow, dynamic systems that have been plagued with symptoms of eutrophication over the past two decades. Extensive research has been conducted over the last 5–10 years to better understand the complex nutrient dynamics of these systems. However, most of these studies have concentrated on nutrient cycling in the water column. Only recently have studies focused on the benthic environment, and most sediment studies have neglected the dynamic nature of the benthos, focusing instead on diffusion as the dominant transport process delivering nutrients to the water column. Although diffusion of nutrients across the sediment–water interface may be important during quiescent periods of sediment deposition and short-term storage, wind events associated with storms throughout the year will resuspend newly deposited sediments resulting in the advective transport of sediment porewater, rich with nitrogen, phosphorus and carbon, into the water column. Sediment resuspension may increase water column nutrient concentrations, and therefore present estimates of nutrient and carbon inputs from the sediments may be too low. This study evaluated short-term sediment dynamics of natural resuspension events and deposition rates in these two estuaries with the use of short-lived radioisotopes, 7Be, 137Cs, and 234Th. Sediment cores at nine sites in the estuaries have been collected at least bimonthly since May 2001. In general, tracers indicate a depositional environment with minimal episodes of removal. The largest sediment removal occurred in August 2001 in the Neuse River where an estimated 2.2 cm of sediment were removed over the previous 6-week period. This removal mechanism essentially advects porewater nutrients into the water column. Calculated advective fluxes of ammonium and phosphate based on this resuspension event were approximately six times greater than the average diffusive flux measured in the same general area of the river. Longer-term deposition rates, using 137Cs, ranged from 1.4 to greater than 5 mm year−1, comparable to earlier studies in the area and agree well with the interpretation of the short-lived tracers. In addition, meteorological (wind speed and direction), turbidity, and bottom current data were collected and indicated that these resuspension events occur when passing fronts developed wind speeds in excess of 4 m s−1 with rapid shifts in direction. Currents exhibited estuarine flow reversals associated with wind events and apparently have some control over the sediment removal processes.

Fate of wastewater-borne nutrients under low discharge conditions in the subsurface of the Florida Keys, USA

We designed experiments to evaluate the fate of the addition of wastewater-borne nutrients injected into the shallow subsurface in the Florida Keys. During three different experiments, either bulk unlabeled phosphate, radio-labeled phosphate 32 3y . 14 1y . PO , or bulk unlabeled nitrate NO was added simultaneously with conservative tracers sulfur hexafluoride and 43 . I-131 into a wastewater injection well on Long Key. Relative concentration changes monitored over time indicated that both phosphate and nitrate acted non-conservatively in the subsurface. Phosphate showed an initial rapid uptake followed by a slower removal, possibly caused by adsorption-desorption reactions. Based on our observations, we estimate that approximately 95% of the phosphate injected into the subsurface could be removed in 20 to 50 h. There was also evidence for some removal of nitrate, possibly due to denitrification. Approximately 65% of the nitrate was removed over several days, suggesting a denitrification rate of 2700 mmol m y3 groundwater h y1 , comparable to estimates of denitrification in other groundwater systems. Collectively, our results suggest that nutrients injected in the subsurface are removed rapidly from solution and thus may not have a significant impact on surface waters. However, these experiments were conducted at 3 . 3

Seepage rate variability in Florida Bay driven by Atlantic tidal height

Atlantic tidal fluctuations drive pressure head variations in shallow offshore wells drilled into the limestone subsurface on both the Florida Bay and Atlantic sides of Key Largo, Florida, USA. We tested the hypothesis that these pressure head variations influence groundwater flow and that flux rate variability is associated with tidal variability. We used an automated Rn monitor to make continuous measurements of 222Rn, a natural tracer of groundwater discharge, in Florida Bay waters. We also deployed three types of seepage meters, including an automated heat pulse meter to collect a continuous record of seepage from the sediments. Drum type seepage meters inserted into soft sediments and fiberglass meters cemented to the rocky bay floor were utilized with pre-filled 4-l bag collectors, and monitored on an hourly basis. Maximum Rn inventories in Florida Bay waters were associated with high tide on the Atlantic side of the island. Modeling of the Rn variation indicated variable groundwater discharge rates with maximum flux occurring at high Atlantic tide. Seepage meter results in Florida Bay were consistent with 222Rn modeling. Florida Bay seepage meter rates showed positive correlation with Atlantic tide, meter 1, r = 0.63, n = 12, p < 0.025 and meter 2, r = 0.67, n = 12, p < 0.025. A seepage meter offshore of the Atlantic side of Key Largo exhibited rates that were inversely correlated with Atlantic tide (r = 0.87, n = 9, p < 0.005) showing negative rates when the tide was high, and positive rates when the tide was low. Overall, our results are consistent with the hypothesis of Reich et al. (2002), that pressure head variations driven by Atlantic tide influence groundwater seepage rate variability in Florida Bay off Key Largo. Effectively, as proposed by Reich et al. (2002), Key Largo functions as a semi-permeable dam separating Florida Bay and the Atlantic Ocean.

The spatial variability of nitrogen and phosphorus concentration in a sand aquifer influenced by onsite sewage treatment and disposal systems: a case study on St. George Island, Florida

Groundwater from a shallow freshwater lens on St. George Island, a barrier island located in the Panhandle of Florida, eventually discharges into Apalachicola Bay or the Gulf of Mexico. Nutrient concentrations in groundwaters were monitored downfield from three onsite sewage treatment and disposal systems (OSTDS) on the island. Estimates of natural groundwater nutrient concentrations were obtained from an adjacent uninhabited island. Silicate, which was significantly higher in the imported drinking water relative to the surficial aquifer on St. George Island (12.2+/-1.9 mg Si l(-1) and 2.9+/-0.2 mg Si l(-1), respectively), was used as a natural conservative tracer. Our observations showed that nitrogen concentrations were attenuated to a greater extent than that of phosphorus relative to the conservative tracer. At the current setback distance (23 m), both nitrogen and phosphate concentrations are still elevated above natural levels by as much as 2 and 7 times, respectively. Increasing the setback distance to 50 m and raising the drainfields 1 m above the ground surface could reduce nutrient levels to natural concentrations (1.1+/-0.1 mg N l(-1), 0.20+/-0.02 mg P l(-1)).