evaluation of sediment dynamics in coastal systems via ...core.ecu.edu › geology › corbettd ›...

www.elsevier.com/locate/jmarsys

Journal of Marine Systems 42 (2003) 83–96

Evaluation of sediment dynamics in coastal systems via

short-lived radioisotopes

Dan Giffina, D. Reide Corbettb,*

aDepartment of Coastal Resources Management, East Carolina University, Greenville, NC 27858, USAbDepartment of Geology, East Carolina University, Greenville, NC 27858, USA

Received 26 June 2002; accepted 11 March 2003

Abstract

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, using137Cs, 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.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Sediments; Nutrients; Radioisotope; Resuspension; Estuary; Benthic flux

Regional terms: USA; North Carolina; Neuse/Pamlico River Estuary

0924-7963/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0924-7963(03)00068-X

* Corresponding author. Tel.: +1-252-328-1367; fax: +1-252-328-4391.

E-mail addresses: [email protected] (D. Giffin), [email protected] (D.R. Corbett).

D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–9684

1. Introduction

The complex sequencing of governing processes

that occur in the sediments (deposition, benthic dia-

genetic, transformation, resuspension/redistribution,

and burial) plays an important role in the overall

water quality of the ecosystem. Subsequent to depo-

sition, bottom sediments and associated constituents,

such as nutrients, are likely to undergo a number of

diagenetic transformations. Decomposition of organic

matter in aquatic sediments proceeds through a well-

established sequence of terminal electron acceptors:

O2, NO3�, MnO2, FeOOH, and SO4

2� (Froelich et al.,

1979). These reactions proceed with those releasing

the most energy occurring first, e.g. the consumption

of O2. These reactions influence pH and oxidation–

reduction potential and ultimately result in an in-

creased concentration of biologically available

nutrients in the porewaters. Physical processes, such

as winds, tides, and groundwater infiltration, can also

play an important part in the overall disposition of

these estuarine sediments. Winds, in particular, can

Fig. 1. Sediment processes in an estuary influence the nutrient load to surfa

both passive (diffusion, desorption, etc.) and dynamic (groundwater adve

develop wave energies that create orbital velocities

sufficient to resuspend sediments and advectively

release dissolved nutrients in relatively shallow water

bodies. Collectively, these physical and biogeochem-

ical processes create a complex and dynamic system

(Fig. 1).

Many nutrient studies that have included sediment

processes have neglected the dynamic nature of the

benthos. The focus has typically been on diffusion as

the dominant transport process. Diffusion is typically

measured at the benthic boundary to evaluate the

transfer of nutrients to the water column (Fisher et

al., 1982; Alperin et al., 2000). However, studies that

compared the nutrient flux associated with stable

(passive) sediments to those recently resuspended

found significantly higher nutrient delivery with the

recently disturbed sediments (Fanning et al., 1982;

Kristensen et al., 1992; Sondergaard et al., 1992; de

Jonge and van Beusekom, 1995). The resuspension of

bottom sediments results in the rapid transport of

diagenetic end-products (nitrogen/phosphorus/carbon)

associated with the porewaters being advected into

ce waters. Nutrients may be introduced to the water column through

ction, resuspension, biological mixing, etc.) events.

D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–96 85

surface waters. This advective transport serves to

increase water column nutrient concentrations, which

could potentially alter water quality of the estuarine

environment.

The Pamlico and Neuse River Estuaries (Fig. 2)

are shallow, dynamic systems that have been plagued

with symptoms of eutrophication over the past two

decades. These symptoms have included nuisance

Fig. 2. Sampling locations in the Neuse and Pamlico Estuaries were selec

environments. Stations are numbered with the lowest number (PR-1, NR-

cyanobaterial and dinoflagellate blooms, bottom wa-

ter hypoxia/anoxia, and fish kills (Bowen and Hier-

onymus, 2000). Nutrient loading, especially nitrogen,

by increased urbanization, industrialization, and

coastal development has contributed to the current

water quality problems. Extensive research has been

conducted over the last 5–10 years to better under-

stand the complex nutrient dynamics of these sys-

ted over various salinities, ranging from oligohaline to mesohaline

1) upriver, increasing sequentially downriver.


tems (Christian et al., 1991; Paerl et al., 1998;

Luettich et al., 2000). However, most of these

studies have concentrated on nutrient cycling in the

water column and have only recently included the

benthic environment as a passive (diffusion) source/

sink of nutrients (Fisher et al., 1982; Alperin et al.,

2000).

This study evaluated whether seasonal and short-

term sediment storage and remobilization could af-

fect the cycling of nutrients and carbon in the

Pamlico and Neuse River estuaries. The following

specific objectives were addressed: (1) determination

of the short-term sedimentation rate at select loca-

tions in the Neuse and Pamlico River Estuaries over

a 12-month period by evaluating the inventories of

natural particle-reactive tracers (234Th, 7Be, 137Cs) in

bottom sediments; (2) to use these tracers to under-

stand the processes and mechanisms that control the

transport and fate of nutrients in these estuarine

sediments; and (3) measure the frequency of sedi-

ment resuspension events utilizing meteorological

data and sensors.

1.1. Natural particle-reactive tracers

The distinction in time scales between temporary

versus permanent storage of particulate material in

bottom sediments is important in determining the

ultimate fate of pollutants in coastal ecosystems. This

difference in time scales can be distinguished quan-

titatively using radiochemical techniques for estab-

lishing geochronologies within bedded sediments,

and for examining rates of sedimentary processes.

Radioisotopes, such as beryllium-7 (7Be) and thori-

um-234 (234Th), can be used to determine fluxes and

rates of internal processes in sedimentary compart-

ments such as rivers, estuaries, and oceans. Beryllium

is produced by cosmic ray spallation reactions with

nitrogen and oxygen in the atmosphere. These reac-

tions occur primarily in the stratosphere and upper

troposphere where charged particles (alpha particles,

electrons, and protons) induce nuclear reactions in

oxygen and nitrogen atoms. Be-7 is delivered to the

surface of the earth through precipitation (wet and

dry) where it quickly adsorbs to particle surfaces and

subsequently is deposited to bottom sediments in

coastal environments (Olsen et al., 1986; Baskaran

and Santchi, 1993). In contrast, 234Th is primarily

produced in the water column through the decay of its

immediate parent U-238. When utilizing radioiso-

topes for investigating natural processes, it is impor-

tant to match the time scale of interest with the half-

life of a particular radionuclide. For the purpose of

this study (i.e. short-term deposition), the half-life of7Be (53.3 days) and 234Th (24.1 days) enables them

to be useful as particle tracers on time scales of days

to months.

Some of the most common uses of 7Be and 234Th

are to measure sediment inputs, exports, resuspension

events, rates of deposition, and sediment mixing

(Feng et al., 1998; McKee and Baskaran, 1999, Olsen

et al., 1985, McKee et al., 1983). An understanding of

these sedimentation processes is important for deter-

mining biogeochemical cycles in these environments.

Sedimentation goes through three processes: deposi-

tion, removal, and accumulation (the end difference

between deposition and removal). Deposition is the

temporary emplacement of particulate material on a

sediment surface and can be measured using 234Th

and 7Be because of their short half-lives. Seasonal

variations and storm events that impact a system can

be quantified in the depositional record with these

radioisotopes. The rapid scavenging of these isotopes

by sediment particles is used to identify areas of

increased deposition by elevated sediment inventories,

e.g. downcore integration of tracer activity (Olsen et

al., 1986). In addition, in an environment with limited

mixing, these tracers will exhibit log-scale reductions

in radioactivity with depth that can be used to deter-

mine short-term deposition and accumulation process

rates (McKee et al., 1983). On a longer-term basis,

year to decade range, sediment accumulation can be

determined with the use of 137Cs or 210Pb. Cesium-

137 is unique and especially helpful in this respect

since areas where minimal mixing occurs will retain

the fallout signature from 1963 (peak 137Cs deposi-

tion) in the sediments, thus providing a distinct

activity maximum during that year. By comparing

deposition to accumulation rates, one can derive the

net erosion rate (sediment export). We have employed7Be, 234Th, and 137Cs in this study to evaluate spatial

and temporal trends in sediment deposition in the

Neuse and Pamlico estuaries. This study represents

the first time these short-lived radionuclides have been

used to evaluate short-term sediment dynamics in

these estuaries.


2. Study area

Originating in the Piedmont region of North

Carolina, both the Neuse and Pamlico River basins

flow through the coastal plain and eventually empty

into Pamlico Sound. The Neuse and Pamlico water-

sheds comprise 16,000 and 11,500 km2, respectively

(Matson and Brinson, 1990) with a mean depth of

approximately 4.6 m. Average annual discharge of

the Neuse at its mouth has been cited as 173 m3 s� 1,

while the Pamlico is 153 m3 s� 1 (Giese et al., 1985).

Flow in the Neuse River is generally highest in

winter and spring with lowest flows occurring in

the summer and fall (Christian et al., 1991). Howev-

er, recent flow measurements near Fort Barnwell, just

upstream of the river’s confluence with the estuary,

indicated peak flows during this study occurred in

July through September 2001 and February and April

of 2002. Both estuaries are oligohaline at their upper

portion and gradually become more saline near their

lower extent (up to 20 ppt or more). Salinity profiles

along these estuaries are very dependent upon dis-

charge and wind events since tidal influence has been

found to be negligible (Luettich et al., 2000). Strat-

ification is very common and highly dependent on

meteorological events.

Bottom sediments in both estuaries are related to the

bathymetry. Simply, the shoulders of the estuary are

typified by medium- and fine-grained sands, while the

deeper regions are predominantly organic-rich muds

with little lithologic variability (Wells and Kim, 1989).

It has been suggested that most of the surface sediments

within these estuaries will be deposited and resus-

pended many times before permanent accumulation

(Wells and Kim, 1989; Riggs et al., 1991). This is due

to the fine-grained nature of the sediments, shallow

depths, and the high levels of wind stress on the basin

(Riggs et al., 1991). Therefore, the importance of

understanding the dynamics of these systems is evident

if we are to fully understand the nutrient dynamics.

All sample locations were located near the center

of the river within the organic-rich muds. Six sites

(Fig. 2) were selected in the Neuse River. The sites

covered a salinity range of 0–10 near the Trent

tributary up to the mesohaline (10–20) waters near

the mouth. Three stations were chosen in the Pamlico

River based on physiographic similarity to stations

sampled in the Neuse and to correspond with the low,

mid, and higher salinities (one at 0–1, one at 5–10,

and one near the mouth).

3. Methods

Sediment cores at nine sites in the estuaries were

collected at least bimonthly beginning May 2001 for

a 12-month period to evaluate these processes and

analyze short-lived nuclides. Cores were collected by

push core from a small boat with a PVC coring

device outfitted with a one-way check valve and a 4-

in.-diameter clear acrylic tube (core tube). The sam-

ple core tube was gently pushed into the bottom

sediments to obtain a vertical column of sediment.

Only cores exhibiting an undisturbed sediment–wa-

ter interface were collected for analysis. Cores were

subsectioned the length of the core (f 19 cm). The

first two subsections were 2 cm thick and the next

subsections were 3 cm each to a depth of 19 cm

total. Extruded subsamples for radiochemical analy-

sis of 234Th, 7Be, 137Cs, and 238U were stored and

returned to East Carolina University. Samples for

water content and porosity were collected onboard

during extrusion. A measured volume of wet sedi-

ment from each interval, typically 20 ml, was trans-

ferred to a preweighed container that was sealed and

brought back to the laboratory. These samples were

oven dried at 60 jC for several days and the

gravimetrically determined water content, correcting

for salt residue, was used to calculate the sediment

dry-bulk density.

Samples from all sediment cores were analyzed for234Th (t1/2 = 24.1 days), 7Be (t1/2 = 53.3 days), and137Cs (t1/2 = 30.2 years) by direct gamma counting.

Samples were initially dried homogenized and packed

into standardized vessels before counting for approx-

imately 24 h. Sample size ranged between approxi-

mately 5 and 40 g, depending on counting geometry

(vial or tin, respectively). Gamma counting was con-

ducted on one of two low-background, high-efficien-

cy, high-purity Germanium detectors (Coaxial- and

Well-type) coupled with a multi-channel analyzer.

Calibration of the detectors was calculated using

several natural matrix standards (IAEA-300) at each

energy of interest (except 7Be) in the standard count-

ing geometry for the associated detector. The counting

efficiency of 7Be (477 keV) was determined by linear


regression of calculated efficiencies for energies be-

yond 200 keV.

Excess 234Th was calculated by taking the differ-

ence between total 234Th activity from that supported

by 238U. Total 234Th (63 keV) was determined by

direct gamma counting, while supported 234Th was

determined by alpha spectroscopy of 238U. Reported

data for short-lived nuclides (234Th and 7Be) are

corrected for the radioactive decay that had occurred

between sample collection and analysis.

In order to assess spatial variability of the short-

lived tracers within an individual field site, several

cores were collected from one site (NR-4) during two

different months (February and April 2002). Cores

were collected within approximately 1 m of each

other. During the first month, the upper 2 cm of each

core was removed and processed as described above.

Fig. 3. Schematic representation of core inventories between sampling ev

subsequent sampling event, typically 6 weeks later for this study. During t

removal (C), or deposition (E).

The 2-cm interval was chosen since a large fraction

(typically >50%) of the activity was found in this

interval. In the following variability study, two equal

intervals (0–2 and 2–4 cm) were collected and

analyzed.

Sediment deposition rates were quantified by mea-

suring the naturally occurring radioisotope 137Cs

(Jeter, 2000; Orson et al., 1992; Robbins and Edg-

ington, 1975). Sediment 7Be and 234Th inventories

were calculated according to the following equation

(after Canuel et al., 1990):

I ¼X

Xið1� /iÞqðxsAiÞ ð1Þ

where I is the total inventory of the sediment core

(dpm cm� 2); Xi is the subsection thickness (cm); /i is

the porosity of the subsection (unitless); qi is the

ents. Month A refers to the first sampling event and Month B is a

he subsequent sampling event, cores may show no change (C or D),

Table 1

Variability studies conducted in the Neuse River estuary

Core 7Be (dpm cm� 2) 234Th (dpm cm� 2)

February 2002

1 0.20 4.46

2 0.30 3.88

3 0.23 3.25

4 0.47 5.01

5 0.41 3.00

Mean 0.32 3.92

S.D. 0.11 0.83

C.V. (%) 34 21

May 2002

1 0.57 2.84

2 0.58 2.80

3 0.30 2.62

Mean 0.48 2.75

S.D. 0.16 0.12

C.V. (%) 33 4.2


sediment density (g cm� 3); and xsAi is the activity

above the level supported by the radioactive parent

(dpm g� 1). Average sediment density was taken to be

2.4 g cm� 3 (Benninger and Wells, 1993). The indi-

vidual subsections are then summed to obtain the total

inventory of the core. Total inventories of the tracers

are then separated into two components: residual and

new inventory (Fig. 3). Simply, cores are initially

collected at each site (Month A, Fig. 3A or B) to

evaluate the sediment inventory of both 7Be and234Th. The two cores show two scenarios: (1) no

new deposition, thus no short-lived tracer present

(Fig. 3A); or (2) new deposition within the mean life

of the tracer (7Be—77 days and 234Th—35 days; Fig.

3B). Subsequent cores collected approximately every

6 weeks (Month B, Fig. 3C–E) are then compared to

the original cores (Month A). Variations in short-lived

nuclides from Month B cores provide evidence of no

deposition (Fig. 3C or D), removal (Fig. 3C), or new

deposition (Fig. 3E). The total inventory (Month B)

can then be separated into two components: the

Residual Inventory, the inventory of the previous

sampling period decay-corrected to the date of present

sampling; and the New Inventory, or the total inven-

tory (present sampling) less the residual inventory.

Therefore, if the total inventory is equal to the residual

inventory, then this would indicate no net sediment

delivery or loss during the sampling period. However,

if the residual inventory is greater than the total

inventory, this is an indication of net sediment remov-

al during the sampling period. Finally, if the residual

inventory were less than the total inventory, this

would indicate net sediment deposition during the

sampling period.

Grain size analysis was performed on all the sites

from one sample period to a depth of 10 cm to provide

the relative variation in surface sediments. Grain size

was determined using a Coulter LS 230 Analyzer.

Previous studies (Baskaran and Santchi, 1993; Feng et

al., 1998) have indicated that variability of these

radionuclides in fine-grained sediments results pri-

marily from resuspension and mixing processes and

not from grain size. Grain size is important in deter-

mining the ‘‘critical erosion velocity’’, that is, the

minimum current velocity at which sediment of a

certain size will be transported. This is also an

important parameter when calculating sediment re-

moval episodes.

4. Results and discussion

4.1. Short-term sediment dynamics

The spatial variability of the short-lived tracers

(234Th and 7Be) is summarized in Table 1. The

coefficient of variation (C.V.) for 7Be was significant-

ly higher than that of 234Th during both experiments.

Most notable was the substantial decrease in variabil-

ity of 234Th during the May experiment when samples

were collected down to 4 cm depth, an indication that

the surface activity is the most variable. A similar 7Be

C.V. and downcore pattern was observed in cores

collected from Cape Lookout Bight, NC (Canuel et

al., 1990). All inventories in this paper are reported as

F 15% and F 33% for 234Th and 7Be, respectively.

For most instances, this is an overestimate of the

propagated counting error.

Total 7Be inventories at most locations (Table 2) in

both estuaries show large variations between sampling

events. The largest variations in the Neuse River are

found in the lower estuary between NR-4 and NR-6.

This area of the lower estuary is most susceptible to

wind-driven sediment disturbances because of the

large surface area and shallow uniform depth that

allow for winds to develop large wind tides (Pilkey et

al., 1998). Interestingly, the Pamlico did not show as

large a variation in total inventory throughout the

estuary as the Neuse, probably associated with the

Table 2

Inventories in dpm cm� 2 for 7Be and 234Th in the Neuse and Pamlico Estuaries

Station May 2001 July 2001 August 2001 October 2001 December 2001 February 2002

7Be 234Th 7Be 234Th 7Be 234Th 7Be 234Th 7Be 234Th 7Be 234Th

NR-1 1.3 45.1 0.9 36.3 1.2 27.3 1.3 45.9 0.7 31.2 1.3 20.4

NR-2 1.8 36.2 1.0 42.9 1.3 36.6 1.8 34.3 0.7 25.1 0.3 24.1

NR-3 1.8 � 50.3 1.6 84.3 1.6 29.3 0.3 52.6 1.8 42.8 1.2 40.8

NR-4 0.9 22.2 3.5 50.8 1.4 54.0 1.5 77.7 1.2 25.5 0.7 43.2

NR-5 1.0 33.0 2.1 69.0 1.1 41.7 0.9 18.7 1.2 34.9 0.5 23.3

NR-6 2.1 17.6 4.6 26.0 1.9 18.1 2.1 13.7 1.4 4.1 0.3 12.0

PR-1 0.9 51.6 3.3 53.1 1.5 47.2 1.2 108 4.2 62.8 0.9 39.8

PR-2 1.6 24.4 1.8 47.6 2.3 27.9 1.2 44.4 2.1 24.9 0.9 23.5

PR-3 1.6 30.1 2.3 44.0 1.9 32.7 0.9 48.1 1.5 11.7 1.9 24.7

Note that 7Be shows large variations in the lower Neuse (NR-4 to NR-6); excess 234Th shows largest variation in the middle reaches of the

Neuse Estuary.


size and dominant direction of wind events (north-

west/northeast).

It has been noted that seasonal variation in 7Be in

the sediment inventories can result from differences in

precipitation throughout the year (Canuel et al., 1990).

Precipitation (Fig. 4) was collected at New Bern, NC,

adjacent to the Neuse River Estuary. River discharge

for the Neuse is also presented to show the relationship

with precipitation. The daily precipitation data show a

fairly uniform distribution throughout the year of the

study with the notable exception of the low precipita-

tion of the October to November timeframe. Compar-

ison of this data to the 7Be total inventory (Table 2)

shows no observable decline in 7Be during the late fall

when precipitation was at its lowest. The reasons that

Fig. 4. Discharge and precipitation for the study year. Precipitation data is

Fort Barnwell on the Neuse River. Note that sediment removals occurred

no decline in 7Be was observed were most probably

due to the half-life of 53.3 days being longer than this

short dry spell and that dry deposition, as small as it

may be, continued through this period. Further, a

major source of 7Be to the estuary are sediments

delivered through basin-wide runoff. The area of the

basin is relatively large compared to that of the estuary,

reducing the effects of short-term rainfall variations.

As with the 7Be inventories, 234Th excess inven-

tories showed the most variability and the greatest

magnitudes in the middle reaches of the Neuse River

(Table 2). These increased excess inventories may be

attributed to increased resuspension and deposition

between the sampling intervals or associated with the

uranium geochemistry within the estuary. Based on

from New Bern, NC and discharge data is from the gaging station at

during periods of increased precipitation and river discharge.


observed linear relationships between dissolved ura-

nium concentration and salinity, uranium is consid-

ered to behave conservatively in many estuarine and

coastal environments (Borole et al., 1977; Martin and

Meybeck, 1979; Toole et al., 1987). In this scenario,

the remaining 234Th activity would appear to have

increased due to the desorption of 238U, rather than

increased adsorption of 234Th. However, it should be

noted that evidence for the removal of dissolved

uranium at low salinities (nonconservative behavior)

has also been observed (Carroll and Moore, 1993).

Comparisons of total 234Th inventory to bottom water

salinity in the estuary, especially the upper portion

where seasonal variation is greatest, did not demon-

strate large variations associated with salinity changes.

However, deciphering the uranium geochemistry of

this estuary was beyond the scope of this project.

Therefore, we believe the 234Th inventory fluctuations

are associated with variations in sediment delivery

and transport. The 234Th results agree with those of7Be and demonstrate the active sediment remobiliza-

tion that occurs in the middle reaches. Further, this

region between NR-3 and NR-5 also appears to be an

area of lower relative sediment accumulation based on137Cs dating.

Other factors that could possibly affect the tempo-

ral distribution of these radionuclides can be related to

the possible differences in their residence times in the

water column and seasonal variability in the distribu-

tion coefficients (Kd), ratio of activity in suspended

sediments to that in solution. Many researchers have

demonstrated the relatively high Kds (f 105) of these

elements, demonstrating their high particle reactivity

(Baskaran and Santchi, 1993; Fisher and Teyssie,

1987; Olsen et al., 1986). Residence times for these

nuclides have been shown to be less than 5 days in

estuarine and coastal waters. In fact, Olsen et al.

(1986) demonstrated that >95% of the 7Be inventory

was located in the sediments. Further, Olsen et al.

(1986) showed that seasonal variations for 7Be were

only noted for differences in atmospheric supply and

not from changes in the water column. For our study,

residence times of these radionuclides in the water

column can be considered irrelevant since they are

much shorter than the radionuclides’ half-lives and

certainly the sampling interval (f 6 weeks). Fluctua-

tions in Kd values are most commonly due to the

amount of total suspended solids (TSS; e.g. Baskaran

and Santchi, 1993). TSS in this study had little

variability and predominantly low values with average

means ranging from 5.5 to 8.6 mg l� 1 with the

exception of one sample period during February

2002. Therefore, it would be expected that Kd values

did not vary significantly during the course of this

study and that variations in the radiotracers can be

attributed to sediment deposition and remobilization.

A comparison of the 7Be/234Th ratio over distance

was also performed to see if there was any correlation.

The only changes observed were coincident with

removal episodes and therefore indicated 7Be as a

better tracer of resuspension events in this system.

This can be attributed to the time scale of sampling

relative to the men life of the nuclides and a greater

source of 7Be associated with the eroded sediments

from the watershed.

4.2. Sediment delivery and remobilization

When the residual activity is calculated and pro-

viding for the new inventory (Fig. 5), sediment

deposition versus removal can be evaluated. It should

be noted that this provides the cumulative (net) result

between the sampling events. There may have been

removal and/or deposition occurring at some point

between the sampling events, but only the overall

effect is obtained. In addition, the differences in the

mean life (35 vs. 77 days for 234Th and 7Be, respec-

tively) of the two tracers may provide additional

insight into the sediment dynamics. In most instances,

both tracers indicate a net deposition between sam-

pling periods. It should be noted that the year of

sampling did not include any major weather events,

i.e. Nor’easters or hurricanes. However, there are

subtle deviations from a depositional system at sev-

eral sites, indicating net removal, particularly with the7Be data.

The largest sediment removal in the Neuse was

observed during August 2001 at the downriver

stations NR-4 through 6 while the second greatest

episode was observed in February 2002 at Stations

NR-2 and once again at Stations NR-5 and NR-6. A

small removal was also observed at NR-2 in Decem-

ber 2001. The largest depositional episode was

observed in July 2001. The only observed removal

in the Pamlico occurred in August at PR-1; however,

this location was primarily depositional throughout

Fig. 5. Calculated new inventories for the Neuse and Pamlico River Estuaries for 7Be (A) and 234Th (B). Removals are indicated by negative

values.


the study. The high amount of sediment deposition at

PR-1 relative to downstream locations is possibly

due to effects from the turbidity maximum being

predominantly located in this area (Postma, 1967).

This is also an area where the river widens thus

current velocities decrease and possibly enhances

settling of the suspended sediment load. This effect

is not seen in the same area of the Neuse where the

turbidity maximum is predominantly located. This

could be due to the confluence of the Trent tributary

and the Neuse’s orientation to dominant wind direc-

tions. Both these forces can enhance current veloc-

ities and produce more mixing and, consequently,

increase suspended load capacity thus precluding the

turbidity maximum zone in the Neuse from being an

area of deposition.

The amount of sediment resuspended in a removal

episode can be calculated using the 7Be inventory lost

in dpm cm� 2 and assuming only the top 5 cm of the

sediments are involved in the disturbance, allowing

for an estimate of the surface activity (dpm g� 1). This

calculation gives the amount of sediment resuspended

assuming the sediment can be transported. Using a

simple relationship developed by Hjulstrom (Pipkin,

1994), where the grain size and current velocity are

known, the critical erosion velocity (CEV) can be

evaluated. Grain size analysis showed primarily silts

and fine sands at surface with downcore median

diameters of 23–155 Am in the Neuse and 19–74

Am in the Pamlico. Other studies have found similar

results for grain size (Wells and Kim, 1989). The

results of the grain size analysis for the top 10 cm did

Table 3

Average sediment deposition and peak Cs-137 activity

Sample

area

Average

depth (cm)

Average sediment

deposition rate

(mm year� 1)

Average activity

(dpm g� 1)

NR-1 >19 >5 –

NR-2 >19 >5 –

NR-3 5.5 1.4 0.95

NR-4 7.5 2 1.90

NR-5 7 2 1.62

NR-6 >19 >5 –

PR-1 11 3 2.59

PR-2 >19 >5 –

PR-3 10.7 3 1.43


not reveal any fining sequences either vertically or

laterally throughout the estuaries. The largest removal

in the Neuse in August 2001 at NR-6 involved a 7Be

inventory loss of 0.8 dpm cm� 2 in the top several

centimeters where the 7Be activity measured was 1.8

dpm g� 1 or approximately 0.4 g cm� 2. Grain size

analysis at this site (NR-6) determined that sediments

primarily consist of unconsolidated silt and very fine

sand. Observed current calculations have shown that

velocities of 20 cm s� 1 are common, which on a

Hjulstrom diagram are sufficient to initiate transport

of these sediment sizes. Accounting for the bulk

density of the sediment removed indicates that the

removal involved the top 2.2 cm of sediment.

As described earlier and can be seen in Fig. 1,

porewaters elevated in nutrients are delivered to the

water column during such a resuspension event. In

order to put this removal into perspective, advective

flux calculations for ammonium (NH4) and phosphate

(PO4) associated with the sediment removal were

made using the porewater concentration of previous

studies (Luettich et al., 2000; Fisher et al., 1982).

Calculated advective fluxes of 17.8 mmol NH4 m� 2

day� 1 and 1.8 mmol PO4 m� 2 day� 1 were deter-

mined for the August sediment removal in the vicinity

of NR-6. The ammonium flux is at least six times the

reported average value of 3.2 mmol NH4 m� 2 day� 1

for a benthic flux from prior studies (Luettich et al.,

2000). The phosphate flux is also about 6 times the

reported value of approximately 0.3 mmol PO4 m� 2

day� 1. Thus it can be seen that the advective impacts

on water quality during resuspension events can be

substantially greater than the effects from passive

diffusion. Further, it should be noted that this maxi-

mum removal occurred in a year of monitoring in

which there was very little storm activity and river

discharges were at a minimum.

4.3. Sediment deposition

Estimates of sediment deposition rates (Table 3)

were measured via 137Cs where the maximum peak

downcore depth was distinguishable, indicating the

1963 fallout signature. In some areas coring was not

performed deep enough (>19 cm) to determine this

peak since activities were still increasing in the

bottom of the sample core. It should be noted that

no evidence of significant macrobenthic bioturbation

was observed. Similar results in this system have been

obtained in other studies (Matson et al., 1983; Cooper,

1998). It is believed that intermittent bottom water

anoxia interferes with biotic life cycles in the finer-

grained sediments of both estuarine systems (Tenore,

1972). Profiles of 137Cs (Fig. 6) can be sharp and

distinct or broad and blunt dependent on the mixing

depth of the sediments (Jeter, 2000). Deposition rates

were determined to range from a low of 1.4 to greater

than 5 mm year� 1 by averaging the depth of the

signature by the interval of time from 1963 to 2001.

These are very similar to rates previously cited of 1–6

mm year� 1 for sediment accumulation in the Neuse

(Benninger and Wells, 1993). Variations in 137Cs

sediment accumulation rates correspond with spatial

variation in sediment remobilization. That is, the

majority of sediment removal/remobilization occurs

in the lower estuary (NR-3 to NR-5 region) and a

distinct maximum can be observed. The upper estuary

(NR-1 to NR-2) is primarily a depositional area and

no 137Cs horizon was noted. The mouth of the Neuse

(NR-6) is an exception and also acts as a depositional

area. This is probably due to a combination of factors

such as a decrease in the influence of river currents

and proximity to greater tidal influence from Pamlico

Sound. Rates in the Pamlico River Estuary ranged

from 0.3 to greater than 5 mm year� 1.

4.4. In situ monitoring of the Neuse River

Meteorological data were collected for the moni-

toring period May 2001 to May 2002 for wind speed

and wind direction from a monitoring platform in the

Neuse River in the area of Cherry Point, NC. Wind

Fig. 7. Typical in situ monitoring period for meteorological events

(September through October 2001). Three frontal systems (A, B,

and C) are noted.

Fig. 6. Cs-137 activity for August 2001 on the Neuse River. Stations

NR-1 and NR-6 show increasing levels at depth; while NR-4 shows

a distinct maximum.


data was obtained through the Neuse River Monitor-

ing Program, Center for Applied Aquatic Ecology,

North Carolina State University. The data indicated

the passage of several cold fronts followed by high

pressure systems that generated winds of 5–17 m s� 1.

Peak events occurred during the winter months.

Depending on the wind direction and its available

fetch, resuspension of sediments due to wave action

occurred which elevated turbidity to as much as

several hundred nephelometric turbidity units (NTUs).

These turbidity events primarily occurred during

fronts with wind speeds z 8 m s� 1. Northeasterly

and northwesterly wind directions have the greatest

fetch and seemed to have the greatest influence. A

typical monitoring period from September through

October 2001 is shown (Fig. 7). The most noteworthy

frontal systems causing increased turbidity during this

period occurred on October 9 (A), October 14 and 15

(B), and October 17 (C). It is evident that even these

frontal systems are by no means large weather events.

However, it appears during each event that a combi-

nation of wind speed approaching 4 m s� 1 and rapid

shifts in wind direction produce rapid increases in

turbidity. This is due to developing sufficient wave

energy to generate orbital velocities capable of resus-

pending bottom sediments. A lag is noticed between

the termination of a front and the decrease in turbidity

accounting for slow settling velocities of these fine-

grained sediments.

Current data was obtained from February through

May 2002 using a Falmouth Scientific, 2D-ACM

current meter. Unfortunately, data from our in situ

turbidity meter was lost during this period due to a

faulty internal battery. However, we were able to

obtain information from an in situ meter monitored

by the Center for Applied Aquatic Ecology, North

Carolina State University. The data show that current

velocity is predominantly 10–20 cm/s and rarely

obtains values of 30–40 cm/s with only slight obser-

vations in the 50–60 cm/s range. Reversals of current

from downriver, easterly flow to upriver, northwest-

erly flow were periodically observed. These reversals

appear to be produced by prolonged northeast winds.

The mechanism for these reversals was previously

observed as being the result of seaward surface water

entrainment which produces upriver estuarine flow of

the deeper, denser, more saline water (Luettich et al.,

2000). Reversals with higher velocities (z 30 cm/s)

appear to be correlated with the larger turbidity

events, which indicates that these frontal passages


are likely responsible for the majority of the resus-

pension events.

5. Conclusions

The short-term sediment dynamics were deter-

mined at select locations in the Neuse and Pamlico

River Estuaries over a 12-month period. Variations in7Be and 234Th inventories are good indicators of

deposition and removal processes in estuarine envi-

ronments. As previous studies have indicated, the

main deposition area for the Neuse River, as indicated

by the short-lived tracers, occurs primarily around

NR-3 and NR-4. Tracer inventories at most locations

in both estuaries showed large variations between

sampling events. The largest variations in the Neuse

River were found in the lower estuary between NR-4

and NR-6 associated with significant sediment

reworking. The largest sediment removal in the

Neuse, calculated as the top 2.2 cm of sediment,

was observed during August 2001 at the downriver

stations, NR-4 through 6. A second significant re-

moval episode was observed in February 2002 at

Stations NR-2, NR-5 and NR-6. These are the areas

most susceptible to wind-driven sediment disturban-

ces. The largest depositional episode was observed in

July 2001 during a low flow period. Interestingly, the

Pamlico did not show as large a variation in total

inventory throughout the estuary as the Neuse, attrib-

uted to the size and dominant direction of wind events

(northwest/northeast). The only observed removal in

the Pamlico occurred in August at PR-1. Flux calcu-

lations for the largest removal showed the importance

for accounting for advective impacts on water quality

from sediment resuspension events.

Cs-137 activity indicated deposition rates of 1.4 to

greater than 5 mm year� 1 which are in close agree-

ment to rates determined more then 10 years ago prior

to the passage of several major hurricanes, including

Floyd. Interestingly, this points to the apparent lack of

any long-term effects on the sediment budget by these

major weather events. Areas of sediment accumula-

tion and removal that were indicated by 137Cs activity

were also in close agreement to those areas indicated

by the short-lived radioisotopes.

Frequency of sediment resuspension events was

measured utilizing meteorological data and sensors.

Turbidity measurements indicated resuspension of

sediments primarily during wind events z 4 m

s� 1. These events occurred mostly as the result of

northwesterly and northeasterly winds since these

have the greatest fetch. Peak events occurred during

the winter months when these wind directions were

prevalent. Current data indicated that reversals with

higher velocities (z 30 cm/s) appear to be correlat-

ed with the larger turbidity events which indicates

that these frontal passages are likely responsible for

the majority of the resuspension events. These data

could provide useful information for future model

development.

Acknowledgements

Financial support for this study was provided to

D.R. Corbett by NC Water Resources Research

Institute under grant No. ECU-70187, NC Sea Grant

under grant no. 1998-0617-0026 and East Carolina

University under the Research/Creative Activity

Grants program. Stan Riggs provided invaluable

information on the geological framework of the

environment, leading toward the initiation of this

study. We gratefully acknowledge Robert Reed of the

Center for Applied Aquatic Ecology, North Carolina

State University for providing meteorological and

turbidity data from several locations on the Neuse

River and Rick Luettich of the University of North

Carolina for providing the use of a current meter. We

would also like to thank Matt Allen, James Frank,

Lorin Gaines, Kevin Miller, and Chris Foldesi for

their help in sample collection and analysis.

References

Alperin, M.A., Clesceri, E.J., Wells, J.T., Albert, D.B., McNinch,

J.E., Martens, C.S., 2000. Sedimentary processes and benthic–

pelagic coupling. Neuse River Estuary MODeling and MONi-

toring Project: Final Report, Draft #4. Water Resources Re-

search Institute, Raleigh, NC.

Baskaran, M., Santchi, P.H., 1993. The role of particles and colloids

in the transport of radionuclides in the coastal environments of

Texas. Mar. Chem. 43, 95–114.

Benninger, L.K., Wells, J.T., 1993. Sources of sediment to the

Neuse River Estuary, North Carolina. Mar. Chem. 43, 137–156.

Borole, D.V., Krishnaswami, S., Somayajulu, B.L.K., 1977. Inves-

tigations of dissolved uranium, silicon, and particulate trace

elements in estuaries. Estuar. Coast. Mar. Sci. 5, 743–754.


Bowen, J.D., Hieronymus, J., 2000. Neuse River Modeling and

Monitoring Project Stage 1: Predictions and Uncertainty Anal-

ysis of Response to Nutrient Loading Using a Mechanistic Eu-

trophication Model. Water Resources Research Institute, Ra-

leigh, NC.

Canuel, E., Martens, C., Benninger, L., 1990. Seasonal variations in

Be-7 activity in the sediments of Cape Lookout Bight, North

Carolina. Geochim. Cosmochim. Acta 54, 237–245.

Carroll, J., Moore, W.S., 1993. Uranium removal during low dis-

charge in the Ganges–Brahmaputra River mixing zone. Geo-

chim. Cosmochim. Acta 57, 4987–4995.

Christian, R.R., Boyer, J.N., Stanley, D.W., 1991. Multi-year dis-

tribution patterns of nutrients within the Neuse River Estuary,

North Carolina. Mar. Ecol., Prog. Ser. 71, 259–274.

Cooper, S.R., 1998. The History of Water Quality in North Carolina

Estuarine Waters as Documented in the Stratigraphic Record.

Water Resources Research Institute, Raleigh, NC.

de Jonge, V.N., van Beusekom, J.E.E., 1995. Wind- and tide-

induced resuspension of sediment and microphytobenthos

from tidal flats in the Ems estuary. Limnol. Oceanogr. 40

(4), 766–778.

Fanning, K.A., Carder, L.C., Betzer, P.R., 1982. Sediment resus-

pension by coastal waters: a potential mechanism for nutrient

re-cycling on the ocean’s margin. Deep-Sea Res. 29 (8A),

953–965.

Feng, H., Cochran, J.K., Hirschberg, D.J., Wilson, R.E., 1998.

Small-scale variations of natural radionuclide and trace metal

distributions in sediments from the Hudson River Estuary. Es-

tuaries 21 (2), 263–280.

Fisher, N.S., Teyssie, J.-L., 1987. Accumulation of Th, Pb, U, and

Ra in marine phytoplankton and it geochemical significance.

Limnol. Oceanogr. 32, 131–142.

Fisher, T.R., Carlson, P.R., Barber, R.T., 1982. Sediment nutrient

regeneration in three North Carolina Estuaries. Estuar. Coast.

Shelf Sci. 14, 101–116.

Froelich, P., Klinkhammer, G., Bender, M., Luedtke, N., Heath, G.,

Cullen, D., Dauphin, P., Hammond, D., Hartmann, B., Maynard,

V., 1979. Early oxidation of organic matter in pelagic sediments

of the eastern equatorial Atlantic: suboxic diagenesis. Geochim.

Cosmochim. Acta 43, 1075–1091.

Giese, G.L., Wilder, H.B., Parker, G.G., 1985. Hydrology of major

estuaries and sounds of North Carolina. U.S. Geol. Surv. Water-

Supply Pap. 2221. 108 pp.

Jeter, H.W., 2000. Determining the ages of recent sediments using

measurements of trace radioactivity. Terra Aqua 78, 21–28.

Kristensen, P., Sondergaard, M., Jeppesen, E., 1992. Resuspension

in a shallow eutrophic lake. Hydrobiologia 228, 101–109.

Luettich, R.A., Martens, C.S., McNinch, J.E., Pearl, H., Pinckney,

J., Peterson, C.H., Wells, J.T., Alperin, M.A., 2000. Sedimentary

processes and benthic–pelagic coupling. Neuse River Estuary

MODeling and MONitoring Project: Final Report, Draft #4.

Water Resources Research Institute, Raleigh, NC.

Martin, J.M., Meybeck, M., 1979. Elemental mass balance of ma-

terial carried by major world rivers. Mar. Chem. 7, 173–206.

Matson, E.A., Brinson, M.A., 1990. Stable carbon isotopes and the

C:N ratio in the estuaries of the Pamlico and Neuse River, North

Carolina. Limnol. Oceanogr. 35, 1290–1300.

Matson, E.A., Brinson, M.M., Cahoon, D.D., Davis, G.J., 1983.

Biogeochemistry of the Sediments of the Pamlico and Neuse

River Estuaries, North Carolina. Water Resour. Res. Inst. Rep.

191. 103 pp.

McKee, B.A., Baskaran, M., 1999. Sedimentary processes of Gulf

of Mexico estuaries. In: Bianchi, T.S., Pennock, J.R., Twilley,

R.R. (Eds.), Biogeochemistry of Gulf of Mexico Estuaries. Wi-

ley, New York, pp. 63–85.

McKee, B.A., Nittrouer, C.A., DeMaster, D.J., 1983. The concepts

of sediment deposition and accumalation applied to the conti-

nental shelf near the mouth of the Yangtze River. Geology 11,

631–633.

Olsen, C.R., Larsen, I.L., Lowry, P.D., Cutshall, N.H., Todd, J.F.,

Wong, G.T.F., Casey, W.H., 1985. Atmospheric fluxes and

marsh-soil inventories of 7Be and 210Pb. J. Geophys. Res. 90,

10487–10495.

Olsen, C.R., Larsen, I.L., Lowry, P.D., Cutshall, N.H., Nichols,

M.M., 1986. Geochemistry and deposition of Be-7 in river-es-

tuarine and coastal waters. J. Geophys. Res. 91, 896–908.

Orson, R.A., Simpson, R.L., Good, R.E., 1992. A mechanism for

the accumulation and retention of heavy metals in tidal fresh-

water marshes of the upper Delaware River estuary. Estuar.

Coast. Shelf Sci. 34, 171–186.

Paerl, H.W., Pinckney, J.L., Fear, J.F., Peierls, B.L., 1998. Eco-

system responses to internal and watershed organic matter

loading: consequences for hypoxia in the eutrophying Neuse

River Estuary, North Carolina, USA. Mar. Ecol., Prog. Ser.

166, 17–25.

Pilkey, O.H., Neal, W.J., Riggs, S.R., Webb, C.A., Bush, D.M.,

Pilkey, D.F., Bullock, J., Cowan, B.A., 1998. The North Caro-

lina Shore and Its Barrier Island. Duke Univ. Press, Durham.

Pipkin, B., 1994. Geology and the Environment. West Publishing,

Minneapolis, MN.

Postma, H., 1967. In: Lauff, G.H. (Ed.), Estuaries, vol. 83. Amer-

ican Association for the Advancement of Science, Washington,

DC, pp. 158–179.

Riggs, S.R., Bray, J.T., Powers, E.R., et al., 1991. Heavy metals in

organic rich muds of the Neuse River estuarine system: Albe-

marle–Pamlico Estuarine Study. US EPA and NC Dehnr, Rept.

No. 90-07. 169 pp.

Robbins, J.A., Edgington, D.N., 1975. Determination of recent sed-

imentation rates in Lake Michigan using Pb-210 and Cs-137.

Geochim. Cosmochim. Acta 39, 285–304.

Sondergaard, M., Kristensen, P., Jeppesen, E., 1992. Phosphorus

release from resuspended sediment in the shallow and wind-

exposed Lake Arreso, Denmark. Hydrobiologia 228, 91–99.

Tenore, K.R., 1972. Macrobenthos of the Pamlico River Estuary,

North Carolina. Ecol. Monogr. 42 (1), 51–69.

Toole, J., Baxter, M.S., Thomson, J., 1987. The behavior of ura-

nium isotopes with salinity change in three UK estuaries. Estuar.

Coast. Shelf Sci. 25, 283–297.

Wells, J.T., Kim, S.Y., 1989. Sedimentation in the Albemarle–Pam-

lico Lagoonal System: Synthesis and Hypotheses. Mar. Geol.

88, 263–284.

evaluation of sediment dynamics in coastal systems via ...core.ecu.edu › geology › corbettd ›...

Documents