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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.
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–9686
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.
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–96 87
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
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–9688
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
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–96 89
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.
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–9690
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.
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–96 91
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.
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–9692
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
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–96 93
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.
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–9694
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
D. Giffin, D.R. Corbett / Journal of Marine Systems 42 (2003) 83–96 95
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.
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