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Resistivity data compiled in OFR
|
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2005-1306 and presented at GSA (abstract below).
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Radon data presented at Estuarine Research
|
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Federation meeting in Norfolk, VA, October 2005
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(abstract below) 2005 Salt Lake City Annual
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|
Meeting (October 16-19, 2005), Paper No. 211-13
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DELINEATION OF NEARSHORE FRESHWATER-SALTWATER
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RELATIONSHIPS IN SUBMARINE GROUND WATER USING
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CONTINUOUS RESISTIVITY PROFILING AND PIEZOMETER
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|
TRANSECTS IN THE NEUSE RIVER ESTUARY BRATTON,
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John F., Coastal and Marine Geology Program,
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|
U.S. Geological Survey, 384 Woods Hole Rd, Woods
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Hole, MA 02543-1598, jbratton@usgs.gov, CRUSIUS,
|
|
John, USGS, Woods Hole, MA 02543, CROSS, VeeAnn
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A., USGS, Woods Hole, MA 02543, and KOOPMANS,
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|
Dirk J., ETI Professionals, Inc, USGS, Woods
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|
Hole, MA 02543 The Neuse River Estuary (NC), a
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|
broad V-shaped water body (avg. ~70 km x 6.5 km
|
|
x 3.6 m) located on the southwestern end of
|
|
Pamlico Sound, suffers from severe
|
|
eutrophication. Several water quality models
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|
have recently been developed to aid in
|
|
management of nutrient loading to the estuary.
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|
To constrain model estimates of the fraction of
|
|
nutrients delivered by direct ground-water
|
|
discharge, field measurements were made in April
|
|
2004 and May 2005. Continuous resistivity
|
|
profiling (CRP) was used to measure electrical
|
|
resistivity of sediments, a property that is
|
|
sensitive to differences in salinity of
|
|
submarine ground water. The 2004 and 2005
|
|
surveys used floating 100-m and 50-m CRP
|
|
streamers, respectively. A total of ~200 km of
|
|
data was collected in the upstream half of the
|
|
estuary and processed using AGI EarthImager 2D
|
|
software. Penetration was ~20-27 m below the
|
|
seafloor (mbsf) for the 100-m streamer, and
|
|
~12-14 mbsf for the 50-m streamer. At four
|
|
transect sites extending up to 70 m from shore,
|
|
piezometers were hand-driven to depths of up to
|
|
4 mbsf in water depths of up to 2.5 m to collect
|
|
ground-water samples for measurement of
|
|
salinity, nutrients, and radon and radium
|
|
isotopes. Data from CRP surveys indicated that
|
|
high-resistivity (fresher) ground water is
|
|
present at depths of ~3-5 mbsf in a zone ~100 m
|
|
wide parallel to shore that becomes narrower
|
|
downstream as the estuary widens and becomes
|
|
more saline. This is consistent with piezometer
|
|
samples that yielded salinities of <1 psu 35-50
|
|
m from shore at some locations. At several
|
|
piezometer sites, ground-water samples were more
|
|
saline than overlying waters, suggesting that
|
|
shallow ground water reflects average annual
|
|
salinities while surface water salinity varies
|
|
seasonally. The depth to fresher ground water
|
|
increases offshore, gradually at first and then
|
|
sharply. In some upstream areas, fresher water
|
|
reappears extending more than 1 km offshore at a
|
|
consistent depth of 3-5 mbsf. A brackish zone
|
|
more than 10 m thick separates the nearshore and
|
|
offshore fresher zones. Changes in underlying
|
|
geology may be partially responsible for this.
|
|
These survey results will be used in combination
|
|
with measurements of radon and radium in surface
|
|
water, as well as seepage meter measurements to
|
|
calculate the quantity of ground water and the
|
|
associated nutrient load being delivered to the
|
|
estuary. ERF 2005 meeting abstract: Submarine
|
|
groundwater discharge to the Neuse River Estuary
|
|
(NC) determined from continuous radon
|
|
measurements Author(s) Crusius, J., US
|
|
Geological Survey Brattton, J. F., US Geological
|
|
Survey Koopmans, D., US Geological Survey
|
|
Spruill, T., US Geological Survey Corbett, D.
|
|
R., East Carolina University Type Poster +
|
|
Summary Session SPS-04 - Identifying,
|
|
Assessing, and Managing Human and
|
|
Climatically-Induced Change of Estuarine
|
|
Ecosystems In many coastal waters submarine
|
|
groundwater discharge (SGD) is an important
|
|
vector for delivery of nutrients. However, SGD
|
|
is frequently diffuse and difficult to quantify.
|
|
In this work we sought to infer locations and
|
|
rates of groundwater discharge to the Neuse
|
|
River Estuary, North Carolina, using
|
|
measurements of radon in surface water and
|
|
groundwater. Radon is an excellent tracer of SGD
|
|
because it: 1) is strongly enriched in
|
|
groundwater relative to surface water; 2) is
|
|
non-reactive; 3) is continuously supplied by
|
|
long-lived parent isotopes; and 4) integrates
|
|
over a wide area. We carried out continuous
|
|
measurements of radon during a two-day period of
|
|
April, 2004 in the surface water offshore of
|
|
Cherry Point where seismic evidence suggested a
|
|
buried paleochannel filled with coarse sediments
|
|
might be a conduit for groundwater discharge.
|
|
Elevated radon activities observed near this
|
|
site are consistent with groundwater discharge
|
|
in this vicinity. Radon activities measured
|
|
continuously at a nearshore site were roughly
|
|
inversely proportional to tidal height.
|
|
Groundwater discharge velocities calculated from
|
|
the radon data averaged 6 cm/d during a two-day
|
|
interval of relatively low tide. These values
|
|
are comparable to estimates based on seepage
|
|
meters. Additional fieldwork is planned for the
|
|
spring of 2005. M Modified abstract
|
|
presented at meeting: Submarine groundwater
|
|
discharge to the Neuse River Estuary (NC)
|
|
determined from continuous radon measurements
|
|
John Crusius; John Bratton; Dirk Koopmans USGS;
|
|
Woods Hole Science Center; 384 Woods Hole Road;
|
|
Woods Hole, MA 02543; jcrusius@usgs.gov
|
|
Timothy Spruill; U.S. Geological Survey, 3916
|
|
Sunset Ridge Road, Raleigh, NC 27607; D. Reide
|
|
Corbett; Department of Geology; East Carolina
|
|
University; Greenville, NC 27858 In many
|
|
coastal waters submarine groundwater discharge
|
|
(SGD) is an important of nutrients. However,
|
|
SGD is frequently diffuse and difficult to
|
|
quantify. In this work we sought to infer
|
|
locations and rates of groundwater discharge to
|
|
the Neuse River Estuary, North Carolina, using
|
|
measurements of radon in surface water and
|
|
groundwater. Radon is an excellent tracer of
|
|
SGD because: 1) it is strongly enriched in
|
|
groundwater relative to surface water; 2) it
|
|
behaves conservatively over the full estuarine
|
|
salinity range; 3) it's continuously supplied by
|
|
long-lived parent isotopes; and 4)
|
|
quantification of discharge based on radon
|
|
measurements integrates over a large area. We
|
|
carried out measurements of radon spanning a 50
|
|
km stretch of the Neuse River during early May,
|
|
2005. Radon was measured continuously from a
|
|
ship using a method similar to that described by
|
|
Burnett et al. (2001). The highest radon
|
|
activities (200 Bq m-3 = 12 dpm L-1) were
|
|
observed in the northern ~20 km of the sampling
|
|
area (north of New Bern). These radon
|
|
activities are roughly 30 times higher than can
|
|
be supported by diffusive inputs from sediments,
|
|
and are therefore attributed to significant
|
|
groundwater discharge in this region of the
|
|
river. A steady-state box model suggests that
|
|
the radon inventories can be explained by
|
|
groundwater discharge that comparable to ~2% to
|
|
10% of the river flow. Reducing the uncertainty
|
|
in this estimate will require reduction in the
|
|
uncertainty in the radon content of groundwater.
|
