Transcript Deliberate Tracer Experiments in the Hudson River and New

Transport & Dispersion in New York Harbor: A High-Resolution SF6 Tracer Study
http://www.columbia.edu/~tc144
Ted Caplow*1, Peter Schlosser1,2,3, David T. Ho2,3
Nicholas Santella, Columbia University
John Lipscomb, Riverkeeper
Megan Garrison, East Side Middle School
Rica Enriquez, Johns Hopkins
*corresponding author: Mudd 918, 500 W 120 St, NY, NY 10027, [email protected]
1Dept. of Earth & Environmental Engineering, Columbia University
2Lamont-Doherty Earth Observatory, Columbia University
3Dept. of Earth & Environmental Sciences, Columbia University
Method
Sulfur hexafluoride (SF6) has been used successfully as a deliberate tracer for rivers, estuaries, and coastal areas, due to its inert
nature, non-toxicity, and extremely low detection limit. An automated, high-resolution SF6 measurement system mounted on a boat was
recently developed (Ho et al 2002). The system has a sampling interval of two minutes and a detection limit of 1 x 10-14 mol L-1.
A single injection of approximately 0.9 mol of SF6 was made in the northern end of Newark Bay on July 14, 2002 to investigate the
hydrodynamics of the Bay, the Kills, and the tidal portions of the Passaic and Hackensack Rivers. The spread of the tracer plume was
tracked for 11 consecutive days following injection (time-series images below). Most measurements were taken at a depth of 1 m, with
deeper profiles at certain locations. Approximately 200 CTD casts were made to examine the halocline and thermocline at different
locations. The study period coincided with dry (~10th percentile of river flow volume over past 20 years) and relatively calm conditions.
High-Resolution, Automated, Real-Time Measurement System uses a
continuous pump, a flow-through membrane contactor, and a gas chromatograph equipped
with an electron capture detector (GC-ECD). The system is deployed on an 11 m wooden
workboat with an enclosed cabin. A laptop computer controls all functions, collects data, and
displays results in near-real time, allowing on-the-fly revisions in sampling strategy.
Results
Initially, SF6 spread quickly through Newark Bay, the Kills, and up into the New Jersey rivers, indicating high-energy mixing driven by
strong tidal currents. Bulk advection of the tracer was almost negligible, indicating very low river flows during the study period and
suggesting long residence times (several weeks) for most dissolved contaminants under these conditions. Ultimately, tidal mixing and
gas loss at the air/water interface dispersed most of the plume. The Kill van Kull (see map below) was the primary dilution pathway.
References: Ho et al 2002, Environ. Sci. Tech. 36, 3234-3241; Nightingale et al 2000, Glob. Biogeo. Cyc., 14, 1, 373-387; Wanninkhof 1992, J.G.R., 97, C5, 7373-7382.
Temperature and Salinity
New York is the third-busiest seaport in
the United States. Most of the shipping
facilities are in Newark Bay and in two
connecting channels: Kill van Kull (8
km long) and Arthur Kill (20 km long).
The boat (Riverkeeper) in Nyack,
NY. The sampling pump attached
to the bow is tilted up for storage.
Vertical Tracer Profile
Fate of Tracer and Conclusions
Gas transfer velocity (k600, cm
Vertical profiles for SF6 were taken at selected times
and locations. The section above (4 profiles), from
the day following injection, suggests that the initial
SF6 mass was concentrated in the upper part of the
water column. Subsequent mixing reduced, but did
not eliminate, this bias.
Tracers in
the Estuary:
Looking Forward
0.90
Total Mols SF6
0.80
Very slow residual advection. Non-volatiles can reside
in the Kills system for weeks, and upriver penetration
can be strong.
s-1):
0.70
-0.30x
0.60
0.50
 Develop predictive
relationships for dispersion
and gas exchange
0.40
0.30
 Tag chronic outfalls (CSO,
industry, leachate)
0.10
6.2 < k600 < 12.6
0.00
0
1
2
3
4
7
8
9
10
11
 Disaster simulation
12
100%
1.00
Total Mols SF6
6
Mass inventory of SF6 in Newark Bays and the Kills. Net loss rate of 26%
per day ( = 0.30) is a combination of gas exchange and tidal flushing.
Gas
Transfer
0.10
Tidal
Flushing
90%
80%
Outgassed
70%
60%
50%
Remaining
40%
30%
20%
Flushed
10%
0%
0.01
0
1
2
3
4
5
6
7
8
9
10
11
12
Days after injection
SF6 concentrations throughout the experiment
showed a slight inverse correlation with depth.
5
Days after injection
Contaminant
Discharge
Typical surface salinity.
 Verification tool for
numerical flow models
y = 0.87e
2
R = 0.98
0.20
Transport direction and rate is dominated by tidal
dispersion at low flow periods, and cannot be
accurately predicted from residual sub-tidal circulation.
Residual
Circulation?
Water is pumped through
the membrane contactor,
where gases are stripped
via counterflow N2.
1.00
High-energy tidal mixing, on the same order as the
Hudson River estuary.
The Arthur Kill displays typical estuarine structure, with a salt
wedge evident at the southern (left) end of the section. Dense
water near the 23 km point in the above figure arrives via the
Kill van Kull, which is considerably less stratified (see below).
SF6 injection. Gas
is bubbled into the
water column via a
perforated hose.
The study area is subject to extensive,
ongoing navigational dredging.
Fraction of initial SF6 mass
A combined SF6 mass inventory was achieved for the “inner harbor” area, including the Kills, Newark Bay, and the lower Hackensack
and Passaic rivers, using detailed bathymetry and daily composites of SF6 measurements. Total SF6 inventory within this area
exhibited exponential decay ( = 0.3 day-1; see figure at lower right). Daily wind data from Newark airport (u10, mean = 4.5 m s-1) was
used to estimate gas transfer velocity over the study period based on quadratic relationships developed by Wanninkhof (1992) and
Nightingale et al (2000) (both formulas yielded k600 = 6.2 cm s-1, normalized to a Schmidt number of 600 using ScSF6 = 819 at Tavg =
24.9 C, Salavg = 24). However, a paucity of data in the literature from this low wind regime, together with results from a recent SF6 study
of the lower Hudson (Ho et al 2002), suggest that the k600 derived from this estimate should be taken as a lower bound.
Log plot of mass inventory. Gas transfer velocity (k600 = 6.2 cm s-1 )
modeled after Nightingale et al. (2002) based on wind at Newark, NJ.
0
1
2
3
4
5
6
7
8
9
10
Days after injection
Tracer fate using modeled gas transfer from figure at left.
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