ENVIRONMENTAL HYDRAULICS AND WATER QUALITY
CONTAMINATED SEDIMENTS: FATE AND TRANSPORT OF CHEMICALS DUE TO REMEDIATION
ACTIVITIES
Joseph Orlins
Assistant Professor, Department of Civil Engineering, Rowan University
Formerly: Ph.D. Candidate and Doctoral Dissertation Fellow, St.
Anthony Falls Laboratory, University of Minnesota
Recent
research at St. Anthony Falls has focused on chemical fate and transport
from contaminated sediments. For many sites with contaminated marine sediments,
bioremediation or capping of the sediments is appropriate. However, for
many other sites, dredging and disposing of the contaminated material is
the most viable remediation option. When these sediments are dredged and
placed in a confined disposal facility (CDF), some of the contaminated
material is entrained into the water column. Once the contaminated sediments
are suspended in the clean overlying water, the chemicals tend to desorb
from the suspended solids due to large concentration gradients between
the sediment particles and the clean water. After the chemicals are in
a “free aqueous” phase, they can volatilize (or ‘evaporate’) to the atmosphere.
Thus, an air pollution problem may arise when engineers try to clean up
a sediment pollution problem.
At present, there are few ways to accurately predict the air quality
impacts associated with cleaning up contaminated sediments. Researchers
at St. Anthony Falls have worked in collaboration with others to develop
ways to quantify these impacts.
RESEARCH OBJECTIVES
Researchers at St. Anthony Falls Laboratory
has worked with collaborators from the Chemical Engineering Department
at Louisiana State University (LSU) to
develop and calibrate laboratory equipment which will resuspend contaminated
sediments under controlled conditions and measure the flux of contaminants
to the water and vapor phases.
The
equipment developed includes a number of square sediment resuspension chambers.
Contaminated sediment is placed in the bottom of each chamber, and then
entrained into the water column by turbulence generated by an oscillating
grid. The flux of chemicals from the sediments to the water and air phases
is then measured. The calibration of the sediment resuspension chambers
included:
-
Quantifying the sediment resuspension caused by different energy inputs
to the system;
-
Measuring the turbulence levels inside the chamber using a 2-component
Laser Doppler Velocimeter (LDV); and
-
Measuring the air-water mass transfer coefficients for the system under
different turbulence levels.
This calibration information will allow researchers at LSU to operate the
resuspension chambers to simulate a variety of natural and man-made scenarios.
EXPERIMENTAL METHODS
Sediment entrainment studies were conducted using cohesive sediments (mud)
from a freshwater lake in Baton Rouge, Louisiana. The sediments were placed
in the bottom of the chamber and allowed to consolidate for a standard
period of time, and then the oscillating grid was started. Water samples
were withdrawn at regular times, and the concentration of total suspended
solids (TSS) was measured for each of the samples. After a steady-state
suspended sediment concentration had been reached, the oscillation frequency
of the grid (and thus the energy input to the system) was increased, and
additional water samples were collected for analysis. This procedure was
repeated for a number of different operating conditions, with the energy
input to the system and the sediment consolidation time varied.
The
mean and turbulent velocity fluctuations inside the tank were measured
using a 2-component LDV system manufactured by TSI.
The system measures the vertical and one horizontal velocity component
of very small particles in the flow as they pass through the intersection
of the laser beams, as shown in the figure at the left. A computer controlled
traversing system allows the measuring volume to be positioned virtually
anywhere inside the sediment resuspension chamber. From the measurements
of the turbulent velocity fluctuations, the total kinetic energy (TKE)
of the turbulence was calculated throughout the chamber for a number of
operating conditions. Representative results of TKE mapping are shown in
the figure below.
RESULTS and CONCLUSIONS
The oscillating grid chambers are capable of suspending cohesive sediments
under a variety of conditions. Steady-state suspended sediment concentrations
are reached fairly quickly (10-30 minutes), and the maximum concentrations
achieved ranged from 0.01 to 3.0 g/l for the sediments tested. The amount
of sediment entrained is a function of the energy input to the system from
the oscillating grid, the sediment consolidation time, and other factors
such as the moisture content of the sediments during consolidation.
The turbulence measurements indicate the flow field in the chamber is
fairly uniform at a given elevation. In addition, the turbulence in the
chamber decays inversely with distance from the oscillating grid, to within
1 cm of the sediments and the free surface. This is as predicted by empirical
relationships in the literature that have been developed for an oscillating
grid without boundary effects.
Researchers at Louisiana State University are currently using these
sediment resuspension / chemical flux chambers to evaluate chemical desorption
and mass transfer rates for a number of organic chemical contaminants.
FURTHER READING
Goldstein, R.J. [1996] Fluid Mechanics Measurements, 2d ed., Taylor
& Francis, Washington DC.
Orlins, J.J. and J.S. Gulliver [1997] "Sediment Resuspension / Chemical
Flux Chamber," Proceedings, WERC/HSRC 1997 Joint Conference on the Environment,
WERC,
New Mexico State University, Las Cruces, NM. pp.51-55.
Valsaraj, K.T.; Ravikrishna, R.; Orlins, J.J.; Smith, J.S.; Gulliver,
J.S.;
Reible, D.D.;
Thibodeaux, L.J. [1997] "Sediment-to-air mass transfer
of semi-volatile contaminants due to sediment resuspension in water,"
Advances in Environmental Research. Vol. 1 No. 2, 145-156.
Valsaraj, K. T.; Thibodeaux, L. J.; Reible, D. D. [1995] "Modeling air
emissions from contaminated sediment dredged materials," Dredging, Remediation,
and Containment of Contaminated Sediments, ASTM STP 1293. (K.R. Demars,
G.N. Richardson, R.N. Yong,, eds) ASTM, Philadelphia. pp. 227-238.
ACKNOWLEDGMENTS
The research described here was funded by the EPA's Hazardous Substance
Research Center/South & Southwest through a subcontract with the Chemical
Engineering Department at Louisiana State University. The photograph of
the dredge at the top of this document was obtained from the U.S.
EPA Great Lakes National Program Office web site.
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