Multimedia Chemical Fate Model for Environmental Dredging
Fabian F. Sanchez
1
; Louis J. Thibodeaux
2
; Kalliat T. Valsaraj
3
; and Danny D. Reible
4
Abstract: Dredging the bed-sediment of rivers, lakes, estuaries, and harbors is a process of remediating contaminated sites. Whether
used alone or in conjunction with in situ capping or natural recovery, its application results in the loss of contaminants to the surroundings.
A steady-state multimedia box model has been developed consisting of a dredge element that initiates chemical release in a flowing stream
and an algorithm for residue transport to air, water, and sediment, both with and without silt curtain containment. The model quantifies
these losses and, commencing with the original in-place contaminant mass, determines the quantity targeted for dredging and the quantity
‘‘delivered to shore.’’The latter is a measure of the efficiency of the dredging operation. The model was developed from experience gained
studying the effectiveness of environmental dredging at three sites: Bayou Bonfouca Slidell, La., Grasse River Massena, N.Y., and
Manistique Harbor Manistique, Mich.. This paper describes the multimedia model theory and algorithm development and demonstrates
its use in the application to the Bayou Bonfouca Superfund site, which was dredged for removal of creosote waste containing 16
polyaromatic hydrocarbons.
DOI: 10.1061/ASCE1090-025X20026:2120
CE Database keywords: Multimedia; Dredging; Models; Chemicals; Sediment; Contamination; Environmental impacts.
Introduction
Toxic and hazardous materials exist in the sediment on the bottom
of rivers, lakes, and estuaries in the United States. This accumu-
lation of contaminants in surface waters represents a long-term
source of contamination to aquatic life. Direct human contact
with contaminated sediments is also a concern if they are present
in a shallow water environment used for recreational purposes.
For contaminated sediments, remediation technologies can be
grouped into four categories: interim control, in situ management,
sediment removal and transportation, and ex situ management
NRC 1998. Interim control is a means of reducing risk of
human exposure by implementing preventive measures warning
signs, fencing, swimming and fishing advisories, and so on. In
situ management deals with the contaminated sediment on-site,
where the technologies are for containment and treatment. A form
of containment is capping, which is the process of placing a layer
of clean sediment such as sand over the contaminated bed. An-
other form of in situ management is monitored natural attenua-
tion, which entails allowing natural processes to reduce the con-
taminant concentration and/or bioavailability.
The two removal technologies involve relocation and handling
of the contaminated sediment. Sediment removal is accomplished
using dredges or earth-moving equipment. Ex situ management is
accomplished by placing the dredged material in landfills, con-
fined disposal facilities, or confined aquatic disposal facilities on
or near the contaminated sites. Alternative and more aggressive
ex situ treatment technologies such as incineration are site spe-
cific, and their implementation depends on the cost and environ-
mental friendliness of the operation and other factors.
Of all the remediation technologies available, the U.S. Envi-
ronmental Protection Agency USEPA mandates that most of the
sites designated for cleanup consider dredging. In fact, ‘‘dredging
has become the default remedy for contaminated sediments’’
GEC 2000. Environmental dredging operations remove sedi-
ments with some known contamination as effectively as possible.
An effective method would include complete removal of the de-
sired sediment with as little environmental risk and consequence
as possible Hayes 1992.
Anticipating extensive use of environmental dredging, it is im-
portant to be able to determine the effectiveness of the operation.
The General Electric Company GEC recently compiled a report
on its effectiveness in controlling risks. The report reviewed
major sediment remediation projects in the United States, their
objectives, the technologies being employed, and the capabilities
and limitations of those technologies GEC 2000. The primary
conclusions drawn from the 54 sites reviewed were 1 environ-
mental dredging has not reduced surface sediment concentrations
to acceptable levels, being higher in some cases that is, Manis-
tique Harbor, Mich.; 2 dredging has not been shown to lead to
quantifiable reductions in fish contaminant levels; 3 dredging
projects are costly and take a long time to complete; 4 dredging
resuspends and releases contaminants into the water column; and
5 dredging technology and experience in large rivers are limited.
From a chemodynamic standpoint, the main disadvantage to
dredging is the resuspension of sediment in the water column.
Sediment resuspension leads to contaminant releases in the water
column and other environmental media air, soil, and sediment.
1
Gordon A. and Mary Cain Dept. of Chemical Engineering, Louisiana
State Univ., Baton Rouge, LA 70803.
2
Gordon A. and Mary Cain Dept. of Chemical Engineering and South/
Southwest Hazardous Substance Research Center, Louisiana State Univ.,
Baton Rouge, LA 70803.
3
Gordon A. and Mary Cain Dept. of Chemical Engineering, Louisiana
State Univ., Baton Rouge, LA 70803.
4
Gordon A. and Mary Cain Dept. of Chemical Engineering, Louisiana
State Univ., Baton Rouge, LA 70803.
Note. Discussion open until September 1, 2002. Separate discussions
must be submitted for individual papers. To extend the closing date by
one month, a written request must be filed with the ASCE Managing
Editor. The manuscript for this paper was submitted for review and pos-
sible publication on February 9, 2001; approved on December 26, 2001.
This paper is part of the Practice Periodical of Hazardous, Toxic, and
Radioactive Waste Management, Vol. 6, No. 2, April 1, 2002. ©ASCE,
ISSN 1090-025X/2002/2-120–128/$8.00+$.50 per page.
120 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE MANAGEMENT / APRIL 2002