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DAMOS Contribution #123 Executive Summary |
The Portland Disposal Site Capping Demonstration Project 1995-1997 Sediments
from the Royal River in Maine, considered suitable for open-ocean disposal,
were sequentially dredged and disposed at the Portland Disposal Site (PDS) as
a proof-of-concept that dredged material could be placed, and capped, in a
deep water open-ocean disposal site. Monitoring protocols developed through
the Disposal Area Monitoring (DAMOS) Program were utilized, as well as a
newly developed tracer technique to track different lithologies of dredged
material on the seafloor. Overall, the Portland Disposal Site Capping
Demonstration Project showed that dredged material can be effectively placed,
capped, and monitored at a deep water disposal site. Recommendations for
improvements to the dredging and disposal operations, as well as to the
monitoring methods, are provided for future project considerations. Disposal
and capping of dredged material is a management technique for the containment
of sediments considered unsuitable for open-ocean disposal (unacceptably
dredged material, or UDM) that has proven successful in Long Island Sound, in
relatively shallow water (approximately 20 m) and over a flat seafloor.
Capping at deep water disposal sites (>40 m) was an unproven management
method due to a variety of factors, including historical difficulties in disposal
barge positioning, and shortage of evidence confirming the formation of
distinct UDM and capping layers. Refinement of dredged material management
techniques and the implementation of the differential Global Positioning
System (DGPS) during disposal and capping operations contributed to our
ability to form, and monitor, discrete mounds in deeper water. This tightly
controlled, closely monitored deep-water capping project has provided
evidence that the technique can be successful in deeper waters. In
order to avoid any potential adverse environmental impact from such a
demonstration, material dredged from the Royal River, Yarmouth, ME, deemed
suitable for unconfined open-water disposal, was used as both
"pseudo-UDM" as well as capping dredged material (CDM). The capping
demonstration was designed to identify reaches (sections) of the Royal River
project that were sufficiently distinct to permit identification of source
materials after disposal. Finer grained sediment removed from the upper
reaches of the river were designated as pseudo-UDM and placed as a discrete
mound at PDS. Material from the lower reaches of the river, characterized by
coarser grained material, was designated as the project CDM and was placed
over the initial pseudo-UDM deposit as a cap. The capped disposal mound was
formed within a basin feature on the PDS seafloor at a depth of 64 m. After
the completion of disposal and capping operations, the newly formed mound was
surveyed and cored to confirm the existence of two distinct layers. This
project design depended upon identifying characteristics of the reaches of
the Royal River that could be analyzed in samples collected after disposal. Based
on the amount of dredged material disposed at the Royal River Project Area
(39,500 m³ pseudo-UDM and 22,200 m³ CDM), the DAMOS Capping Model predicted
the formation of a conical pseudo-UDM deposit approximately 1.2 m high with
flanks extending up to 250 m from the central point of disposal, and a 20 cm
cap. Although the volume of cap material was smaller than for normal projects
(generally a minimum thickness of 50 cm), the areal distribution of both
pseudo-UDM and CDM observed in the demonstration, was relatively consistent
with the model predictions. An
important component of the Portland Disposal Site Capping Demonstration
Project was the identification of tracers within the Royal River that could
be used to identify the sediment on the seafloor at the PDS. Prior to the
excavation of sediment, 30 vibracores from three reaches (upper, middle, and
outer) of the Royal River were collected and analyzed for a variety of
potential tracers. Although no single tracer was identified that was both
unique to one reach of the river and commonly observed in all collected
samples, a statistical method of combining several biological and
mineralogical parameters was found to be suitable for classifying the
material types. The sediment fine fraction (63-500 µm) was selected as
providing the most statistically robust data. Monitoring
at the Royal River Project Area in the southeast corner of the PDS utilized
standard DAMOS techniques, including single-beam bathymetry, side-scan sonar,
REMOTS® sediment-profile images, as well as grab and core sampling. Results
of the monitoring surveys showed that a discrete dredged material mound was
detected on the seafloor within the Royal River Project Area. An accumulation
of pseudo-UDM was detected to the south and southeast of the disposal buoy
position, located in the relatively flat-bottomed basin targeted for disposal.
Accurately detecting dredged material deposition in the surrounding area of
more complex topography by single-beam bathymetry alone was complicated by
survey artifacts. In this case, sediment-profile images and core data were
key to mapping the footprint of both UDM and CDM on the seafloor. The
grab and core samples collected from the disposed dredged material were
analyzed for the environmental tracers selected after analysis of Royal River
Cores. The statistical tracer data were able to show a discernible difference
between the CDM, pseudo-UDM, and ambient material. The presence of historical
dredged material at the project area complicated the analyses, as historical
material shares biological characteristics with both native, ambient sediment
(recolonization by benthic species, settling of planktonic species), and with
recent dredged material (presence of freshwater species). Statistical
analyses showed that tracers successfully identified disposed dredged
material layers collected from different regions of the estuary, but material
from the middle reach had many overlapping characteristics that complicated
the interpretation. The biological indicators were found to be more
statistically robust than the mineralogical indicators. Differences in
species composition of the microorganism populations corresponded to the
contrasts among the brackish habitats of the three reaches of the Royal
River.The statistical overlap of the pseudo-UDM and CDM samples collected in
cores EXECUTIVE SUMMARY (continued) and grabs from the disposal mound was
consistent with the sequence of disposal operations. The
results of the demonstration project provided recommendations for future cap
monitoring projects in deep water disposal sites, including suggestions for
modifications to both the monitoring protocols and to dredging and disposal
operations. For areas of complex bottom topography, a higher resolution
single-beam bathymetric survey grid (5 to 10-m lane spacing) or multibeam
bathymetry is required to provide more precise depth information over a wider
area of seafloor. For the demonstration project, the low volumes of dredged
material, and the complications in the dredging and disposal schedule,
contributed to uncertainty in the data interpretation. Operational complications
that may occur with a larger project will have less of an impact, because
larger volumes reduce the overall monitoring error. Finally,
the tracer technique that was selected demonstrated promising results in
tracking dredged material at a subaqueous disposal site. Several
recommendations were made to improve the method, including selecting tracers
with the narrowest range in the dredging area, and sampling and analyzing the
baseline (ambient and historical dredged) material prior to disposal of
project material. |
Updated: Monday, 21 December 1998