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More than Mud
Sediment is composed of shell, sand, silt and
clay particles in various proportions depending upon the morphology
(form and structure) of the estuary. Associated with these inorganic
materials are many geological and chemical components, such as organic
carbon, nutrient compounds, heavy metals, and hundreds of biogenic
(created by biological processes) and anthropogenic (man-made) organic
compounds. The biological components of sediments such as bacteria
and benthic (sediment-dwelling) invertebrates decompose organic
material brought to the estuary by runoff and incorporate many organic
molecules into the sediment.
Sediments in estuaries like Galveston Bay come
from three sources: inland areas, seas, and erosion of Pleistocene
deposits on the slopes of the estuarine bed. They are carried into
the Bay via tributaries, rainwater runoff and tides. All of these
sources and means of flowing into the Bay exist in different proportions
at various times in the Galveston Bay system, and have been drastically
influenced by subsidence (loss of land elevation due to groundwater
and petroleum withdrawal) in the Houston-Galveston area during the
last 30 years. Other major influences, such as construction of dams
on the San Jacinto and Trinity rivers, and land use changes in the
watershed (land area drained by a river or stream, which in this
case also includes Galveston Bay) have had a profound influence
on the rate and flow of sediments into and out of Galveston Bay.
The Role of Sediments
Bay sediments serve as a foundation upon which organic matter and benthic organisms mix to store the products of water column photosynthesis (carbon compounds created by sunlight interacting with green plants in the water) and consume portions of the nutrients that enter the estuary. The benthos serve as a food source for finfish and shellfish as well as a regulator of the estuary ecosystem by emphasizing nutrient regeneration (continuous cycle of organic and inorganic phases of the food chain). Sediments also serve as anchorage for the roots of macrophytic vegetation (large plants) as well as a nutrient source. Finally, estuarine sediment plays a major role in regulating the ability of the most productive habitats on earth, to product plant material that can be consumed by organisms higher on the food chain.
Status in Galveston Bay
Distribution of sediments in an estuary is
heavily influenced by the bathymetry (depth patterns) of the system.
The greatest depth of Galveston Bay, excluding channels, which range
from 9 to 40 feet deep, is 12 feet, Trinity Bay is approximately
8 feet deep in the center, East Bay ranges from 4 to 8 feet deep
and West Bay ranges from 4 to 6 feet deep at uniform sampling stations
throughout.
Land-surface subsidence in the area where Buffalo
Bayou and the San Jacinto River join has altered the bathymetry
of that area, making the Bay deeper. The Houston Ship Channel traverses
Galveston Bay for nearly 50 miles, and a barge canal (the Gulf Intracoastal
Waterway) runs through the system from East Bay through West Bay.
These channels need periodic maintenance to remove sediments carried
in by currents, tides and erosion. Movement of ships contributes
to the shifting of sediments in and around channels. Channel maintenance
requires dredging, during which sediment is removed and disposed
into the open Bay or confined areas. Additionally, the redistribution
of this dredged material by currents and wave action has altered
the natural sediment base of Galveston Bay.
The dominant sediment types in the Galveston
Bay complex are mud, muddy sand, and sandy mud. Mud, composed of
silt and clay, is distributed in the central area of Trinity Bay,
the northwestern part of Galveston Bay, and in the northern region
of East Bay. Most bay margins are sand that gradually becomes sandy
mud toward the deeper portion. Sand occurs in the tidally influenced
areas of Bolivar Roads (which is in the plume of the Mississippi
River, and receives sediments transported southwest by longshore
currents), San Luis Pass, and along the delta head of Trinity Bay.
Many oyster reefs are located throughout the bays and they contribute
detrital shell (shell debris) around the reefs.
Recent investigations of contaminant impacts
on sediments have documented that some maintenance dredged material
disposal and produced water (oil and gas drilling by-product) discharges
from oil and gas separators have environmentally degraded certain
areas of Galveston Bay. Sites sampled in the upper Houston Ship
Channel, Tabbs Bay, and selected dredged material disposal areas
show high levels of some heavy metals and polycyclic aromatic hydrocarbons
(toxic organic compounds derived from fossil fuels and their combustion),
reduced benthic diversity, and increased sediment toxicity (poisonous,
or toxic quality).
Other studies demonstrated that metals, polycyclic
aromatic hydrocarbons (PAHs), polychlorinated biphenyls, or PCBs
(toxic volatile organic compounds released by plastics and electrical
equipment), and dioxins (highly toxic manufacturing by-products)
have contaminated crabs, oysters, fish, and fish-eating birds from
selected sites in the Galveston Bay system.
The news is not all bad. Despite the impact
of the large amount of contaminants entering the Bay, most sediment
samples collected from sites designed to be representative of the
bay system contain contaminants at levels that are low compared
to bays near other industrialized areas in the United States.
This fact sheet is one of a Galveston
Bay fact sheet series produced through a cooperative agreement between
the Galveston Bay Foundation and the U.S. Fish and Wildlife Service.
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