Part
2, Sec. 2C7
Human Activities - Sand and Gravel Mining
7.
Sand and Gravel Mining
Within
the past decade, the Boston metropolitan area has experienced
significant and rapid economic growth, which has in turn encouraged
substantial industrial, commercial, and residential development.
Pressures on both the housing industry and transportation systems
to meet the demands of this growth have resulted in increased
consumption of and demand for sand and gravel resources, for
use as aggregate in construction activities.
Recently,
three large-scale public works projects have been initiated
in this area: the construction of a new secondary wastewater
treatment facility by the Massachusetts Water Resources Authority
(MWRA); reconstruction of the Central Artery, the major highway
through Boston, by the Massachusetts Department of Public Works
(MDPW); and construction of the Third Harbor Tunnel, also being
undertaken by MDPW. These projects will create additional demand
for construction aggregate.
Stubblefield
and Duane (1988) identify two principal areas in Massachusetts
Bay and surrounding waters where sand and mixed aggregate are
known to occur in significant quantities. The first is in the
inshore waters off Boston Harbor between Hull and Plymouth.
Fitzgerald, et al. (1990) in characterizing these
deposits, provide a very speculative estimate of the total volume
of material in three potential deposits within this area as
approximately 4.8 million cubic yards (3.7 million cubic meters).
The second area is Stellwagen Bank. Setlow (1973) estimated
that the volume of material (predominantly sand) on or adjacent
to the Bank was 114.7 million cubic yards (87.7 million cubic
meters). Sands account for over 90% of the Bank feature's composition
(BOM 1987).
More
recently, Stellwagen Bank has been identified by the Minerals
Management Service (MMS) as a potentially favorable area for
possible mining activities, primarily for sand deposits (MMS,
1987). Environmental considerations were not, however, factored
into the MMS analysis of site suitability. The Bureau of Mines
made particular reference to the possibility that concerns over
environmental protection "could have significant adverse effects
on any dredging and processing operations". (BOM, 1987). Several
small deposits of gravel and coarse sand occur on top of the
Bank, which could be individually exploited.
The
distribution of gravel and sand on Stellwagen Bank is provided
in Figures 11 and 12, respectively.
Most
of the sand and gravel resources on the Bank occur in less than
130 feet, indicating the feasibility of recovery using currently
available mining technology. While a number of small gravel
deposits have been identified immediately east of the Bank,
and in waters off of Cape Ann, these areas are considered too
deep to make the deposits economically recoverable (MMS, 1987).
Other factors which make Stellwagen Bank a desirable source
for sand and gravel are its proximity to Boston (approximately
30 miles east of Boston Harbor), and its occurrence in Federal
waters, making the area potentially available for leasing (Hassol,
1987).
Sand
and gravel resources are unconsolidated deposits classified
as "industrial materials" by MMS (Cruickshank, et al.,
1987). These deposits may be collected directly either at or
under the seafloor. While numerous methods have been developed
to exploit offshore sources of mineral aggregate, current mining
technologies applicable to sand and gravel deposits on Stellwagen
Bank would likely include individual variations of two basic
methods: scraping the surface and excavation of pits and tunnels
into the surface. Variations in methodology could include both
trailing suction dredges (scraping), or anchored suction dredges
(excavation). It is likely that the latter method would be used
at Stellwagen Bank, depending on water depths at operating locations.
Similar methods are routinely used for mining of sand and gravel
at depths of less than 100 feet (30.48 meters).
In
general, the environmental effects of offshore sand and gravel
mining include: destruction of the existing benthic biota; resuspension
of fine sediments; and alteration of the surface profile (Hurme
and Pullen, 1988). To date, however, there have been few studies
thoroughly assessing the effects of offshore mining activities.
During the late 1970's, the New England Offshore Mining Environmental
Study (NOMES) addressed the impacts of commercial-scale mining,
although the study was terminated prior to actual test mining.
The NOMES project identified several possible results of offshore
hydraulic mining for sand and gravel, including: formation of
stagnant water-filled excavation pits, causing in turn coastal
erosion or penetration of freshwater aquifers; harm (or benefit)
to fisheries, depending on the physical nature of the bottom
surface following excavation; introduction, via discharge plumes,
of pollutants and undesirable nutrients, causing interference
with filtering, feeding, and respiratory functions of marine
organisms; direct smothering of benthic species; loss of food
sources and habitat; lowered photosynthesis and oxygen levels;
and degraded appearance of the water itself. Unavoidable changes
in bathymetry and bottom type may also cause alterations in
population and migration patterns (Hurme and Pullen, 1988).
Of
particular concern at Stellwagen Bank are impacts to fish, invertebrates,
and marine mammals resulting from mining operations. The sandy
substratum is especially important to sand lance, the primary
forage fish for cetaceans. Sand lance burrow into the Bank's
sandy substratum during the day, and may also burrow for longer
periods of inactivity during the late summer. (S. Katona, 1991).
In
terms of finfish, there is general agreement among the sources
consulted (DeGroot, 1979; ICES, 1981; MMS, 1987; Hurme and Pullen,
1988; Oulasvirta and Lehtonen, 1988) that individual adult fish
are unlikely to be affected by mining operations, as they are
likely to avoid the disturbed site. Early fish life stages are
less mobile, however, than adults and more sensitive to elevated
suspended sediment concentrations. DeGroot (1979) has determined
that dredging and construction of a sand island would cause
damage to fisheries of the area, as well as a sizeable permanent
economic loss to commercial fisheries (Dfl. 10,000,000, or approximately
$ 5.3 million in 1990 dollars). In a study of the effects of
sand extraction on herring in the Gulf of Finland (Oulasvirta
and Lehtonen, 1988), while results did not indicate that Baltic
herring were affected by the mining operation, the catch in
a trapnet set nearby was significantly reduced over that of
years previous to the operation.
Some
of the direct impacts to organisms that live on or in the sediment
can be mitigated through various operational practices, such
as avoiding overlap of dredging paths. However, as there are
increased costs generally associated with such practices, and
compliance can be difficult to ensure, reliance upon operational
practices as a mitigating factor can be of limited value.
Data
on potential effects of offshore mining on marine mammals are
almost nonexistent. A recent study associated with locating
a dredged material disposal site in Cape Cod Bay (Battelle,
1987) stated that evidence available on suspended sediments
indicated that elevated levels would have no effect on whales.
This conclusion was based on the speculation that whales often
live in turbid environments (inshore waters during winter months,
or offshore of glaciers); and certain species are known to feed
on organisms in or on the surface of the sediment. However,
secondary effects may be significantly more important than direct
impacts. Bowhead and beluga whales have been observed altering
their swimming patterns within 2.4 miles of a dredging operation,
a change in behavior thought to be associated with the noise
generated by the dredging operation (DOI, 1983b, reported in
MMS, 1987). Similar effects also have been observed in grey
whales off the California coast (MMS, 1987).
Impacts
to principal prey species of marine mammals also may be important.
Both zooplankton and phytoplankton can be affected by exposure
to elevated suspended sediment (MMS, 1987), possibly causing
some secondary impacts to marine mammal predators. If fish actively
avoid dredging plumes, whales in the area may have to exert
more effort in feeding or other behavioral changes. The available
information presently is inadequate to allow any conclusions
to be drawn about this issue, beyond suggesting that a potential
for adverse impacts exists.
There
is additionally some concern about physical effects on the Bank
feature resulting from a substantial mining operation. Stellwagen
Bank is biologically productive because the Bank feature causes
upwelling to occur, bringing nutrient-rich waters to the surface.
Any change in the physical characteristics of the Bank could
alter the pattern of upwelling. Even small alterations in the
characteristics of the circulation and upwelling could have
profound effects on the biology of the Bank. In addition, there
is some indication that the Bank feature is very important in
propagating internal waves in Massachusetts Bay (Gardner 1990).
Internal waves seem to be important in affecting both the Bay's
general circulation, and its primary and secondary production
in surface waters. Any change in the Bank feature caused by
a mining operation could affect the wave propagation properties
of the Bank. However, further investigation and analysis are
necessary to develop a better understanding of the relationship
between the Bank and Massachusetts Bay.
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