United States
Environmental Protection
Agency
Environmental Research
Laboratory
Narragansett Rl 02882
Research and Development
EPA-600/S3-81-050 Mar. 1982
Project Summary
Nearshore Marine Trace
Metal Geochemistry
Michael L Bender
This publication summarizes a
number of studies aimed at under-
standing several aspects of the near-
shore geochemistry of trace metals
which give information on conse-
quences of trace metal pollution
attending nearshore waste disposal.
The studies included the distribution
of dissolved and particulate trace
metals in the Hudson River Estuary;
pore water nutrient, carbon and metal
geochemistry in Narragansett Bay;
trace metal adsorption in Narragansett
Bay sediments; and benthic fluxes of
transition metals out of Narragansett
Bay sediments.
The key findings are that dissolved
Mn, Ni, Cu, Cd and Zn tend to remain
dissolved during passage through an
estuary; that reducible metals (Mn and
Fe) are released at rapid rates from
reducing estuarine sediments; and
that other divalent cations forming
insoluble sulfides are sequestered in
reducing nearshore sediment pore
waters.
This Project Summary was devel-
oped by EPA's Environmental Research
Laboratory, Narragansett, ft/, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The disposal of wastes at a nearshore
site may affect the environment by
introducing heavy metals (among other
pollutants) at subtoxic and toxic levels.
Since certain wastes (notably sewage
plant effluent, sewage sludge, and
dredge spoils) have high metal concen-
trations, pollution of the affected envi-
ronment is likely, and it is important to
assess the impact of this pollution on
the ecosystem. There are three steps to
such an assessment: determining the
fate of a pollutant metal, determining
which metal species (i.e., free ionic,
organically bound, particulate, etc.)
have deleterious biological effects, and
determining the deleterious effects of
different metal concentrations.
This study has been concerned with
understanding the fate of manganese,
iron, nickel, copper, cadmium and zinc
in the estuarine and nearshore envi-
ronment. In particular, three central
issues have been addressed: 1) how
these metals are partitioned between
the dissolved and particulate phases, 2)
to what extent they are removed from
the water column into sediments, in the
nearshore environments, and 3) under
what conditions these metals are re-
leased from sediments.
A related problem was also addressed.
Benthic nutrient regeneration is a major
source of nutrients to nearshore waters.
The dumping of polluted solids into
these waters may, by impacting the
benthic community, decrease the benthic
fluxes of nutrients and lower the fertility
of the overlying waters. The study
results give insights into the question of
whether nearshore dumping is likely to
be a problem in this respect.
Conclusions
Estuarine conditions vary greatly as a
function of season, fertility (i.e., bloom
vs. non-bloom conditions), and river
runoff. There are also major variations
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from one estuary to another. Hence
caution must be used in extrapolating
results from this study of two estuaries,
concentrating on summer conditions, to
other seasons and systems.
The results reveal that about half of
the dissolved copper, and more than
half of the dissolved nickel, cadmium
and zinc, entering the Hudson River at
the harbor remain in the dissolved form
until removed from the estuary by
advection (water flow). Thus it is clear
that adsorption, salting out, and other
processes do not purify the water of
these elements. They remain largely
dissolved, presumably the state most
toxic to organisms. On the other hand,
the mass balance could not be done well
enough to rule out the possibility that up
to half of the dissolved nickel, cadmium,
and zinc entering the Hudson accumu-
lates in sediments. In the case of copper,
data show that about this fraction is
removed from solution into sediments.
Manganese is apparently slowly removed
from nearshore waters by oxidation
onto particulates. Where estuarine
residence times are on the order of a
month and temperatures are warm,
most of the dissolved manganese will be
removed into sediments.
The benthic flux of iron, manganese,
nickel, copper and cadmium out of
anoxic nearshore sediments is consistent
with the relative solubility of these
metals in oxic and anoxic environments.
Apparently as a result of insoluble
sulfide formation, nickel, copper and
cadmium have low pore water concen-
trations and are released at low rates to
the overlying waters. Manganese and
iron have high concentrations in anoxic
pore waters. They are rapidly released,
although iron, which is very quickly
oxidized in bottom water, shows a low
net flux. These results suggest that
release of nickel, copper, cadmium and
other metals from dumped dredge spoils
can be minimized by disposal of spoils
under low energy, high productivity
waters.
Nutrient benthic fluxes at highly
polluted and relatively clean Narra-
gansett Bay sites are similar, indicating
that pollution does not inhibit benthic
bacterial decomposition and nutrient
regeneration.
Recommendations
With regard to preventing toxic metal
release, the best type of nearshore
environment for dumping dredge spoils
and metal rich wastes are low energy
environments underlying productive
waters. The low energy will obviate
sediment dispersal, and the high
productivity will supply biogenic debris
and insure maintenance of a reducing
environment in which most heavy metals
are sequestered within sediments. Pol-
luted sediments are likely to be at least
as highly reducing as clean sediments
at a potential dump site. Thus a site
where sediments are currently anoxic
very near the interface (certainly within
the top centimeter) would appear to be
suitable for dumping.
The conclusions are based on very
limited sampling and must be tested by
more field work on pore water chemistry
and metal benthic fluxes.
The effect of pollutants on benthic
nutrient regeneration has not been
adequately considered. While these
results do not indicate that this is a
problem, the possibility should be eval-
uated by more extensive work.
Michael L Bender is with the Graduate School of Oceanography, University of
Rhode Island, Kingston, Rl 02881.
Earl Davey is the EPA Project Officer (see below).
The complete report, entitled "Nearshore Marine Trace Metal Geochemistry,"
{Order No. PB 82-109 372; Cost: $11.00, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
South Ferry Road
Narragansett, Rl 02882
-tf U S GOVERNMENT PRINTING OFFICE, 1982 - 559-017/0703
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