United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development
EPA-600/S3-81-046 Feb. 1983
Project Summary
Early Diagenesis and Chemical
Mass Transfer in Lake Erie
Sediments
Gerald Matisoff, J. Berton Fisher, and Wilbert Lick
Vertical profiles of pore water and
sediment solids chemistry were ob-
tained from two sites in Lake Erie.
Samples were collected using both
gravity coring and pore water "peeper"
techniques. In general, concentra-
tions of nutrients and toxic metals in
sediment solids decreased with in-
creasing depth. Comparisons of pore
water "peeper" data to gravity core
data showed that "peeper" data
provides higher resolution near the
sediment-water interface. Modifica-
tions of the present "peeper" are
required to adequately sample easily
oxidizable materials (e.g., ammonia,
ferrous iron).
The thermodynamic tendency of
metal phosphate and carbonate miner-
al phases to precipitate in Lake Erie
sediments has been calculated by
means of an ion-pair model of the
interstitial water chemistry. The cal-
culations suggest that detrital calcite,
aragonite, and dolomite should be
dissolving in the sediments, but that
iron and manganese carbonates should
be precipitating. Regenerated phos-
phate should be reacting with calcium,
iron, manganese, and lead to form
authigenic mineral phases. Whitloc-
kite (Ca3 (PChb) and not hydroxyla-
patite (Ca5 (PO4)3 OH) is the predicted
mineral phase controlling phosphate
solubility. Zinc and cadmium are
apparently controlled by other mech-
anisms, perhaps by sulfide phases,
mixed mineral phases, adsorption
and/or ion exchange equilibria.
Rates of anaerobic decomposition
of Lake Erie sediments from one
locality were determined for seven
depth intervals at three temperatures.
Sealed sediment sections were in-
cubated under anoxic conditions and
the interstitial waters were sampled
over a period of approximately 200
days. Concentration increases of
bicarbonate, phosphate, ammonium,
calcium, magnesium, iron, and man-
ganese in pore water within any given
depth interval followed zeroth order
kinetics and exhibited Arrhenius
temperature dependency. The rates
and energetics of these fermentation
reactions are only slightly less than
those reported from sediments under-
going sulfate reduction. The observed
release rates decrease exponentially
with depth in the sediment due to a
corresponding decrease in the amount
of oxidizable organic matter and acid
hydrolyzable mineral phases.
This Project Summary was developed
by EPA's Environmental Research
Laboratory, Duluth. MN, to announce
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
Lacustrine sediments are known to
play an active role in the biogeochemical
cycling of materials. Freshwater sedi-
ments act as both a source and a sink for
many biologically important materials,
notably nutrients, such as phosphorus,
-------
carbon, nitrogen, sulfur, and silicon.
Further, sediments are known to play an
important role in resulting cycles of
trace metals, radionuclides, and xeno-
biotics. Because of this, knowledge of
the early chemical diagenesis of sedi-
ments, that is those reactions occurring
during and after burial, is essential to an
understanding of materials cycling in
freshwater environments.
This study focused on the chemistry
of nutrient and metal release during
early diagenesis, rates of nutrient and
metal regeneration, rates of materials'
movement across the sediment-water
interface, and the effects of bioturbation
on materials' cycling.
The experimental approach was used;
the nutrient and metal regeneration
was studied in laboratory microcosms.
Field observations of pore water chem-
istry and sediment solids chemistry
were used to verify and test laboratory
results.
Modeling indicates that 60% of the
observed bicarbonate release is the
direct result of organic decomposition,
that 20% of the release is from the
dissolution of calcium carbonate mineral
phases, and that the remaining 20% is
from the dissolution of magnesium,
iron, and manganese carbonate mineral
phases.
Kinetic modeling of the observed
production rates accurately predicts the
vertical profiles of calcium, magnesium,
iron, and manganese, but cannot
quantitatively account for all the
concentration differences of the nutrient
elements carbon, nitrogen, and phos-
phorus In addition to decomposition,
increased depositional flux also appar-
ently accounts for significant changes in
concentrations of the nutrient elements
in the near surface sediments. Con-
sideration of organic decomposition in
the calculation of anthropogenic loading
of nitrogen to Lake Erie sediments
decreases the estimate of anthropogenic
loading by about a factor of two.
Estimates of anthropogenic loadings of
labile materials (carbon, phosphorus,
sulfur) to lake sediments cannot ignore
organic decomposition.
The flux of nutrients and metals from
Lake Erie sediments to anoxic overlying
water was studied in laboratory micro-
cosms. Three cases were investigated:
1) homogenized sediment without
worms, 2) homogenized sediment pre-
conditioned by tubificid worm activities,
and 3) natural lake cores. Flux estimates
were made using both direct (concentra-
tion changes in the overlying water) and
indirect (pore water concentration
gradients) techniques.
Sediments preconditioned by the
activities of tubificid oligochaetes ex-
hibited a higher flux of ammonia, but a
lower flux of iron, soluble reactive
phosphorus, and soluble reactive silica.
The presence of tubificids had no effect
on bicarbonate flux. Comparison of
direct and indirect flux estimates
showed that both types varied widely. In
general, indirect flux estimates were
higher than direct flux estimates.
Reduced fluxes of iron and phosphorus
in the presence of tubificids probably are
due to their continuous subduction of
surficial oxidized material prior to
anoxia. The reduction of silica flux in the
presence of worms cannot be presently
explained. Fluxes observed in the
natural lake core experiment were
similar to those observed in the homo-
genized sediment with tubificids. Mineral
equilibrium calculations performed for
the pore water data collected in these
experiments showed that the laboratory
microcosms provided a reasonable
representation of chemical conditions
in Lake Erie sediments.
A one-dimensional time dependent
reaction-transport model which con-
siders production, adsorption, and
diffusion was found to adequately
predict ammonia and bicarbonate
profiles in laboratory microcosms
containing homogenized Lake Erie
sediment and no tubificids.
Conclusions
Thermodynamic modeling of intersti-
tial waters is a useful technique for
suggesting possible mineralogical
controls on trace metals. Application of
such a model to Lake Erie pore waters
reveals that iron and manganese
carbonates, phosphates, and sulfides
are all forming in the sediments as well
as chlorophyromorphite, a lead phos-
phate. No mineralogical controls for zinc
and cadmium were clearly identified.
The inclusion of organic complexes or
complexes yet to be discovered in the
thermodynamic model will not signifi-
cantly improve the results. Additional
complexing only serves to lower the ion
activity products and hence the satura-
tion indices. Predicted supersaturation
would be decreased, but not by more
than about a factor of two. Undersatura-
tion of zinc and cadmium phases would
increase. Mixed and sulfide mineral
phases are the most likely mineralogical
controls on zinc and cadmium. It is also
possible that the controlling reactions
could be adsorption or ion exchange
equilibrium. Until more sophisticated
techniques for examining sediment
solids are employed and until existing
thermodynamic data is critically compiled
and adopted, no further progress can be
made on this problem.
Knowledge of the rate of anaerobic
decomposition of organic matter and
subsequent release of nutrients and
metals to pore waters is essential to an
understanding of early diagenesis and
chemical mass transfer in sediments. In
Lake Erie, anaerobic decomposition
proceeds via fermentation reactions,
primarily methane fermentation. The
rates and energetics of these fermenta-
tion reactions are only slightly less than
those reported from sediments in which
sulfate reduction is the primary di-
agenetic pathway.
Both direct and indirect estimates of
the flux of ammonia, iron, soluble
reactive phosphorus (SRP), soluble
reactive silica (SRS), and bicarbonate
from lake sediments to anoxic overlying
water exhibit a high degree of variability.
Further, indirect flux estimates for redox
sensitive materials (i.e., ferrous iron and
SRP) may grossly underestimate the
actual flux. The initial flux of iron and
phosphorus from sediments to anoxic
overlying water is strongly dependent
on conditions at the sediment-water
interface prior to anoxia in the overlying
water. Sediments preconditioned by the
activities of tubificid oligochaetes
exhibited a higher flux of ammonia but a
lower flux of iron, SRP, and SRS. The
presence of worms had no effect on
bicarbonate flux. The higher flux of
ammonia in the presence of worms
appeared to be due to a worm-caused
ammonia source in the upper zone of
sediment. Reduced fluxes of iron and
phosphorus in the presence of tubificids
is most likely due to their continuous
subduction of surficial oxidized material
prior to anoxia. The reduction of SRS
flux in the presence of worms cannot
presently be explained.
Inclusion of diffusive loss of trace
metals from sediments in mass balance
calculations shows that Cu, Pb, and Zn
are lost from sediments in roughly the
same molar ratio as they accumulate in
sediments. Even if the loading of Cu, Pb,
and Zn to Lake Erie were to increase
exponentially for the next 100 years, the
concentration of these metals in the
lake's waters would increase by only a
factor of three to five.
Consideration of organic decomposi-
tion in the calculation of anthropogenic
-------
loading of nitrogen to Lake Erie sedi-
ments decreases the estimate of anthro-
pogenic loading by about a factor of
two. Estimates of anthropogenic loadings
of labile materials (carbon, phosphorus,
sulfur) to lake sediments cannot ignore
organic decomposition.
A one-dimensional time dependent
reaction-transport model which con-
siders only production, adsorption, and
diffusion was found to adequately
predict ammonia and bicarbonate
profiles in laboratory microcosms
containing homogenized Lake Erie
sediment and no tubificids. More
complex models are required for other
parameters (iron, phosphorus, silicon)
and situations (homogenized sediment
with tubificids, real lake sediments).
Gerald Matisoff, J. Barton Fisher, and Wilbert Lick are with Case Western
Reserve University, Cleveland, OH 44106.
David M. Dolan is the EPA Project Officer (see below).
The complete report, entitled "Early Diagenesis and Chemical Mass Transfer in
Lake Erie Sediments," (Order No. PB 82-247 602; Cost; $16.50; 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
6201 Congdon Blvd.
Duluth. MN 55804
U.S. GOVERNMENT PRINTING OFFICE: 1983— 659-O17/O9OO
3
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
------- |