ANTHROPOGENIC INFLUENCES OF MERCURY CONCENTRATIONS IN

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ANTHROPOGENIC INFLUENCES OF MERCURY
CONCENTRATIONS IN SNAG LAKE SEDIMENT Kathleen
J. Gosnell, William M. Landing, Department of
Oceanography, Florida State University,
Tallahassee, FL. Jane Caffrey, University of West
Florida, Pensacola, FL.
ABSTRACT Mercury (Hg) is a naturally occurring
element, however, industrial activities and the
burning of fossil fuels have elevated
concentrations towards toxic levels in
environmental systems. Mercury concentrations in
coal tend to be up to four orders of magnitude
greater than natural gas or fuel oil. Coal-fired
power plants are known to be one of the greatest
sources of anthropogenic mercury into aquatic
ecosystems. Sediment samples were taken from
Snag Lake, which lies in close vicinity of a
coal-fired power plant in northern Florida, and
analyzed for total mercury content in order to
assess impacts of increased coal burning on
measured mercury levels. A timeline of
approximately 60 years was established with lead
210 (Pb-210) measurements. Snag Lake has a
constant rate of sedimentation displaying an
exponential rise in mercury content as
anthropogenic activities increase environmental
loading.
METHODS Several lake cores were taken from the
Escambia River delta region, an area located in
the vicinity of several known mercury sources
(Figure 3). The Snag lake core indicated a
constant sedimentation rate, which yields a
better quality record of deposition and trace
metal accumulation. Sedimentation chronology was
determined by Pb-210 regression analysis (Figure
5). Lead is deposited in the environment at a
constant rate with other pollutants such as
mercury. Lead 210 experiences a constant rate of
decay which provides an approximate analysis of
sediment age (t1/222.3 yr). Sediment and
rainwater mercury concentrations were determined
with Cold Vapor Atomic Fluorescence Spectrometry.
In this method Hg2 is reduced to gaseous Hg0
and purged onto a Au-coated quartz sand trap
using Hg-free argon gas. The Hg0 signal is
recorded as peak area in counts. A standard
regression line is used to calculate the
concentration of mercury in each sample in ng/L
(Figure 4). Sediment of approximately 0.05-0.15
g was extracted from each depth interval and
analyzed individually using microwave bomb
digestion. A set acid matrix is added to each
bomb in order to release total mercury into
measurable liquid form. Microwave digested
samples are diluted with Hg-free water,
centrifuged, and analyzed for total-Hg. The dry
concentration of mercury in each sediment sample
than calculated as ng/g (ppb) based on the mass
of sediment, dilution, and the percentage of
water in each sample interval (Figure 6).

Molino
Pace
1x109 g/yr
Ellyson
INTRODUCTION Human activities associated with
fossil fuel combustion have been shown to
increase local, regional and atmospheric
deposition of mercury. Current atmospheric
emissions of mercury are estimated to be three
times historical levels (Figure 1 and Figure 2).
Gulf Power Companys Crist plant, a coal-fired
power plant located in the study region of
Pensacola (Figure 3.), provides a known source of
mercury to the Escambia river delta. Other
sources of mercury in the region include chemical
plants, a paper mill and automobiles. This study
was undertaken in order to investigate local
point source influence on mercury and trace metal
deposition. Through sediment deposition a link
can be established between recently elevated
mercury levels and historic levels. Modern
mercury levels and sources are deduced from
measuring individual rain samples for total
mercury over the last couple of years (Figure 4).
Snag Lake sediment mercury is linked to
anthropogenic emissions in the vicinity of the
core sample site. Once lake sediment intervals
are dated using Pb-210, a robust record of Hg
mass accumulation can be established from present
values to levels dated over 60 years ago.

• RESULTS
• There is a seasonal flux of mercury into the
  Pensacola region, exhibiting the majority of
  increased concentrations during the spring and
  summer months (Figure 4). At first glance it
  appears that the Pace site consistently exhibits
  higher amounts than the Ellyson or Molino sites,
  however, Ellyson and Molino display peak
  deposition rates for certain months depending on
  the wind patterns influence on rainfall. Mercury
  concentrations in the Snag Lake core indicate a
  clear increase in recent levels compared to past
  deposition (Figure 6). Core sediment intervals
  indicate minor elemental deposition 60 years ago,
  followed by a slightly higher interval from 40 to
  10 years ago. This increase coincides with the
  period that the Crist Power plant began
  production. The sediment record indicates a
  sharp enhancement in mercury levels for the last
  10 years.
• It is important to note that total-Hg levels in
  rain samples are displayed in concentrations of
  ng/L (wet concentrations), while sediment values
  are calculated in ng/g (dry concentrations).
• Measured values for total-Hg at the three rain
  sites range from approximately 3 ng/L to 35 ng/L.
  These sites additionally yield an Hg-flux range
  from 36 ng/m2 to over 4000 ng/m2.
• Sediment Hg concentrations have increased from
  past levels of 69 ng/g to modern values of 439
  ng/g.
• DISCUSSION
• Anthropogenically elevated mercury
  concentrations released into watersheds is a
  proven environmental problem. Elemental mercury
  is known to have a very long half-life of up to 2
  years in the troposphere before it is oxidized
  and washed out of the atmosphere. In this
  extended period there is ample time for mercury
  to mobilize and contaminate pristine sources in
  addition to local watersheds. Monthly rainwater
  measurements provide a clear record where it is
  possible to correlate sources of elemental
  mercury deposition into lake sediment. Although
  a relationship is clearly present between
  atmospheric deposition as indicated by Hg
  concentrations in rainwater, these presented
  values were analyzed after the sediment core was
  taken.
• Measured sediment levels indicate a significant
  increase of atmospheric mercury added to the
  Escambia watershed through coal-fired power
  plants and other anthropogenic trace metal
  sources.
• Lakes can exhibit different background levels of
  mercury and other sediments. Geological
  characteristics of the watershed, hydrologic flow
  patterns and sediment reworking can impact
  measured levels. Snag Lake provided a clear
  Pb-210 profile, indicating a constant rate of
  atmospheric deposition to the lake, and minimal
  disturbance and sediment mixing. Mercury
  concentrations correspond well to the Pb-210
  depth profile.
• .

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Fig 4. Monthly total-Hg concentrations measured
for 3 rain sampling sites.
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Years of Sediment Accumulation History of Organic
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Landers, D.H., et. al,. (1998) Using Lake
Sediment Mercury Flux Ratios to Evaluate the
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Deposition in Arctic and Boreal Ecosystems.
Atmospheric Environment, 32, pp. 919-928. 4.
Fitzgerald, William F. (1995) Is Mercury
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