This study analyzed mercury concentrations and organic matter levels in soil samples from three clear-cut sites in the White Mountains before and after cutting. At all three sites, mercury levels significantly decreased after cutting but then increased again after another year. Organic matter levels showed mixed responses, decreasing at one site but remaining steady at another. The results suggest clear-cutting impacts mercury and organic matter levels but their relationship is complex, raising questions about how other environmental factors influence changes over time. Further analysis of methylmercury levels and samples from different elevations aims to provide more insights.

1. ANALYSIS OF MERCURY CONCENTRATIONS IN THE SOIL OF THREE
CLEAR CUT SITES IN THE WHITE MOUNTAINS
David Johnston ‘16, Cassia Armstrong ’18, Steve Leo ‘17
Professor Jonathan Gourley ,
Environmental Science Department, Trinity College, Hartford, USA
Abstract:
In past studies, a positive relationship has been found between the percentage of
organic materials in soil and concentrations of mercury. Studies have also shown
that less densely forested areas are correlated to increases in mercury
concentrations. In clear cutting, these observations seem to contrast each other, as
forest loss means more exposure to airborne fallout, but also decreases in organic
content. This study focuses on the concentrations of mercury and organic matter
in three separate sites in the White Mountains National Forest in New Hampshire
and Maine. These sites are named Milstone, Hogsback, and Douglas Brook,
which were cut in 2014, 2014, and 2015 respectively. Samples were gathered
prior to the clear-cuts, and annually since the clear-cuts. Samples were freeze
dried, and mercury concentrations were determined using a Milestone Direct
Mercury Analyzer (DMA). Organic matter concentrations were determined using
the loss on ignitions method (LOI). Thus far, data shows significant decreases in
mercury concentrations between 2013 and 2014 for Millstone and Hogsback. For
the Hogsback site, organic matter significantly decreased as well, however for
Millstone the percent organic matter remained relatively the same showing only a
slight decrease after being clear-cut. Between 2014 and 2015, the concentration of
mercury showed a significant increase for Millstone. The data for 2014 and 2015
of Hogsback is currently being compiled. This study is a comparison of results
overtime, so it is to be determined whether this trend will continue in future site
samplings. Another focus of this summer was determining a method for
methylmercury extraction so as to provide further insight into the binding
characteristics of mercury to soil and its environmental effects. The final results
of this study will be shared with the USDA Forest Service to further
understanding on how current clear-cutting methods affect soils.
Introduction:
•The USDA Forest Service proposed four alternatives regarding forest
management on the White Mountains. The alternative chosen called for the
addition of approximately 34,500 acres of wilderness, and the harvest of 281,300
acres. (USDA Forest Service, 2005).
•High-elevation forests receive elevated atmospheric deposition of anthropogenic
pollutants (Blackwell, Driscoll, 2015)
•Airborne emissions are the largest source of mercury in the United States and
globally. Atmospheric concentrations of mercury are three times higher than
preanthropogenic levels (Driscoll et al., 2007)
•The Northeast U.S receives high concentrations of mercury deposition from
anthropogenic sources such as electric utilities, incinerators, and industrial
processes. Forest regions are particularly sensitive to mercury deposition, and can
lead to high Hg concentrations in fish. (Driscoll et al., 2007)
•Mercury has a vast array of cognitive, developmental, physical, and behavioural
effects on both humans and other living organisms, and can experience
biomagnification in ecosystems. (Bank, 2012).
•Characteristics that contribute to the cycling of mercury in watersheds include
slope, soil thickness, pH, wetland percentage, types of vegetation, and carbon
content (Bank, 2012).
•High carbon conent increases the soil's retention of mercury, and therefore alone
can predict 44% mercury variability. Organic carbon, fine fraction, and C/N ratios
in soil can predict mercury levels with close accuracy. (Gruba et al., 2014).
•Other factors in affected mercury fluxes in soil include moisture, temperature,
and solar radiation, which are all affected by canopy disturbances due to clear
cutting (Mazur et al., 2014).
Methods: Samples were collected in Whirl-Pak bags using a transect method
(using a Garmin Montana 650). Pre-cut data was collected in 2013, and post-
clearcut in 2014. The samples were then freeze dried in a Labconco FreeZone 6
to remove any moisture. For the analysis of mercury concentrations, an
instrument called a direct mercury analyzer (Milestone's DMA-80, or DMA) was
used. Between 0.0100 g and 0.0200 g of soil was added to each boat using a
sterilized spatula that was rinsed with deionized water and dried with a clean
paper towel between each sample addition, and then placed in the appropriate slot
in the DMA. This mass was then labeled and recorded in the instrument, and the
method used for each sample was called "stream sediment." This method was set
to take ten seconds to heat the sample up to 200°C, left for ten seconds before
taking one minute and ten seconds to heat up to 650°C, then left at this
temperature for three minutes before let to cool. The resulting mercury
concentration was then recorded, and the file saved. Each sample was analyzed
three times on three different days.
2013 2014 2015
2013 2014 2015
0
0.1
0.2
0.3
0.4
Numberofsamplesasfraction
oftotal
Range of Mercury level in ppb
Milstone
O horizon Mercury
Precut 2013 Postcut 2014 Postcut 2015
0
0.2
0.4
0.6
Numberofsamplesasfraction
oftotal
Range of Mercury level in ppb
Milstone
B horizon Mercury
Precut 2013 Postcut 2014 Postcut 2015
0
0.1
0.2
0.3
Numberofsamplesasfraction
oftotal
Range of Mercury level in ppb
Hogsback
O horizon Mercury
Precut (2013) Postcut 2014 Postcut (2015)
0
0.1
0.2
0.3
0.4
Numberofsamplesasfraction
oftotal
Range of Mercury level in ppb
Hogsback
B horizon Mercury
Precut (2013) Postcut 2014 Postcut (2015)
Milstone
Hogsback
Douglas Brook
Results:
Milstone: Figure 1 shows GIS maps and histographs to provide visual
representations of the changes in mercury over time. It can be observed in the
GIS maps an apparent decrease in mercury concentrations. This is also
represented in the shift left and spread of concentrations shown in the
histographs between 2013 and 2014. A t-test was performed on these data points
and it was determined that this change was in fact significant for the O-horizon
(t = 5.6, P <0.0001) as well as the B-horizon (t =5.65, P <0.0001). Between
2014 and 2015, an overall shift right on the histographs for both horizons
indicates that there was an average increase in mercury concentrations. This
change was also determined to be significant (t=-2.33, P= 0.0219). For Milstone
there was very little change in the average percent organic matter from the 3
years in the O horizon as well as the B horizon. For the 2015 data the O horizon
ranges from 0-100 percent organic content with the highest percent organic
content between 60-80. The B horizon falls between 0-40 with all the samples
falling in that range.
Hogsback: Figure 2 shows GIS maps and histographs to provide visual
representations of the changes in mercury over time. It can be observed in the
GIS maps an apparent decrease in mercury concentrations. This is also
represented in the shift left and spread of concentrations shown in the
histographs between 2013 and 2014. A t-test was performed on these data points
and it was determined that this change was in fact significant for the O-horizon
(t = 4.71, P = 6.49 x10-6 ) as well as the B-horizon (t = 4.13, P = 5.66 x10-5 ).
Between 2014 and 2015, an overall shift right on the histographs for both
horizons indicates that there was an average increase in mercury concentrations.
This change was also determined to be significant (t= -3.83, P= 0.000274). For
Organic matter in the O horizon there was a significant drop in average percent
organics after the initial clear cutting (t=8.15, P<0.0001) followed by a
significant rise in percen organics between 2014 and 2015. For the 2015 data the
percent organic content ranged from 0-100 in the O horizon where as in the B
horizon it ranged from 0-60.
Douglas Brook: Figure 3 shows GIS maps and histographs to provide visual
representations of the changes in mercury over time. It can be observed in the
GIS maps an apparent decrease in mercury concentrations. This is also
represented in the shift left and spread of concentrations shown in the
histographs between 2013 and 2015. A t-test was performed on these data points
and it was determined that this change was in fact significant for the O-horizon
(t= -4.93 = , P <0.0001) but not for the B-horizon (t =-1.47, P = 0.145). For
Douglas Brook there was a decrease in 2015 in the O horizon in the average
percent organic matter after the initial clear cutting event. However with the B
horizon there was an slight increase in average percent organic matter. For the
2015 data the O horizon was spread more consistently than the B horizon in
terms of percent organic content. The O ranged from 0-100 and the B ranged
from 0-60.
Conclusion:
For all three sites it was determined that after a clear-cut, a significant decrease
in the average mercury concentrations occurred. For Hogsback and Douglas
Brook, after another year the concentrations of mercury increased. This occured
in both the O and B horizons. For Hogsback and Douglas Brook, this pattern
was consistent with the results for the change in % organic matter as well for the
O Horizons. However, Milstone did not experience a significant decrease in %
organic matter, despite the mercury concentrations changing rather dramatically.
While the results of this research answer many questions in how organic matter
and mercury concentrations change in soils following a clear cut, these results
also raise questions about the relationship between these two components, and
whether slope, elevation, and direction play a crucial factor in these
observations.
These questions have led to the initiation of two new projects. One of these
projects is determining a method for extracting MeHg from the samples to
further understanding of the relationship between organic matter and organic
variations of mercury, while the other project is collecting samples at different
elevations to examine the relationship between elevation and mercury
concentrations. These elevated samples will be analyzed for MeHg as well.
0
0.1
0.2
0.3
0.4
Numberofsamplesasfractionof
total
Range of Mercury level in ppb
Douglas Brook
O Horizon Mercury
2013 2015
0
0.1
0.2
0.3
0.4
Numberofsamplesasfractionof
total
Range of Mercury level in ppb
Douglas Brook
B Horizon Mercury
2013 2015
Figure 1: Millstone GIS maps of the Milstone clear-cut site (left) displaying the spatial distribution and change in mercury over time, and histographs (right) displaying the
distribution of mercury concentrations over time. The bar graph on the right shows the change in average percent organic matter over time for both horizons.
Figure 2: Millstone GIS maps of the Hogsback clear-cut site (left) displaying the spatial distribution and change in mercury over time, and histographs (right) displaying the
distribution of concentrations over time. The bar graph on the right shows the change in average percent organic matter over time for both horizons.
Figure 3: Millstone GIS maps of the Douglas Brook clear-cut site (left) displaying the spatial distribution and change in mercury over time, and histographs (right) displaying the
distribution of concentrations over time. The bar graph on the right shows the change in average percent organic matter over time for both horizons.
0
20
40
60
80
100
120
O Horizon
2013
O Horizon
2014
O Horizon
2015
B Horizon
2013
B Horizon
2014
B Horizon
2015
AverageOrangics(%)
Horizon
Milstone
% Organic Matter in O and B
Horizons
0
20
40
60
80
100
120
O 2013 O 2014 O 2015 B 2013 B 2014 B 2015
AverageOrangics(%)
Horizon
Hogsback
% Organic Matter in O and B
Horizons
0
20
40
60
80
100
120
pre cut O O horizon 2015 pre cut B B horizon 2015
AverageOrangics(%)
Horizon
Douglas Brook
% Organic Matter in O and B
Horizons