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Constructing and Evaluating a Monolith of an Active Acid Sulfate Soil with a Duripan

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Abstract

Soil monoliths are powerful tools for soil education and for representing soils as they are found in the field. They preserve and display major soil features and are relatively portable (Belohlavy 1994). The monolith described in this poster was collected from an active acid sulfate soil located along a road cut near the Smithsonian Environmental Research Center in Edgewater, Maryland. The key features of this profile include a silica-cemented duripan, a sulfuric horizon, as well as prominent concentrations of jarosite and iron oxide, glauconite, and sulfidic materials. The scalped surface (down to the duripan) is thought to be an anthropogenic feature, resulting from the creation of a now-abandoned road, prior to the construction of Maryland Route 468 (Acid sulfate soils of the U.S. Mid-Atl./Ches. Bay region 2006).
Constructing and Evaluating a Monolith of an Active Acid Sulfate Soil with a
Duripan
Jaclyn C. Fiola, Barret M. Wessel, and Martin C. Rabenhorst
Department of Environmental Science and Technology, University of Maryland
Contact: Jaclyn Fiola, fiola.1@osu.edu
ABSTRACT
Soil monoliths are powerful tools for soil education and for representing
soils as they are found in the field. They preserve and display major soil
features and are relatively portable (Belohlavy 1994). The monolith described
in this poster was collected from an active acid sulfate soil located along a
road cut near the Smithsonian Environmental Research Center in Edgewater,
Maryland. The key features of this profile include a silica-cemented duripan, a
sulfuric horizon, as well as prominent concentrations of jarosite and iron oxide,
glauconite, and sulfidic materials. The scalped surface (down to the duripan)
is thought to be an anthropogenic feature, resulting from the creation of a
now-abandoned road, prior to the construction of Maryland Route 468 (Acid
sulfate soils of the U.S. Mid-Atl./Ches. Bay region 2006).
CONSTRUCTION
To create a monolith that best
represented the soil, it was important to
include the duripan as closely and
accurately as it occurred in situ. During the
extraction process, duripan fragments were
collected using a spade and rock hammer.
Soil from between the duripan plates was
collected, and the rest of the profile was
removed using standard monolith extraction
techniques. The duripan was later
reassembled from fragments and fit into
place by using a rock saw to cut flat
surfaces (Fig. 6). The pieces were then
affixed to the monolith board with epoxy and
the gaps between the rocks were filled with
soil material (Fig. 7).
Figure 9. Finished
Monolith with labels
Figure 1. Soil profile before monolith extraction with knife for scale. pH values were
measured on 2/6/15 and are reported in both Fig.5 and 6. The oxidized-unoxidized
boundary occurs between Bsej2 and BCse around 80 cm
REFERENCES
1. Acid sulfate soils of the U.S. Mid-Atlantic/Chesapeake Bay region.
For 18th World Congress of Soil Science. (2006).
2. Belohlavy, F., : Making soil monoliths using white glue as a
fixative. Soil Survey Horizons 35(3): 74-80. (1994).
3. Fanning, D. S. et al. An acid sulfate perspective on
landscape/seascape soil mineralogy in the US Mid-Atlantic region.
Geoderma 154, 457-464, doi:10.1016/j.geoderma. 2009.04.015.
(2010).
4. Rabenhorst, M.C. and T. M. Valladares. Estimating the depth to
sulfide-bearing materials in upper cretaceous sediments in landforms
of the Maryland Coastal Plain. Geoderma. 126:101-116. (2005).
CLASSIFICATION
Classifying this soil proved to be difficult since it does not fit any existing soil
series and there are no other reported instances of duripans in the region. This
site is mapped as Collington and Annapolis soils. Both series are Typic
Hapludults; they are old enough to have developed an argillic horizon and lack
anything resembling a duripan. However, the parent material is glauconitic
fluviomarine deposits, suggesting that this soil may have previously had an
Annapolis or Collington soil above it before the area was developed and the soil
was scalped.
Due to the root-limiting layer (duripan) within 36cm, the family particle-size
control section extends from the mineral surface to the root limiting layer which
limits the control section to the 6cm of the Ase horizon. The scalped surface and
duripan are
CD
42041
42048
42059
42103
42183
1
2
3
4
5
6
Ase
Bseqmj1
(UG)
Bseqmj1 (G)
Bseqmj2
(UG)
Bseqmj2 (G)
Bsej1
Bsej2
BCse
pH
pH 3.5
Table 1: Soil morphological description at field site where the soil monolith was
collected.
Horizon Depth
(cm)
Description
Ase 1-6 5Y 4/2 loamy fine sand; moderate granular structure
Bjseqm1,
2
6-47 Duripan with many prominent jarosite concentrations
Bjse1 47-65 2.5Y 4/1 sandy loam; weak subangular blocky structure;
common prominent jarosite intercalations
Bjse2 65-80 2.5Y 3/1 sandy loam; weak subangular blocky structure;
common prominent jarosite and iron oxide intercalations
BCse 80-108 2.5Y 2.5/1 sandy loam; massive structure
ACKNOWLEDGEMENTS
Keegan Rankin and Chris Seitz were also involved in the
construction of this monolith. Thanks to Dr. Needelman for his
instruction and the initial idea for this project in his class. Also many
thanks to Dr. Rabenhorst for the use of his lab equipment and all of
his input and expertise.
CONCLUSION
This monolith of an active acid sulfate soil is a work of
art and science that will be a useful tool for soil education
and for representing the soil as it is found in the field. It
displays major soil features such as a silica-cemented
duripan, jarosite concentrations, iron oxide intercalations,
sulfidic materials, and an oxidized-unoxidized zone
boundary.
The monolith and moist incubation pH data can be used
to raise awareness about the implications of disturbing
potential acid sulfate soils. According to the data, every
horizon became more acidic when oxidized. The pH of the
bottom BCse horizon dropped from 5.5 to 1.86. If this soil
was moved or the upper soil scalped and exposed to
oxygen, the soil would be capable of dropping to a pH that
is unfit for plants or most human activities and
construction. At the very least, we hope the monolith will
prompt people to think about their soils (Fig. 9).
Figures 2 & 3. (2) Linear jarosite and iron oxide intercalations (concentrations) in the Bsej2
horizon and (3) Circular iron and jarosite concentration perhaps surrounding an old root
channel.
Figure 4. An image of the duripan layer along
the side of the road cut. Note the sparse
vegetation.
EVALUATION – MOIST INCUBATION
According to Soil Taxonomy, a sulfuric horizon must be at least 15cm thick
with a pH of 3.5 or less. Further, the pH can be 4.0 or less if sulfide or sulfur-
bearing minerals are present and there is evidence that the low pH is caused by
sulfuric acid. The presence of jarosite in the Bseqmj and Bsej horizons is enough
evidence to call them sulfuric. Without jarosite, the layer directly underlying the
horizon must contain sulfidic materials. Sulfidic materials are “mineral or organic
soil materials that have a pH value of more than 3.5 and that become
significantly more acid when oxidized.” These materials usually accumulate
under estuarine or marine conditions and persist even when the water recedes.
Exposure to aerobic conditions, for instance by human activity, results in
oxidation of the sulfides leading to the pH drop. To test for the presence of
sulfides, a moist incubation was conducted on material from each horizon
following guidance in Soil Taxonomy. The measured pH values are reported in
Figure 8. According to the definition, the sulfuric horizon extends from the bottom
of the Ase to the top of the BCse horizon.
Figure 8: pH of horizons during moist incubation. “UG” refers to an unground piece of
duripan. “G” refers to duripan that was ground using a ball mill. The dashed line at pH 3.5
indicates which horizons meet that initial definition of sulfuric horizon.
INTROUCTION
The collection of this monolith began as a project for Dr. Brian Needelman’s
Soil Morphology, Genesis, and Classification class. The site, at the intersection
of Muddy Creek Road (Rt. 468) and Mill Creek Road in Edgewater, Maryland
(38°53'08.51"N, 76°34'08.06"W), is used as an example acid sulfate soil for
field trips and other educational outings. The monolith and supplementary data
will provide additional information for future trips and instruction.
A morphological description of the soil was completed in the field before
extracting the monolith and is summarized in Table 1. The surface includes a 1-
2cm covering of decomposing pine needles (Oi horizon) underlain by the Ase
horizon. The duripan begins at 6cm and extends to 47cm, but was split into two
distinct horizons based on color differences caused by an increase in glauconite
with depth. The duripan is composed of ~95% channers and flagstones that are
interlocked but otherwise not bound to one another. These "plates" are coated
with a thin layer of jarosite (<1mm) and consist of matrix-supported glauconitic
wacke with ~50% subrounded glauconite fine sands.
DIAGNOSTIC & INTERESTING FEATURES
Jarosite, Iron Oxide Concentrations, and Glauconite
Both the Bsej1 and Bsej2 horizons contain concentrations of jarosite occurring
as intercalations (linear/planar concentrations) which run roughly parallel to the
soil surface (Fig. 2). The Bsej2 horizon is distinguished by the presence of iron
oxide intercalations in addition to the jarosite intercalations, as well as iron oxide
coatings on peds. Interestingly, there were circular iron coatings which suggest
the past presence of roots (see Fig. 3). The oxidized-unoxidized zone boundary
occurs at 80cm, under which jarosite and iron concentrations are not found. The
entire profile contains a pelletized form of the mineral glauconite as sand-sized
grains that formed in a marine environment (Fanning et al., 2010). In the mid-
Atlantic area, pyrite has been shown invariably to occur in the unoxidized zone of
Tertiary and Cretaceous sediments containing glauconite (Rabenhorst and
Valladares, 2005).
The Duripan
Soil Taxonomy defines a duripan
as “a silica-cemented subsurface
horizon with or without auxiliary
cementing agents” and requires it to
meet 4 criteria: (1) The pan is
cemented in more than 50 percent of
the volume of the horizon (we
estimate it to be closer to 95%) (2)
The pan shows evidence of the
accumulation of opal or silica. (3)
Less than 50 percent of the volume of
air-dry fragments slakes in 1N HCl
even during prolonged soaking, but
more than 50 percent slakes in
concentrated KOH or NaOH or in
Figures 6 & 7. (6) Using a diamond
rock saw to cut pieces of duripan & (7)
reconstructing the duripan on the
monolith board
Bjseqm
1
Ase
Bjseqm
2
Bjse1
Bjse2
BCse
important features that should be included in the
classification. Thus, the proposed classification is: Sandy
mixed subactive mesic Scalpic (proposed, Fanning)
Duric (proposed) Sulfudept.
pH Profile
Typical of acid sulfate soils, the
pH increased with depth (Fig. 5).
The pH is consistently low in the
Bsej1 & 2 horizons, but the relatively
unweathered material of the BCse
has higher pH. This is the location of
the oxidized-unoxidized boundary.
Note: pH values are reported on the
monolith.
alternating acid and alkali (4) Because of lateral continuity, roots can penetrate
the pan only along vertical fractures with a horizontal spacing of 10 cm or more.
(The few plant roots that survived the low pH were only present in the upper 6cm
of the soil profile). Without looking at a photomicrograph, we speculate that the
pH was low enough to allow silica in the mineral structure to cement this duripan
layer. Previous studies of this site have shown opal-CT in X-ray diffraction patterns
(Acid sulfate soils of the U.S. Mid-Atl./Ches. Bay reg. 2006)
Soil pH Weathering
Profile
Figure 5: initial pH in water (1:1 v/v) of each
horizon with depth (before moist incubation).
2 22 42 62 82 102 122
0
20
40
60
80
100
120
pH
depth (cm)
pH 5.5
pH 2.9
pH 3.0
pH 2.8
pH 3.4
pH 4.2
(Unoxidized)
5. Soil Survey Staff. Soil taxonomy: A basic system of soil classification for making and
interpreting soil surveys. 2nd edition. Natural Resources Conservation Service. U.S.
Department of Agriculture Handbook 436. (1999).
pH of Horizons during Moist Aerobic Incubation
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