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Stop 1Central Maryland Research and Education Facility at Upper Marlboro
Maryland Agricultural Experiment Station Farm
Prince Georges County, MD
Background
The surficial geology of an extensive portion of the Southern Maryland Coastal Plain
consists of sediments that have significant amounts of a sulfide mineral, pyrite, in the unoxidized
or unweathered part of the regolith. These sediments are mainly of Tertiary (Nanjemoy,
Marlboro Clay, Aquia Fms) and Late Cretaceous (Magothy, Matawan/Monmouth Fms) ages and
occur at the surface throughout most of Anne Arundel County and to a lesser extent in Prince
Georges County (Fig. 1-2). Only the Nanjemoy, Aquia, & Matawan/Monmouth Fms are
glauconitic
This stop is situated within the outcrop area of the glauconitic Aquia Formation of
Paleocene age. The soil to be examined is identified as the Annapolis (previously Collington)
series, which is the most extensive glauconitic soil in Maryland. Glauconitic soils occur in six of
Maryland's Coastal Plain counties and occupy over 47,000 hectares.
Acid sulfate problems associated with the exposure of sulfide-bearing soil materials in
the inner Maryland Coastal Plain have been documented for many years. The microbially
mediated processes of sulfide and iron oxidation and hydrolysis are essentially the same as those
which occur in coal mining areas in the Appalachian provinces of Western Maryland, which lead
to problems of extremely acid soils and acid mine drainage. On the Coastal Plain, similar
problems arise when construction activities expose sulfide-bearing horizons deep within the
regolith.
.
Figure 1-1.
56
Coastal Plain Geology – Stop 1
Kp: Potomac Group
Kmo: Monmouth Formation
Kma: Matawan Formation
Ta: Aquia Formation (Paleocene)
Tn: Nanjemoy Formation (Eocene)
Tc: Calvert Formation (Miocene)
QTu: Upland Deposits (Quaternary)
QTl: Lowland Deposits (Quaternary)
Cre
tace
ou
sTe
rtia
ryQ
uat
.
DC
Figure 1-2
57
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
0 0.1 0.2 0.3 0.4
De
pth
(cm
)%S
Ap1
Ap2
BE
Bt
BCj
Cj1
Cj2
Figure 1-3 (Data from D. P. Wagner dissertation) shows
the distribution of sulfur in the soil profile (near pit 1A).
Nearly all of the sulfur in the subsoil of this profile is
present as jarosite, and because only small amounts of
sulfur were present in surface horizons (upper 50 cm), no
attempt was made to fractionate sulfur in the surface
horizons. It is interesting to note that even though the
highest jarosite concentrations were measured (Phil
Snow PhD dissertation – differential extraction and XRF
techniques) in the argillic horizon (0.22% S), field
observations failed to detect jarosite, This is in contrast
to other profiles described in which field observations
were able to detect jarosite concentrations later
determined in the laboratory to be as low as 0.02 %
jarosite-S. Perhaps masking action by pedogenic
processes leading to argillic horizon formation tend to
destroy or cover over discrete jarosite mottles. Partial
hydrolysis of jarosite to iron oxide could also mask the
conspicuous yellow color of the mineral.
Pyrite Associated with Glauconitic Materials
Glauconite is A 2:1 phyllosilicate of the mica group that
has ferrous and ferric Fe in the octahedral position
(Fanning et al., 1989a). Typically, glauconite occurs as Figure 1-3
fine sand size pellets that are green to black in color. In Maryland, glauconite is found in Tertiary sediments of the Piney Point, Nanjemoy, and Aquia Formations and in Cretaceous sediments of the Monmouth and Matawan Formations. These sediments dip to the southeast and occur at or near surface along the western part of the Coastal Plain in a band approximately 30 km wide. This band begins about 15 km east of the fall line and extends along a NE to SW direction. In Maryland, approximately 50 000 hectares (120 000 acres) have been mapped in soil series developed in glauconitic sediments exposed at the surface. These soils are mainly in fine-loamy and clayey families of Typic and Aquic Hapludults and Typic Endoaquults. Many of these soils have >20% glauconite by weight in the <2 mm fraction (some >40%) and thus are in glauconitic mineralogical families, although some contain <20% and would be in mixed, rather than glauconitic, families.
The soil/geologic column in glauconitic sediments can be divided into an upper oxidized zone,
which contains oxidized forms of Fe such as goethite and jarosite, and a lower unoxidized zone
which contains pyrite (Wagner, 1982; Wagner et al., 1982; Valladares, 1998). While pyrite has
also been found in some lignitic layers in Cretaceous sediments in Maryland, it is generally not
present except in the glauconitic materials. The boundary between the oxidized and unoxidized
zones is usually quite abrupt, and occurs at depths ranging from between approximately 2 and 8
m under most natural land surfaces.
58
A
B
Micrographs of thin sections from samples collected from the oxidized zone (A) and from the unoxidizedzone (B) of the Nanjemoy formation in Charles County, MD. Note the presence of black pyrite around and within some of the glauconite grains (green) in the unoxidized zone. Plane polarized light. Frame length approximately 1.2 mm.
Micrograph of thin section from a sample collected from unoxidized zone of the Nanjemoy formation in Charles County, MD. Note the presence of (light) pyrite around and within some of the glauconite grains (green). Combined reflected and plane polarized light. Frame length approximately 1.2 mm.
Light Microscopy
Observations of thin sections using plane
polarized light show that the glauconite
in the oxidized zone appears yellowish
green (5Y-2.5GY 6/6-6/8) in color, while
in the unoxized zone the glauconite
appeared to be more bluish green (5GY
5/8-6/8) in color. This difference in color
has been attributed to a difference in the
ratio of ferrous to ferric Fe in the mineral
structure (Fanning et al., 1989b; Stucki
and Roth, 1977). Furthermore, pyrite can
be clearly seen in the unoxidized zone as
opaque grains under transmitted light and
as yellowish colored grains under
reflected light (Fig. 1-4). While some of
the pyrite occurred distinctly apart from
the glauconite, a portion of the pyrite was
intimately associated with glauconite.
This pyrite occurred both as embedded
grains within the matrix of glauconite
and also within fissures and other voids
in the glauconite. Pyrite is absent from
samples of the oxidized zone. However,
there were within the oxidized materials,
zones of Fe oxide or oxyhydroxide
concentrations visible as coatings and
soft masses.
Electron Microscopy Figure 1-4 Figures 1-5A and 1-5B show the
accumulation of pyrite on one face of a fracture through a glauconite grain. The backscattered
image of the minerals in this figure illustrates the compositional difference between the pyrite
(more electron dense) as the light colored phase, and the glauconite (less electron dense).
Essentially all of the pyrite associated with the glauconite demonstrates a euhedral habit,
suggesting that this portion of the pyrite was authigenic (formed post-depositionally).
Pyrite also occurs in the sediment in phases not associated with glauconite. Clusters of euhedral
pyrite having an octahedral habit have been observed, with some crystal and clusters ranging up
to >20 /un in size (Fig. 1-6A). The delicate euhedral structures showed no evidence of abrasion
or rounding which indicates that the grains also formed post-depositionally, as forms of this
nature would not tolerate the forces of sedimentary transport.
59
SEM of pyrite from the unoxidized zone of the Nanjemoyformation which were separated from the nonmagnetic (non-glauconitic) fraction using heavy liquids. Pyrite occurred both as clusters of euhedral crystals (A) and also with a framboidalhabit (individual microcrystals show pyrithohedral habit) (B).
A
B
Scanning electron micrograph of euhedral pyrite (light area) on an exposed fissure in a glauconite grain (A) from the unoxidized zone in the Nanjemoy formation. B is a higher magnification of the inset area grain shown in A (backscatter electron emission). Rabenhorst, M. C. and D. S. Fanning. 1989. Soil Sci. Soc. Am. J. 53:1791-1797.
A
B
Framboidal pyrite was also observed in the heavy fraction (Fig. 1-6B). Pyrite framboids have
been reported in materials formed under a variety of environments including tidal marsh and
estuarine sediments and in coal. While other habits have been reported, the individual
microcrystals within the framboid shown in Fig. 1-6B appear to be pyritohedral in shape. Pyrite
framboids have been clearly shown to occur in sediments as both allogenic grains having formed
in one environment and being transported to another, and as authigenic grains forming in place.
The framboidal nature of this portion of the pyrite is not therefore indicative of its origin in these
sediments.
Figure 1-5 Figure 1-6
60
There are two soil pits open for examination (Stop 1A and Stop 1B) (Fig 1-1). Due to the great
depth to sulfides, the sulfide bearing zone is not exposed in the pits. At Stop 1B we will try to make an
auger boring deep enough to reach the sulfides demonstrating the presence of sulfidic materials at depth
in these landscapes (at Stop 1A, the depth to sulfide is estimated to be approximately 8 m below the
surface).
The exposed Annapolis profile (1A) is an example of a post-active acid sulfate soil. Unlike the
soil to be observed at Stop 1B, this soil occurs in a well drained upland position and is much more typical
of the majority of post-active acid sulfate soils in the Coastal Plain. As indicated in the following
description made of a profile located near to the pit exposed for this tour (but note that this is not the same pit/pedon), Jarosite mottles occur within 1 m of the land surface.
Table 1A- Pedon description of Pedon S79MD16-1 (S79 MD 033-1 by FIPS) Annapolis (formerly Collington) loamy fine sand (taxadjunct) - Nearby to where the pit for Stop 1A is located - but not the same pit/pedon.Inceptic Hapludult, fine-loamy, mixed, mesic
Horizon Depth (cm) Description
Ap 0-30 Dark brown (10YR 3/3) moist, pale brown (10YR 6/3) dry, loamy fine sand; weak medium granular structure in upper 5 cm, massive below 5 cm; friable; pH 4.8; abrupt smooth boundary.
E 30-37 Yellowish brown (10YR 5/4) to dark yellowish brown (10YR 4/4) loamy fine sand; massive; friable; pH 4.8; clear smooth boundary.
BE 37-54 Yellowish brown (10YR 5/4) to dark yellowish brown (10YR 4/4) fine sandy loam; weak medium subangular blocky structure; friable; pH 4.6; clear smooth boundary.
Bt 54-68 Dark yellowish brown (10YR 4/4) and olive (5Y 5/4) where indurated, sandy clay loam; weak coarse prismatic breaking to moderate medium subangular blocky structure; friable to firm and very firm where indurated; thin nearly continuous brown (7.5YR 4/4) clay films on ped faces and fossil casts; pH 4.6; clear smooth boundary.
BCj 68-100 Olive (5Y 4/3) fine sandy loam; common medium yellow (5Y 7/6) jarosite concentrations, and common medium strong brown (7.5YR 5/6) iron oxide concentrations where slightly indurated; weak coarse subangular block structure; friable; pH 4.5; gradual smooth boundary.
Cj1 100-130 Olive (5Y 4/3) fine sandy loam; common medium yellow (5Y 7/6) jarosite concentrations; massive; friable; pH 4.4; gradual smooth boundary.
Cj2 130-190+ Olive (5Y 4/3) fine sandy loam; common to many medium yellow (5Y 7/6) jarosite concentrations; massive; friable to firm where partially indurated; pH 4.2.
Location Prince Georges County, Maryland; University of Maryland Tobacco Research Farm near westernmost buildings, east side of farm lane, 35 meters east of tobacco drying barn. Approximately 38.858031, -76.779382
Vegetation Grass
Parent Material Glauconitic, sulfidic sediments of the Aquia Formation of Paleocene age
Physiography Coastal Plain Upland
Elevation 30 m
Slope 2% northern aspect
Drainage Well drained
Permeability Moderately slow
Moisture Moist
Groundwater 7 meters
Described by D. P Wagner and D. S. Fanning 8/29/78
Remarks An auger boring at this site did not encounter sulfidic strata within a depth of 8 m. Caving of the hole prevented deeper observation. This soil is a taxadjunct of the Collington series, which is classified as a Typic Hapludult
61
Pedogeomorphic modeling of the depth to sulfide-bearing materials - Aquia Formation
Based on the M.S. Thesis of Terry M. Valladares
While the approximate geographic extent of sulfide-bearing sediments is known, the
depth at which sulfides occur is highly variable, ranging from 2 m to over 15 m. Because
geomorphic factors control the depth to sulfidic “unoxidized zone” materials, soil-geomorphic
landscape models were developed to estimate the depths at which these potentially hazardous
materials occur in various portions of a landscape. Information regarding the depths at which
sulfides occur in various landscapes would prove useful to those who engage in land
development and highway construction activities. This would allow earth-moving activities to
be better managed to avoid disturbance of sulfidic materials and thereby minimize the potential
hazard of developing acid sulfate conditions.
Materials in the unoxidized zone generally have moist Munsell chromas of 1 or less and values
of 4 or less, that is, they are usually black or dark grey in color. The boundary between the
oxidized and unoxidized zone is almost always very sharp or abrupt and is therefore easily
distinguished in the field. The depth at which this boundary occurs in the natural landscape or
the depth at which the sulfide-bearing materials is encountered is primarily a function of
topographic variables which control the depth of geologic oxidation and weathering. As shown
in Fig. 1-7, sulfide-bearing materials have pHs in the range of 4.5 to 7.5.
There are sharp increases in chromium-reducible sulfur (CRS) contents across the abrupt
morphological boundary determined from field observations (based on differences in color),
indicating that accurate determinations of the boundary between oxidized and sulfide-bearing
strata could be made in the field (Fig. 1-8). CRS contents for both Upper Cretaceous and
Tertiary sulfide-bearing materials vary between 0.3 and 2.1% (0.6 to 4.0% pyrite). As shown in
Fig. 1-9, moist incubation of fine sandy loam Aquia, Nanjemoy sulfide-bearing materials
-200
-150
-100
-50
0
50
3 4 5 6 7Dis
tan
ce f
rom
bo
un
dar
y (
cm)
pH
Aquia (Paleocene)Nanjemoy (Eocene)
Changes in pH across the morphological boundary (zero line) between the oxidized and unoxidizedzones in the aquia and Nanjemoy formations. Borings sampled had depths to sulfides ranging from 2.6 to 11.1 m (Aquia) and 4.1 to 8.1 m (Nanjemoy).
Figure 1-7
62
-200
-150
-100
-50
0
50
0 0.5 1
Dis
tan
ce f
rom
bo
un
dar
y (
cm)
% CRS
Aquia (Paleocene)Nanjemoy (Eocene)
Chromium reducible sulfur (CRS) contents across the morphological boundary (zero line) between the oxidized and unoxidized zones in the Aquia and Nanjeoy formations.
2
3
4
5
6
7
0 10 20 30 40 50 60
pH
Time (days)
Aquia (Paleocene)Nanjemoy (Eocene)
Graphs of moist, aerobic incubation pH vs time for samples from the Nanjemoy and Aquia sulfide-bearing samples. Samples were collected between 5 and 150 cm below the boundary between the oxidized and unoxidized zones, and samples contained between 0.4% and 1.1% chromium-reducible sulfur (CRS).
demonstrate that they are also sulfidic or potentially acid (pHs dropped to values between 2.3
and 3.0 by the end of the incubation).
Figure 1-8
Figure 1-9
63
Highly significant regression models indicate that point relief (the difference in elevation between the point of interest and the lowest point or hydrologic base point in the landscape unit) could explain at least 75% of the variability in depth to sulfides (Fig. 1-10). These models result in a reasonably good agreement between observed and predicted depths to sulfides on model validation data sets. Fig. 1-11 shows spatial patterns of the depth to sulfides (estimated using the quantitative model) for the Aquia geomorphological setting.
The Upper Marlboro site is a very broad interfluve with undulating topography and
several small hills rising to an elevation of about 36 m (Fig. 1-11). On the western edge of the
Upper Marlboro landscape, the area slopes steeply downwards to the Western Branch of the
Patuxent River. This site is almost entirely covered by the Aquia Fm except for erosional
remnants of Pleistocene terrace deposits at the surface along transects 2, 3, and 4 (Figs 1-12, 1-
13, 1-14).
y = 2.71e0.06x
R² = 0.75
y = 0.89x - 14.65R² = 0.96
y = 0.75x + 2.72R² = 0.83
0
2
4
6
8
10
12
14
16
18
0 10 20 30
Dep
th t
o s
ulf
ides
(m
)
Point relief (m)
Matawan/Monmouth Fms Nanjemoy & Marlboro Clay Fms
Aquia Fm
Talbot Fm
Figure 1-10
64
Figure 1-11
65
Transect 2
Transect 3
As shown in the cross-sections of transects 2, 3, and 4 (Figs 1-12, 1-13, 1-14
respectively), the boundary between oxidized and sulfide-bearing strata occurs at a relatively
constant elevation (about 26.5 m) in the portion of the landscape furthest from the Western
Branch of the Patuxent River. This section of the landscape also has thin remnants of
Pleistocene terrace deposits at the surface. These terrace deposits are of fluvial origin and are
probably of Late Pleistocene age (Glaser, 1981). Both pH and chromium reducible sulfide
(CRS) data (generally equivalent to pyrite in these landscapes and systems) are provided in Figs
1-7 and 1-8.
Figure 1-12
Figure 1-13
66
Transect 4
Transect 5
Along transect 5, there is a gradual sloping downwards (≈1.5% slope) of the sulfide surface
toward the highly dissected backslope areas of Western Branch (Fig. 1-15). Therefore, sulfides
occur at shallower depths in areas further away from the highly dissected Western Branch
channel. Sulfides are not found even at depths of 12.0 to 12.5 m at UM 1-1 (summit) and UM 1-
2 (shoulder) (Fig. 1-11). The entire UM 1-2 profile is highly oxidized and indicates the edge
weathering effect typical of steep valley backslopes (Richardson and Daniels, 1993). Depth to
sulfides is more strongly affected by the Western Branch channel at distances of up to 0.6 km
from the channel. Sulfides occur at greater depths with decreasing distance from the channel and
is related to the highly oxidized nature of profiles (“red edge effect”) that occur close to steep
valley backslopes in otherwise nearly level landscapes. Thus, at the Upper Marlboro site,
proximity to the Western Branch channel has more effect on the depth to sulfides than local
variations in topography.
Figure 1-14
Figure 1-15
67
Figure 1-16. Left – A zone of silica-cemented shell-casts in a Bqm horizon beneath a Bt horizon in a monolith (from the
collection of soil monoliths in H. J. Patterson Hall of the University of Maryland in College Park, MD) of the Annapolis
soil series variant (Typic Hapludult; fine-loamy, glauconitic, mesic), a post-active acid sulfate soil collected near to Stop
1A. Based on other XRD work in similar settings, the silicified/indurated material is thought to be cemented by opal-CT.
This zone shown is for the 40 cm to 75 cm depth.
Right – A large piece (35 cm by 22 cm) of the silica-cemented shell casts material from the same soil near Stop 1A. This
material had a pH of about 4.0 (in water). It likely had a pH of about 8 when the shells were present, when the silica may
have been deposited after being released into solution by active acid sulfate weathering higher in the soil-geologic column
and when pyrite was still present in these soil materials. The past presence of pyrite is inferred by the presence of
jarosite concentrations on some of the shell casts.
68
Stop 1B
Figure 1-17. Location of Stop 1B which occurs along transect 2 (Valladares) roughly midway between points 2-1 and 2-2.
Transect 2
Approximate Location of Stop 1B
69
Tab
le 1
-2 P
edo
n d
escr
ipti
on
of
Ad
elp
hia
pit
sto
p 1
B. (
Ori
gin
al d
escr
ipti
on
19
98 w
as in
th
e ge
ner
al v
icin
ity;
up
dat
ed
fro
m a
uge
r b
ori
ng
on
Ju
ne
13,
201
6 b
y M
. Rab
enh
ors
t an
d D
. Sm
ith
.)
Ho
rizo
n
Dep
th
(cm
)
pH
Ju
ne
201
6
Text
M
atri
x C
olo
r
Red
ox
Feat
ure
s St
ruct
ure
C
on
sist
. B
ou
nd
. O
ther
Co
mm
ents
D
epl.
Co
nc.
A
0-6
fsl
10
YR3/
3
1-2
gr
vfr
cs
Ap
8
-24
fsl
10
YR3/
4
1 s
bk
fr
cs
BE
24
-50
fsl
10
YR4/
4
Bt
50
-75
cl
10
YR4/
4
cmf
2.5
Y5/3
cm
d
7.5
YR3/
4,4
/6
2m
sbk
fr
cs
10%
wea
ther
ed (
kao
linit
ized
) fe
ldsp
ar 7
.5YR
6/4
BC
7
5-1
00
sl
10
YR4/
4
cmf
2.5
Y4/2
cm
f 7
.5YR
4/4
, 4
/6
1co
pl /
1
msb
bk
vfr
cs
30%
wea
ther
ed (
kao
linit
ized
) fe
ldsp
ar 7
.5YR
6/4
;
BC
g1
100
-14
0lf
s 5
Y4/2
m
m&
co d
1
0YR
4/6
1
cop
l/
1m
sbk
fr
cs
35%
wea
ther
ed (
kao
linit
ized
) fe
ldsp
ar 7
.5YR
6/4
; so
me
we
ath
ered
fra
gmen
ts a
re
wh
ite
2.5
Y7/2
BC
g2
140
-17
5lf
s 5
Y4/2
M
(45
%)m
d
7.5
YR3
/4
1m
&co
sbk
vfr
BC
g3
175
-24
0lf
s 5
Y4/2
cf
p 7
.5YR
4/6
m
ost
ly in
u
pp
er p
art
BC
gj
240
-30
0lf
s 5
Y4/2
M
(20
%)
md
5Y6
/4
Co
nce
ntr
atio
ns
are
jaro
site
BC’
300
-32
24
.11
lfs
7.5
YR
3/4
, 4/6
m
m&
co d
5
Y4/2
Cgs
e 3
22-3
40
4.9
8
lfs
10
Y2.5
/1
c(20
%)m
d
7.5
YR3
/3
Sulf
ide
Bea
rin
g
Co
nsi
sten
ce: v
fr –
ver
y fr
iab
le; f
r –
fria
ble
; St
ruct
ure
: gr –
gran
ula
r; s
bk –
sub
angu
lar
blo
cky;
pl –
pla
ty; m
a –
mas
sive
; co
– c
oar
se; m
– m
ediu
m;
Red
ox
Feat
ure
s: A
BU
ND
AN
CE
f –
few
; c –
co
mm
on
; m –
man
y; S
IZE
f –
fin
e; m
– m
ediu
m; c
o –
co
arse
; CO
NTR
AST
f –
fai
nt;
d –
dis
tin
ct; p
– p
rom
inen
t;
Text
ure
: fsl
– f
ine
san
dy
loam
; cl –
cla
y lo
am; s
l – s
and
y lo
am; l
fs –
loam
y fi
ne
san
d;
70
Figure 1-18. June 13, 2016 boring at Stop 1B showing contact with unoxidized zone (BC’ over Cgse) at 322 cm.
71
0.00
0.20
0.40
0.60
0.80
1.00
1.20
3
3.5 4
4.5 5
5.5 6
6.5 7
050
100150
200250
300350
400450
pH - Sept 1995
pH - O
ct. 1998
pH June 2016
CRS
pH
%CRS
Depth (cm)
Figure 1-19. Percent CRS and pH measured above and below the boundary between
the oxidized and unoxidized zones in a boring near to Stop 1B. The pH measurements
were made in the Fall of 1995, in the Fall of 1998 and on June 13, 2016. The 3 borings
were probably made within 30 m of each other.
72
References - Mid-Conference Tour
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Coppock, C. & Rabenhorst, M. C. 2003. Subaqueous soils of Rehoboth Bay, DE: Soil mapping in a coastal lagoon. In: Annual Meeting Soil Science Society of America. Denver, CO.
Demas, G. P. 1998. Subaqueous soil of Sinepuxent Bay, Maryland. Ph.D. Dissertation. In: Natural Resource Sciences and Landscape Architecture. University of Maryland, College Park.
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