HYDROGEOLOGY OF ARABY BOG: A PRISTINE WATER SOURCE
By Tony Fleming, Indiana Licensed Professional Geologist #1675
Araby
Bog is a biologically diverse and largely undisturbed example of a rare type
of wetland known as a Magnolia Bog, or seepage bog. The bog is located in the lower Mattawoman Creek watershed of
northwestern Charles County, Maryland, and is unusual in being situated atop a
rolling, gravelly inland, at the head of a high valley that stands 100 or more
feet higher in elevation than the adjacent Mattawoman valley.
The sandy surface sediment and the soils derived from it are highly
permeable, and a large percentage of the landscape is level or gently sloping,
which allows nearly all of the precipitation that falls in the bog basin to
recharge the ground-water table. The reason for the existence of the bog in this location is
suggested by subsurface data from soil borings and local wells, which indicate
that the bog is probably situated over a buried hill on the eroded upper
surface of the Nanjemoy Formation, which is composed of permeable silty clay
that retards the leakage of water downward out of the overlying gravel.
The watershed of Araby Bog is approximately 75 acres in size, and all
of the shallow ground water within it discharges in the bog, where it
coalesces into a remarkably clear, cold outlet stream with an estimated
outflow from the bog of 100,000 gallons per day.
All of this water ultimately flows into the Chesapeake Bay via
Mattawoman Creek and the Potomac River. In
short, Araby Bog is an outstanding example of the type of wetland whose proper
function is critical to protecting and enhancing the Chesapeake Bay and its
tributaries.
All
of the ground water that feeds the bog recharges very locally, chiefly on
flat, gravelly terraces and divides at the head of the bog, and on several
sandy ridges that define the basin. All of these areas are currently forested,
which greatly enhances recharge and reduces surface runoff: Unfortunately, major parts of these recharge areas are in
imminent danger of dense residential development.
Though the state of Maryland has declared the bog a "wetland of
special concern", any protection afforded by this designation does not
extend beyond the confines of the bog floor to the recharge areas that are
critical to the bog's continued existence. This is akin to saving Niagara
Falls while allowing the St. Lawrence River to be diverted out of its channel
just upstream.
Dense
residential development in me bog-shed is likely to have the following
impacts:
A
bog that produces 100,000 gallons or more of pristine water every day is
precisely the type of wetland the state of Maryland has touted as being
crucial to the health of the Bay, yet the regulatory apparatus appears to be
oblivious or indifferent to the fact that the bog and its output of clean
water cannot be maintained without also maintaining the quality of the
watershed that feeds it. "Stormwater
management" is a common buzzword in the regulatory response to these
kinds of developments, but mitigation efforts based solely on engineered
approaches are fraught with scientific uncertainty, are difficult to prove to
be effective, have virtually no follow-up compliance monitoring to assess
their functionality, and are thus unlikely to blunt the effect of the
development on the wetland and water quality.
This is especially true of schemes that attempt to maintain
ground-water levels by directing stormwater to artificial recharge ponds,
particularly when the artificial recharge has to be directed via subsurface
flow to a precise point (i.e., the bog) at considerable distance away.
All evidence at this site indicates that natural recharge is spread
evenly across the landscape, and is not focused at a few discrete points, as
would be the case with artificial recharge.
Geologic evidence also indicates that there are very course gravel
lenses and other features embedded in the subsurface that act as preferential
conduits for ground-water flow, but it is virtually impossible to predict the
trends of these buried features, much less that a large amount of ground water
artificially recharged at a few points will actually flow to its intended
destination. Even subtle
modifications to surface topography during development can greatly alter local
shallow ground-water flow paths.
The
best outcome would be to avoid any development in the bog watershed, but if
development is to occur, it would be far preferable that it be based on large
(several acre) lots where the forest is largely left intact during
construction.
By Tony Fleming, Indiana Licensed
Professional Geologist #1675
Araby Bog is a biologically diverse Magnolia Bog located in
lower Mattawoman Creek watershed in northwestern Charles County, Maryland.
It is one of only a few remaining Magnolia Bogs, and probably the least
disturbed. The bog is located
approximately 1 mile south of Mason Springs and a similar distance immediately
southwest of the valley of Mattawoman Creek.
The location of Araby Bog and the surface watershed that defines its
basin are shown on the topographic map in figure 1.
Magnolia Bogs are acidic, somewhat fen-like wetlands that
occur on gravel terraces of the inner Atlantic Coastal Plain (Shetler, 2000;
Thomas, 2000); they are also called seepage bogs (Fleming and others, 2001).
Magnolia Bogs are supported primarily by ground-water discharge that
originates from adjacent sandy and gravelly uplands, and they in turn support
a characteristic and varied plant community (McAtee, 1918; Shetler, 1970,2000;
Fleming and others, 2001; Simmons and Strong, 2001).
This report describes the hydrogeologic setting of Araby Bog and the
potential impacts of impending residential developments in the watershed on
the hydrologic integrity of the bog. Fieldwork for this report was performed
in late February and late March, 2004.
The topographic setting of Araby Bog is shown in figure 1,
which also identifies some of the major landforms in and around the immediate
bog basin. Araby Bog occupies the
head of a short, unnamed tributary that flows north-northeast into Mattawoman
Creek. The main "bog",
as used here, is an approximately 6-acre reach along the uppermost part of
this tributary characterized by areas of standing water that coalesce with and
are traversed by several small, free-flowing stream channels.
The channels merge downstream into a unified main stem at the outlet of
the bog. Fringing the main bog
along most of its length are irregular, low-lying areas characterized by
abundant seepage, which are typically on the order of a few inches to 2 feet
higher in elevation than the main bog. With
a slight rise in elevation, these seepage areas blend into broad,
terrace-like, colluvial aprons that rise gently to the toe slopes of the
adjacent ridges. A small
tributary hollow joins the main bog near its outlet.
The tributary lacks definitive bog hydrology and vegetation, but it
does exhibit a few small seepage areas and a small, ephemeral stream channel
in its lowermost part.
Except at its narrow outlet, the bog is entirely surrounded
by topographically higher areas that range from just a few feet above the bog
floor to 160 feet above mean sea level (a.m.s.l) The most prominent of these fringing uplands are ridges
composed of sand and/or gravel, which define the limits of the surface
watershed at most places, and are present immediately adjacent to the lower
half of the bog. The head of the
bog (marked by a prominent spring) merges imperceptibly into a broad, dry,
virtually flat interfluve (divide) that extends southward almost 2,000 feet to
the head of a south-flowing ravine, and eastward about 1,000 feet to a
different bog at the head of another northeast-flowing ravine.
This same landform wraps around the southwest side of the bog, forming
a broad terrace-like embayment in the sandy ridge that fringes this edge of
the basin. The area occupied by
the bog (defined by standing water) and adjacent seepages is estimated to
comprise about 12 acres. The
total surface watershed of the bog is estimated to be approximately 75 acres.
The floor of the bog and the adjacent seepages range in
elevation from ~100 to 120 feet a.m.s.l, and thus stand between 90 and 100
feet higher in elevation than the valley of Mattawoman Creek a mile away at
its confluence with the unnamed tributary that flows from the bog.
The surface of the upland containing the bog typically is gravelly, and
its margins are strongly dissected and defined by many short, steep slopes
leading down to major drainages such as Mattawoman Creek.
The water table might be expected to be fairly deep in this
hydrogeomorphic setting, yet Araby Bog is a perennial
wetland, based on the characteristics of the soil and vegetation (Summons and
Strong, 2001), and on observations made by various investigators and nearby
residents at different times of the year (Rod Simmons, personal
communication). This relation is
suggestive of some sort of subsurface stratigraphic control (e.g., clay
layers) on the water table.
Regional Geologic Setting: The geologic interpretation of
Araby Bog presented below is based on several sources of information,
including: the 1:62,500 Geologic Map of Charles County (McCartan, 1989); the
1:24,000 Topographic and Geologic Maps of the Port Tobacco Quadrangle (Glaser,
1984); field observations made by the author in numerous soil pits and cores,
and small surface exposures; and 10 nearby well construction reports obtained
for this project from the Maryland Department of the Environment (Appendix A).
Both of the above geologic maps show the upland around the bog to be
underlain by upland gravel deposits (also known as "Brandywine
Formation" on older maps) of Pliocene age that range from a few feet to
50 feet thick. These upland
deposits represent stream deposits associated with the ancestral Potomac River
and consist chiefly of medium and coarse sand that is commonly quite gravelly.
The sand and gravel are composed almost entirely of siliceous rock
types resistant to chemical weathering, such as vein quartz, quartzite,
sandstone, and chert. Fine-grained
sediments such as silt and clay are reported to be only a minor component of
the upland gravels in this area, whereas the upper part of the unit is
reported to be composed at places of loamy sand and sandy loam.
Underlying the upland gravels are interbedded fine to medium sand
(greensand in part) and gray-green silty clay of the Nanjemoy Formation. As shown in the geological maps, the elevation of the contact
between the Nanjemoy and the upland gravel below the study area ranges from 90
to 110 feet a.m.s.l., which is basically the elevation of the bog floor.
The contact, however, is an erosional unconformity that was eroded by
running water for a period of 11-13 million years before the overlying gravel
was deposited, and so probably exhibits a fair amount of local relief at
places, especially where channels were cut into the Nanjemoy during deposition
of the overlying gravel unit.
Geologic Interpretation: A hydrogeologic map of the
bog and vicinity is presented in figure 3.
The upland areas adjacent to the bog are universally composed of sand,
with or without gravel. Several
of the ridges adjacent to the lower two thirds of the bog-shed are composed of
loamy sand that is mainly medium-grained and slightly clayey, but ranges from
coarse and slightly pebbly, to fine and silty.
The ridge between the main bog and the tributary hollow and the ridge
just north of the lowest part of the bog are good examples.
The loamy sand always occupies the hilltops, and is probably some kind
of low-energy stream deposit, such as overbank sediment derived from reworking
of the upland gravel by younger streams.
In contrast, coarse gravel and sand deposited by high-energy streams
underlies the uplands in the upper third of the bog and appears to extend
southward across the interfluve to the limits of the bog watershed.
Coarse gravel also forms the ragged ridges west of the bog.
Gravel in the 1-2-inch size is common in these areas, and some cobbles
up to 4 inches were observed. The active stream channels within the bog itself
contain a thin veneer of alluvium derived from this coarse material, whereas
toeslopes and adjacent terraces consist chiefly of colluvial fine and medium
silty sand with scattered fine pebbles.
As noted in the previous section, the relatively elevated
topographic setting of the bog appears to suggest that the seepage may be
related to the presence of a shallow confining unit, specifically one or more
bodies of fine-grained material below the floor of the bog.
Several lines of evidence obtained during this investigation indicate
that a thick, extensive body of silty clay underlies the sand and gravel
throughout most, if not all of the upland area that encompasses the bog
(figure 2):
1) several soil borings in the tower part of the bog
and tributary hollow penetrated massive, greenish gray, silty and sandy clay
within 0-2 feet of the surface;
2) small outcrops of this same material were observed
in the streambed, several hundred feet downstream from the outlet of the bog.
3) an old earthen dike near the bog outlet, now
breached, is composed chiefly of clay, which appears to have been locally
derived;
4) all of the well records examined (Appendix A)
report a very robust clay unit immediately below the surface sand and gravel
The clay is variously described as "gray clay", "green
clay", "blue marl", and/or "clay and sand", ranges
from 30 to more than 50 feet thick without interruption by any large sand
units, and overlies another 15-50 feet of "pink clay".
The well records indicate that the top of the clay sequence lies at
90-105 feet a.m.s.l. and is most commonly between 90 and 95 feet.
All of the well records have known locations that can be identified on
the topographic map (figure 1), however, the wellheads and their elevations
were not physically located in the field.
Therefore, the well elevations should be considered estimates with an
accuracy of ±10 feet (the contour interval of the topographic map).
5) the topographic map indicates that all of the
streams that originate on the northern part of this upland begin at similar
elevations (100-120 feet a.m.s.l.) This relation suggests that the clay
sequence is probably present and acting as a low-permeability layer below the
upland gravel throughout the map area.
Based on the regional geologic information, the silty clay
units described above are interpreted to be the top of the Nanjemoy Formation,
which apparently consists chiefly of fine-grained sediment in this area.
The top of the silty clay is also interpreted to slope gently to the
north-northwest, because it appears to be somewhat higher in several borings
in the bog as compared to the well records to the north and west (figure 2).
This suggests that the bog may overlie a buried hill on the eroded
surface of the Nanjemoy Formation. Additional well records to the south and
east of the bog need to be field located and evaluated to confirm this
hypothesis, however. The
underlying "pink clay" described in most of the well records in
Appendix A is probably the reddish Marlboro Clay (Glaser, 1984).
The upper part of the soil profile was described at 18
locations, along three transects (figure 1) oriented to maximize exposure to
different positions in the landscape. Transects
A and C are perpendicular to the principal direction of drainage in the main
bog, whereas transect B lies longitudinally along the bottom of the tributary
hollow. The latter transect was
selected in order to better define the hydrology, soils, and role of the
tributary hollow. Otherwise, the
primary purposes of the soil descriptions were to identify the geologic parent
material, estimate water table depth, and observe the nature, distributions,
and depths of hydric soil indicators at various positions in the wetland
landscape. At all locations, a
pit approximately 18 inches in diameter and 12-16 inches deep was dug by hand;
a soil probe was used to extract soil cores below the base of the pits as
needed. In addition, small
excavations dug by the developer of the property were used to evaluate soil
conditions at two of the localities (profiles 9 and 11).
The full profile descriptions appear in Appendix B.
Hydric soil indicators were observed in all of the profiles
established in the floor of the bog, in several profiles in the lower reach of
the tributary hollow, and in the tower parts of some profiles on terrace/toeslope
positions near the main bog (figure 3)1. Typical hydric indicators found include: 1) thin surface muck
on sandy or silty mineral soil; 2) redoximorphic features such as mottling and
iron hydroxide precipitates on macropore surfaces; 3) gleyed, reduced, and/or
iron-depleted B horizons; 4) slight hydrogen sulfide odor; and 5) standing
water visible in the profile. Many
or all of the indicators were observed in profiles in the bog floor, whereas
indicators 2 and 3 were most common in the other landscape positions,
typically in the lowest parts of the profile examined.
In contrast, upland profiles typically exhibited intensely leached,
extremely acid profiles characterized by bright brown, red, and yellow colors,
and well developed Bt (argillic) horizons at places. The hydrogeomorphic relationships of soil profiles can be
visualized in the cross sections in figure 4.
The hydrodynamics of Araby Bog are primarily a function of
the site geology, topography, and soils.
The water-table configuration and interpretation of ground-water flow
directions presented in figures 3 and 4 are based on these factors as well as
on observations of surface water elevations and water-table depth inferred
from the soil profile characteristics and locations of seepage and springs.
As described below, Araby Bog appears to be entirely ground-water fed,
and could, therefore, be considered an unusual type of mineral-poor fen.
Since fens are typically defined by their mineralized status, however
(e.g., Tiner, 1999), it is probably best to characterize Araby as an acidic
seepage bog (e.g., Fleming and others, 2001).
The soils in this area are intensely leached of their bases, and
consist chiefly of silica; iron, manganese, and a small amount of sulfur
appear to be the chief ground-water solutes.
Therefore, the ground water contributes little in the way of minerals
or nutrients to the bog.
Ground-water flow is focused almost entirely in the upland
sand and gravel, and is primarily in a south-to-north direction, parallel to
regional surface topography and local drainage. Vertical flow below the upland gravel is severely constrained
by the underlying Nanjemoy Formation and Marlboro Clay, which collectively
form a major confining unit that greatly limits downward leakage from the
shallow flow system. Instead,
ground water tends to flow horizontally along the top of the underlying silty
clay, essentially acting as a perched system2.
Available evidence suggests that the top of the silty-clay confining
unit slopes gently to the north and west below the bog, further amplifying the
northerly ground-water flow direction in the overlying sand and gravel. If
the well records (#'s 1 and 2, Appendix A) at the two homes adjoining the west
side of the bog are correct, a significant sand-and-gravel-filled channel has
been entrenched into the top of the Nanjemoy Formation below the area just
west of the bog.
All available topographic, geologic, soils, and hydrologic
evidence collectively points to the following hydrogeologic model for Araby
Bog. Rainfall and snowmelt seep
into the permeable surface sediment along the forested upland areas that flank
the bog. Judging from the near
total absence of ravines and gullies on even the steepest ridges, it appears
that a large percentage of total precipitation that falls in the basin
recharges the water table, rather than generating overland runoff.
The percolating soil water moves downward until it encounters the water
table perched on the top of the underlying confining unit.
The shape of the water table below the basin is expected to be a
subdued replica of the land surface above it, with a gentle slope away from
the surrounding uplands and towards the low area occupied by Araby Bog and the
tributary hollow. Ground water
flows down the slope of the water table, away from the uplands where it
recharges and towards the seepage areas around the bog, generally following
the slope of the land surface. As the elevation of the land surface diminishes toward the
bog (and/or the top of the underlying confining unit approaches the surface),
the water table becomes increasingly shallow and eventually intersects the
land surface, resulting in the numerous diffuse seeps and springs that
characterize the area. Shallow
soil borings taken at several places in the lowland close to the bog provided
abundant evidence of this process in the form of saturated runny sand and silt
and strongly gleyed (hydric) soil colors; at somewhat greater distances from
the bog, the soil showed heavy mottling, indicating the frequent presence of a
seasonally high water table. The
seepage coalesces into several small, perennial, gravel-bottomed stream
channels that crisscross the main bog. These
streams gradually merge into a single, well-defined channel near the outlet of
the bog. The surface outflow from
the bog appears to be on the order of 100,000 gallons per day (~70 gallons per
minute), and is probably greater when shallow ground-water outflow is also
considered. The water at the
outlet is crystal clear, with no suspended sediment or other characteristics
(e.g., off color or odor) that would indicate anything other than pristine water quality. This is not surprising given the undisturbed, forested
landscape of the watershed. The
bog thus provides a significant contribution of clean water to the Mattawoman
Creek watershed, and ultimately to the Chesapeake Bay ecosystem
Protecting
the source of the ground water that discharges into the bog is of utmost
importance to the long-term integrity of the ecosystem.
All other things being equal, the intensity of ground-water recharge is
primarily a function of the permeability of the soil and near-surface
sediment, and the surface slope, with much greater amounts of recharge
occurring on flatter areas underlain by coarse soil and sediment, and
relatively less taking place on steep slopes and/or poorly permeable soil and
sediment. The two most favorable
locations for ground-water recharge in the Araby basin are: 1) the broad,
nearly flat interfluve that occupies a large area above the head of the bog,
and is underlain by coarse gravel, with a shallow to moderate (5-20 feet)
water table depth, and 2) the terraces that fringe the bog, especially along
the west side, which are underlain by sand and are also relatively gentle.
It is no coincidence that the majority of seeps and springs are
concentrated adjacent to those areas. Additional,
significant ground-water recharge also is associated with the sandy ridges
throughout the basin, as evidenced by the fact that many of the ridges that
directly abut the wetland are characterized by seeps at their bases.
In view of the highly permeable appearance of the gravel, the travel
time to the bog of ground water that recharges in the most distant parts of
the basin is probably only a matter of weeks.
A large amount of recharge is also likely to occur directly in the
flat-bottomed tributary hollow, which has a sandy or gravelly surface layer,
shallow water table, and poorly developed surface drainage. Shallow ground
water moves down this hollow and discharges into the lowermost part of the bog
near its outlet3.
Nearly all
of the aforementioned recharge areas are covered by mature forest, which is
estimated to increase ground-water recharge by a factor of 1,000 to 10,000
over surface runoff, relative to other land uses (Freeze and Cherry, 1979;
Fetter, 1992). A crucial factor
in this watershed is that the development is proceeding by removing 100% of
the forest cover. The residential
developments occurring in the bog shed (figure 5) are concentrated in
hydrogeomorphic units G5 and S4 (figure 3), and so will result in a
significant percentage of impervious surfaces, grass, and other non-forest
land cover in favored recharge areas. The
wide distribution of seeps along the length of the bog is explained by the
even distribution of recharge over the sandy landscape.
Attempts to mitigate runoff from development frequently employ various
forms of seepage ponds, catchments, and the like, which focus unnaturally
large amounts of recharge at specific, isolated points in the landscape.
Even if these schemes succeed in maintaining the overall quantity of
recharge, they change the way recharge is distributed over the landscape and
will likely alter greatly the ground-water flow patterns in the basin in ways
that cannot be entirely foreseen. This
is because the shallow flow system, and the subsurface geology that controls
it, are not resolved in sufficient detail to predict the impacts of altering
the distribution of recharge (rates) in the landscape.
One outcome that seems virtually certain is that the ground water
recharge along the east and northeast sides of the basin, where major
development is planned, will be significantly reduced in volume, i.e., the bog
will lose a major, but currently unquantified, amount of its total
ground-water budget. Another
potential, and closely related, outcome is that certain seeps along the bog
will likely dry up, whereas others may greatly expand (or new seeps will
appear), as the artificially recharged ground water finds preferential flow
paths along beds or zones of high hydraulic conductivity (e.g., very coarse
gravel lenses). In an extreme
case, too much ground-water discharge concentrated in a small area could
overwhelm the ability of the wetland to diffuse surface flows, and result in
greater channelization and downcutting of the streams that cross the bog.
Also, any increase in overland runoff directly entering the bog floor
(e.g., stormwater management schemes in adjacent developments) will probably
greatly alter the delicate hydrologic balance that exists in the wetland by
enlarging the surface streams and causing them to downcut into the wetland,
thereby lowering the water table and seriously compromising the hydrologic
continuity of the bog.
Fetter,
C.W., Jr., 1992. Applied Hydrogeology. Chas Merrill Publishers, Toronto. 488
pp.
Fleming,
G.P., Coulling, P.P, Walton, D.P., McCoy, K.M., and Parrish, M.R. 2001.
The natural communities of Virginia: classification of ecological
community groups: Natural Heritage Technical Report 01-1. Virginia Dept. of
Conservation and Recreation, Division of Natural Heritage. Richmond, VA
Unpublished report.
Freeze,
R., and Cherry, J., 1979. Groundwater. Prentice
Hall, Inc. Englewood Cliffs, N.J. 604 p.
Glaser,
J., 1984. Geologic Map of the Port Tobacco Quadrangle, Prince Georges and
Charles Counties, Maryland. Maryland Geological Survey. Scale 1:24,000.
McAtee,
W.L., 1918. A sketch of the natural history of the District of Columbia.
Bulletin of the Biological Society of Washington No. 1.
McCartan,
Lucy, 1989. Geologic Map of Charles County, Maryland. Maryland Geological
Survey. Scale 1:62,500.
Shetler,
S.G., 1970. The Suitland Bog. Atlantic Naturalist, v. 25, p. 65-68.
Shetler,
S.G., 2000, List of plants recorded in or near Araby Bog. Unpublished report.
Simmons,
R.H., and Strong, M.T., 2001. Araby Bog—a globally rare Magnolia Bog in
Charles County, Maryland. Unpublished report.
Thomas,
L.K. Jr., 2000, The Araby Bog. Unpublished report.
Tiner, R.W.,
1999, Wetland Indicators. CRC-Lewis Press, Boca Raton, FL, 392 pp.
1-The
floor of the main bog is mapped as the Fallsington sandy loam in the Charles
County Soil Survey. This soil
series is listed as a hydric soil on the state and national lists maintained
by the NRCS. The sandier hydric
profiles noted above resemble this series, and generally correspond to unit G1
on figure 3, whereas the soils found on unit C1 are mostly much heavier in the
B horizon.
2-The
degree of saturation of the confining unit itself or sand units below it, is
unknown, so it cannot be stated with certainty that the shallow system at
Araby is truly perched, though water levels in deep confined aquifers in this
area are tens or hundreds of feet below the base of the confining unit,
suggesting that a perched condition probably does exist.
3-The
hydrogeologic role of the ridge that separates this hollow from the main bog
is uncertain. The surface sediments along the ridge appear to consist of fine
loamy sand with little gravel, but it is conceivable that the ridge may be
composed of gravelly coarse sand at depth. The steep sides and narrow crest
suggest that a considerable amount of the precipitation that falls on it may
run off into the adjacent hollows, which would create transient ground-water
mounds along the toestopes of the ridge as the runoff infiltrates the water
table below. But if little
recharge occurs directly below the ridge, then during periods of low rainfall,
no water table mound, or divide, may be associated with the ridge, and ground
water could flow beneath it from the tributary hollow, which is slightly
higher in elevation, to the main bog. A more precise interpretation of this
feature requires much more detailed ground-water monitoring beyond the scope
of the present study.