By Owen P. Bricker, Wayne L. Newell, and Nancy S. Simon
U.S. Geological Survey
Open-File Report 03-346
The accompanying report was presented at a poster session at the Gordon
Conference on Catchment Science: Interactions of Hydrology, Biology and Geochemistry ,
July 2003, Colby-Sawyer College, New Hampshire.The study was conducted in cooperation by the
USGS Eastern Earth Surface Processes Team and Water Resources National Research Program.
Bog iron deposits occur at a number of localities in the Pocomoke River basin
(Figure 1) (Singewald, 1911). The most extensive deposits are situated
along Nassawango Creek northwest of Snow Hill, a town on the Pocomoke River. After the
discovery of these deposits an iron furnace was built in 1830 on the west side of Nassawango
Creek, five miles northwest of Snow Hill, at a location known as the Furnace. The furnace
exclusively smelted bog iron obtained along Nassawango Creek for a mile or so north of the
furnace site; iron was produced from 1830 to 1850 at a rate of approximately 700 tons per year.
Smelting technology of that period used mollusc shells for lime flux and locally burned charcoal
for fuel; they were unable to remove phosphorous and the iron was rendered brittle when cold
("cold short") because of the high phosphorous content inherited from the ore. For that reason,
and because of the limited size of the deposits and the discovery of the much larger and higher
quality ore deposits in other parts of the country, production ceased about 1850. The bog iron
deposits have been forming throughout the Holocene and are still forming today. Singewald (1911)
stated, that their chief interest today lies in the fact that the deposits are now forming and
at such a rate as to be observable. "Deposits which were once exhausted are again workable
after an interval of a few years" (for more on rate of formation of bog ore at different
locations see Starkey, 1962 and Moore, 1910). In general, bog ores consist primarily of iron
oxyhydroxides, commonly goethite (FeO(OH)). The ores in the Nassawango, in addition to goethite
and other ferric oxyhydroxides, contain a significant amount of magnetite.
Figure 2 shows the surficial deposits in the Pocomoke River watershed. Sandy,
quartz-rich barrier beach type deposits occupy east (ocean) side of the watershed. To the west
are organic-rich fine sand to clayey silt estuarine/back bay deposits.
Figure 3 is a map showing the distribution and extent of ditching in the Pocomoke
River basin. Drainage ditches have been dug since colonial times. However, most of the ditches
were dug during the middle of the 20th century and are actively maintained. There is more ditching
in the areas of fine grained sediment where natural drainage is poorer. The water table in the
basin before ditching was close to the surface and interfered with agricultural practices. Ditching
lowered the groundwater table, but caused bypassing of the natural riparian wetlands, swamps and
tidal marshes where processing of nutrients and trapping of sediment occurred. Prior to ditching,
there were approximately 800 miles of natural drainage through areas that permitted long contact
time of water with environmental materials. Ditching created an additional 1200 miles of water
ways and substantially decreased the contact time for the waters with natural materials that mitigated
nutrient concentrations and trapped sediment. The shorter residence time may limit the concentration
of dissolved iron. By contrast, the Nassawango Creek has long reaches with natural channeling and
limited ditching along the tributaries. This original configuration may result in the apparent
higher concentration of dissolved iron and the thicker present day accumulation of ore.
Geomorphological models of bog iron accumulation in the Nassawango Creek watershed
are shown in Figure 4.
Iron-bearing groundwater typically emerges as a spring (Figure 5). The iron is
oxidized to ferric hydroxide upon encountering the oxic environment of the surface. "Bog ore"
often combines goethite, magnetite and vugs or stained quartz. It is not clear whether the magnetite
precipitates upon first contact with oxygen, then oxidizes to ferric compounds, or whether the
ferric compounds are reduced when exposed to anoxic conditions upon burial beneath the sediment
surface and reoxidized upon exhumation at the surface (Figure 6).
Figure 5 and Figure 6
Study of the reduction-oxidation potential of spring waters in the watershed
showed that most waters are consistent with equilibrium between dissolved ferrous iron and solid
ferric hydroxide (log K=-37.1). Two of the measurements (Figure 7) are consistent with equilibrium
between dissolved aqueous iron species. It is not known why the electrode responded to the
dissolved species in these two cases rather than the dissolved-solid equilibrium that most
measurements reflected, since all of the springs looked similar in physical characteristics.
The study has shown that Pocomoke River tributaries in the central part of the
watershed exhibit turbidity even during dry years (Figure 8). The turbidity results primarily
from iron oxyhydroxide floc due to precipitation of ferric compounds as ferrous-rich ground
water emerges as river base flow. Note that the internal generated turbidity has nothing to do
with runoff and sediment transport from upland areas.
The ground water of the Pocomoke basin is rich in reduced iron. This is
particularly true in the Nassawango sub-basin where bog iron deposits along the flood plain of
Nassawango Creek were dug in the mid-1800's to supply an iron smelter near the town of Snow Hill.
The rate of bog iron formation was so rapid that areas could be re-mined in a matter of few
years (Singewald, 1911). Bog iron is still forming in this area, and in other parts of the
Pocomoke basin. Ground water has been measured with ferrous iron concentrations in excess of
20 ppm. When this water emerges at the surface or is discharged into the river system it rapidly
oxidizes to an amorphous particulate iron oxyhydroxide which in time crystallizes to goethite.
The iron in this system is important for at least two reasons: 1) iron oxyhydroxides strongly
sorb phosphorous and many trace metals. Early reports on the composition of the Nassawango bog
ore indicate that it commonly contained 10% P which made the pig iron smelted from this ore
brittle when cold (Singewald, 1911); 2) the iron precipitating in the rivers causes turbidity
which reduces light penetration to rooted aquatic vegetation and may impact other organisms,
for instance, by coating gills and interfering with oxygen transfer. The first effect will play
a role in the behavior and cycling of P in the system, while the second effect will impact
biota in the system. In the fall of very dry years (1999 and 2001), we found the rivers in the
central part of the Pocomoke basin quite turbid although there had been no storms to wash
sediment-laden runoff into the rivers. Samples of the particulate matter creating the turbidity
were iron-rich and displayed a weak x-ray diffraction pattern of goethite. There also seemed to
be some organic material, probably algae, contributing to the turbidity, but this has not yet
been investigated. Whatever the mix of materials that cause the turbidity, they are authigenic
in the rivers and are not contributed by runoff. If all of the sediment erosion and runoff could
be eleminated, it would have no effect on the turbidity generated by these chemical processes.
Any practice recommended to reduce suspended sediment in these waters must take authigenic
precepitates into consideration. Best management practices for sediment control in the watershed
will have no effect on the turbidity resulting from oxidation of ferrous ions. Our project
studies at other locations in the Chesapeake Bay have observed that vivianite
(Fe3(PO4)2* 8H2O) is forming in sediment at heads of
tributary estuaries where tidal water is non-saline and anoxic. Presumably, the phosphates
originate from degradation of organic materials in the anoxic environment. Vivianite is
commonly encountered in modern sediments we have cored from similar depositional environments.
Vivianite in the estuarine sediments of the Kent Island Formation and Omar Formation may be the
source of phosphates in early bog iron that was first mined in Nassawango Creek.
Moore, E.J. 1910. The occurence and origin of some bog iron deposits in the district of Thunder Bay, Ontario. Economic Geology 5, p. 528-537.
Mixon, R.B., Berquist, C.R. Jr., Newell, W.L., and Johnson, G.H. 1989. Geologic map and generalized cross sections of the Coastal Plain and adjacent parts of the Piedmont, Virginia. U.S. Geological Survey Miscellaneous Investigations Series Map I-2033, 3 sheets.
Owens, J.P., and Denny, C.S. 1978. Geologic map of Worcester County. Maryland Geological Survey, Baltimore, MD.
Owens, J.P., and Denny, C.S. 1984. Geologic map of Somerset County. Maryland Geological Survey, Baltimore, MD.
Singewald, J.T. Jr. 1911. Report on the iron ores of Maryland. Maryland Geological Survey Special Publication, Volume IX, Part III. The Hopkins Press, Baltimore, 337 p.
Starkey, J.A. Jr. 1962. The bog ore and bog iron industry of south New Jersey. N.J. Acad. Sci.
7, p. 5-8.
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For questions, please contact Wayne Newell or Owen Bricker.[an error occurred while processing this directive]