Bog Iron Formation in the Nassawango Creek Watershed, Maryland, USA

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Bog iron formation in the Nassawango Creek watershed, Maryland, USA

O. P. Bricker, W. L. Newell & N. S. Simon U.S. Geological Survey, USA

Abstract

The ground water of the Pocomoke River basin is rich in reduced iron. This is particularly true in the Nassawango Creek sub-basin where bog iron deposits along the flood plain of the Nassawango Creek were stripped in the mid-1800’s to supply an iron smelter near the town of Snow Hill, Maryland. The rate of bog iron formation was so rapid that areas could be re-stripped in a matter of few years. 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, 2) the iron oxyhydroxides precipitating in the rivers cause 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 two 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. The materials that cause turbidity are internally generated in the rivers and are not contributed by runoff. Any practice recommended to reduce suspended sediment in these waters must take internally generated sediment into consideration. Best management practices for sediment control in the watershed will have no effect on the turbidity generated by internal processes. Keywords: bog iron, ferrous ion, ferric ion, redox, floodplain, phosphorous, turbidity, ferric oxyhydroxide.

1 Introduction

Colonial American iron works were developed at many locations across the middle Atlantic Coastal Plain. One was located in eastern Maryland on Nassawango Creek, a tributary of the Pocomoke River, Figure 1. The furnaces refined iron from rapidly accumulating deposits of bog ore, commonly goethite. Today, the chemistry of the groundwater and surface water discharging into black water streams periodically creates turbidity events that limit light transmissivity in the water, impacting ecological function. These events are particularly effective during drought when clastic sediment input is essentially nil. This natural phenomenon can inhibit the efficacy of various land-use practices intended to limit the impact of sediment loads on aquatic biota.

Figure 1: Index map showing location of Pocomoke River watershed on the Delmarva Peninsula of the Atlantic Coastal Plain (derived from an unpublished map of the surficial geology and geomorphology of the Atlantic Coastal Plain by W.L. Newell and others, 2002).

2 Description of study area

The Pocomoke River drains 710 sq. km. of the Delmarva Peninsula into the Chesapeake Bay. All elevations in the watershed are less than 25 meters and the mean elevation is about 8 meters; much of the land surfaces are low relief, poorly drained, paleo-estuarine terraces. The Pocomoke is a sluggish, low gradient blackwater river bordered by extensive tidal and forested riparian wetlands. Tidal influence extends nearly half way up the river. In recent decades, agricultural practices have developed more than 1800 linear kilometers of

Geo-Environment, J. F. Martin-Duque, C. A. Brebbia, A. E. Godfrey & J. R. Diaz de Teran (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-723-X Geo-Environment 15 drainage ditches in the Pocomoke watershed. Natural sediment storage and nutrient mitigation in riparian wetland buffer zones is now by-passed and the water quality of the downstream estuary is impacted. Nassawango Creek is a major Pocomoke tributary on the western side of the valley. An abandoned colonial iron furnace is sited in the middle reaches of Nassawango Creek. The entire economy of the furnace was locally derived. Charcoal was made in the surrounding forests, shells for lime flux were barged up the creek and the bog ore was stripped from the flood plain bottoms. As in other Atlantic Coastal Plain bog-iron works, it was widely recognized that groundwater springs, seeps, and shallow wells were charged with dissolved ferrous iron that precipitated in the oxidizing environment at the surface, providing a renewable resource. Today, the Nassawango bog ore continues to accumulate in the forested wetlands on the floodplain of the creek. Ongoing work documenting the flux of sediments and nutrients in the Nassawango valley has revealed a reach of the creek where the provenance, deposition, reworking and concentration of these bog ores, valuable during the colonial period, can be set forth within the context of the hydrogeomorphic system.

Figure 2: Geologic map of Pocomoke River watershed (Source: Owens and Denny [1, 2]; Mixon et al. [3]).

3 Geology and geomorphology

Figure 2 shows the distribution of Neogene and Holocene surficial deposits that characterize the Pocomoke Valley. The uplands are underlain by deeply weathered arkosic fluvial sand and gravel of the Pensauken Formation, part of an enormous Miocene delta that forms the core of the Delmarva Peninsula. The sand and gravel cap is the recharge area for most of the groundwater of central Delmarva. Much of the upland area, especially in the Nassawango headwaters, is mantled by two to six meters of wind blown sand of the Parsonsburg Formation; it occurs as a continuous blanket and as isolated dunes. The wind blown sand is porous, providing rapid infiltration of meteoric water, and contributes to the groundwater the Nassawango system. The upland core of Delmarva is bordered by coast-wise scraps and terraces, created during Pleistocene sea level high- stands. The Omar and Kent Island Formations are part of this sequence. Locally, bay bottom sand and mud of these formations occurs in restricted, reducing, estuarine environments rich in organic material and sulfides. Apparently, the paleo-estuarine substrate of the Nassawango watershed concentrated dissolved iron from leaching of the Pensauken gravels. Iron may also have been derived and deposited from older Miocene and Pliocene deposits beneath the Pensauken; the subsurface distribution of these materials and the groundwater flow paths is not well defined.

4 Hydrology

Baseflow of the Nassawango Creek is fed primarily by water from the unconfined aquifer; other lesser sources include stormflow, shallow soil water, and surface runoff. Dating of groundwater at localities farther north on the Delmarva Peninsula suggest that groundwater contributing the baseflow of rivers in similar areas ranges from less than 10 to more than 50 years in age depending on the flow path of the water. (Dunkle et al. [4]; Ekwurzel et al. [5]; Böhlke and Denver [6]). During August, 1998, the Pocomoke River and its tributaries presented near record low discharges but were quite turbid with light tan particulate matter. The summer had been dry with no storms that would have flushed sediment into the river. Filtrates of suspended sediment were collected from the Pocomoke River and Nassawango Creek and examined using x-ray diffraction for mineral identification. The suspended material from both sites exhibited weak goethite patterns indicating that the source of turbidity was precipitates. Water in the rivers at that time was entirely baseflow fed by iron rich groundwater. The ferrous iron oxidized on exposure to the atmosphere at the river surface, creating ferric oxyhydroxides and the observed turbidity. Upper reaches of the river were clear and the zone of turbidity corresponded to the outcrop pattern of paleo- estuarine terrace sediments in the watershed. The extreme low water stage of the Nassawango also presented an opportunity to observe the sedimentology and stratigraphy of the Holocene flood plain deposits as substrate and habitat for the accumulation of the bog ore (Figure 3).

Figure 3: a) Accumulation of “bog ore” in the vadose zone of modern alluvium on the flood plain of Nassawango Creek downstream from gaging station on Highway 12 west of Snow Hill, MD. Note: 1 – pre-colonial alluvium, 1600 AD; 2 – post-colonial alluvium; 3 – surficial alluvium, post-ditching; b) Exposure of Nassawango Creek flood plain stratigraphy during extended period of near record low flow (July- September,1999).

5 Mineralogy

Commonly, bog ore consists of a mixture of ferric oxyhydroxide minerals including goethite, lepidochrosite, and ferric hydroxide (commonly called limonite). All of these compounds contain iron in the trivalent state. We found that the deposits in the Nassawango, in addition to the ferric oxyhydroxides, contain magnetite which contains both ferric and ferrous iron. This appears to be the first reported occurrence of low temperature formation of magnetite in bog ore deposits on the Mid-Atlantic Coastal Plain. The photomicrograph (Figure 4a)

Geo-Environment, J. F. Martin-Duque, C. A. Brebbia, A. E. Godfrey & J. R. Diaz de Teran (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-723-X 18 Geo-Environment shows ore from the period of furnace operation. The dominant mineral, as determined by x-ray diffraction, is goethite. The macro-photograph shows recent bog iron from the flood plain of Nassawango Creek as shown in Figure 4b. Bog iron at this location is extensive and is exposed along the banks and tributaries at depths of several centimeters to tens of centimeters beneath organic-rich sediment.

Figure 4: a) Transmitted light photo-micrograph is about 0.5 cm on the vertical scale. Black areas of goethite may include magnetite that has been identified by X-ray diffraction of powdered ore. Light areas may be vugs or stained quartz; b) Samples of “bog ore” from Nassawango Creek show vugs lined with goethite around massive “ochre”.

In soft bog ore deposits of the Belgian Campine, Stoops [7] found goethite, vivianite, and siderite and pyrite in close proximity. The deposit was layered and the goethite grew in situ; siderite, vivianite, and pyrite occurred at depth in anoxic environments. De Geyter et al. [8] reported similar associations from Holocene and Neogene ores of northern Belgium. They attributed high

Geo-Environment, J. F. Martin-Duque, C. A. Brebbia, A. E. Godfrey & J. R. Diaz de Teran (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-723-X Geo-Environment 19 phosphorous contents of the Holocene ores to a vivianite source. As opposed to the Belgian bog ores, the bog ore at the Nassawango site is primarily goethite with some magnetite and amorphous ferric hydroxide (perhaps ferrihydrite). The samples have a high phosphorous content (up to 10 % by weight), but vivianite has not been observed in these ores. (Vivianite is potentially available in the Pleistocene estuarine muds of the substrate). In the modern agricultural environment, the high phosphorous content of the Nassawango ores is attributed to the scavenging of phosphorous by ferric oxyhydroxides. In ponds on the flood plain and in the river itself, ferric hydroxide (ferrihydrite) rapidly precipitates from ferrous-rich groundwater as the water emerges into the oxic surface environment, it then inverts to goethite. The magnetite has been observed in the cores of granules surrounded by goethite. Once the ore is exposed to the atmosphere, it oxidizes rapidly to goethite. It is not yet clear whether the magnetite forms from reduction of goethite when bog ore is buried in an anoxic environment after formation, or if it forms directly as ferrous-rich groundwater migrates toward the surface and encounters oxygen in the soil zone. The reactions for the precipitation and maturation of Nassawango bog ore are presented in Figure 5.

Figure 5: Reactions postulated to be taking place in the emerging spring waters.

6 Water chemistry

Water associated with bog iron formation in the Nassawango is undersaturated with respect to siderite and the phosphates and sulfides of iron, but ground water is very rich in dissolved ferrous iron. The groundwater samples that we analyzed contained up to 20 ppm Fe2+ and the Nassawango Creek baseflow had concentrations up to 5 ppm. These concentrations are well above saturation levels for Fe(OH)3 under the ambient conditions of the system and this compound precipitates and , with time, inverts to goethite. Plotted on Figure 6 (based on log K for Fe(OH)3 = 37.1 from Langmuir, [9]) are a number of Eh-pH measurements made in springs, seeps and residual ponds

Geo-Environment, J. F. Martin-Duque, C. A. Brebbia, A. E. Godfrey & J. R. Diaz de Teran (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-723-X 20 Geo-Environment on the flood plain of the Nassawango Creek. The majority of the measurements 2+ fall on the Fe aqueous - Fe(OH)3 solid boundary. Two of the measurements fall 2+ along the Fe aqueous - Fe(OH)3 aqueous boundary. It is not clear why these measurements respond to the aqueous-aqueous rather than aqueous-solid boundary since the settings for all of the measurements were similar and Fe(OH)3 was actively precipitating at all of the locations.

Figure 6: Eh-pH diagram showing measurements of a number of spring waters. Most waters are consistent with equilibrium between dissolved ferrous iron and solid ferric hydroxide (log K=-37.1). Solid lines represent an activity of total dissolved iron of 10-5; dotted lines represent an activity of total dissolved iron of 10-3.

7 Formation of the Nassawango Bog iron ore

The mechanism of formation of the bog ore on the Nassawango flood plain is probably similar to that of bog ore at other localities. Water from springs and seeps, high in dissolved ferrous iron, emerges at the surface where it encounters an oxygen-rich environment. The reduced iron is oxidized to ferric hydroxide that forms deposits and coatings downstream from the point of emergence under normal flow conditions. Floodwaters also carry iron-rich floc. After high waters inundate the flood plain of the Nassawango, the bottoms of residual ponds are covered with orange, colloidal iron oxyhydroxide. With desiccation, the oxide

Geo-Environment, J. F. Martin-Duque, C. A. Brebbia, A. E. Godfrey & J. R. Diaz de Teran (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-723-X Geo-Environment 21 darkens and hardens into a thin layer and converts to goethite. The next high water event erodes the flood pain, breaking this layer into millimeter-scale clasts. These grains are transported and concentrated in channel and flood plain deposits. Additional iron-rich water, wicked to the surface by capillary attraction, precipitates forming pisolitic crusts. There are two plausible hypotheses for the formation of magnetite. As the pisolitic material is buried and exposed to sub-oxic or anoxic conditions, some reduction may take place converting the oxyhydroxides to magnetite. The material is a porous, cemented agglomeration of the original clasts cored with magnetite and covered with web-like cement of goethite. Upon exhumation and exposure, the magnetite re-oxidizes to ferric oxyhydroxides which subsequently converts to goethite. Alternatively, as iron-rich groundwater migrates upward from sandy substrate, it encounters an environment enriched on O2. The first iron oxide compound to precipitate may be magnetite that further oxidizes to goethite as PO2 increases toward the surface.

Figure 7: a) Iron-rich groundwater discharge from spring on Nassawango Creek flood plain; b) Turbid baseflow discharge of Nassawango Creek during extended period of near record low flow (July-September, 1999).

As shown in Figure 7, a 20 cm layer of the bog iron has formed in the top of a coarse, clean white sand 50 to 75 cm below the surface of the Nassawango flood plain. This sand deposit is post-colonial agricultural sediment. Locally it is overlain by black, organic-rich muck containing scattered granules of iron oxides. Crerar et al. [10] investigated the biogeochemistry of bog iron in the New Jersey Pine Barrens. They found that the ores consisted primarily of goethite and ferrihydrite. Abundant microscopic evidence was presented for the presence of iron fixing bacteria in these ores; bacteria were considered essential to the precipitation of iron in that environment. Crerar et al. [10] believed that the rate of oxidation of Fe, without the influence of bacteria, would not be significant considering the pH and chemical composition of the waters, implying that without the effect of iron fixing bacteria, bog deposits could not form at significant rates. We see little evidence of iron bacteria either optically or by SEM at the Nassawango site. The formation of Nassawango bog ores is due primarily to inorganic oxidation when groundwater rich in ferrous iron emerges into the oxic, surficial environment.

8 Summary

• Studies of Nassawango ores in the modern environment have described the hydro-geomorphic setting for surficial geochemical processes. • Low temperature magnetite has been recognized in sedimentary iron ores of the Atlantic Coastal Plain for the first time. • Phosphorous sorption on bog ore was the bain of “good” iron-ware. Today, the process, providing a phosphorous sink, may be an unrecognized benefit for mitigating nutrient loading from agricultural lands. • Turbidity from precipitates can impact ecosystem function during periods of extreme low flow.

References

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[5] Ekwurzel, B., Schlosser, P., Smethis, W.M. Jr., Plummer, L.N., Busenberg, E., Michel, R.L., Weppernig, R, and Stute, M., Dating of shallow groundwater: Comparison of the transient tracers 3H / 3He, chlorofluorocarbons and 85Kr. Water Resources Research, 30(6), pp. 1693-1708, 1994. [6] Böhlke, J.K., and Denver, J.M., Combined use of groundwater dating, chemical, and isotopic analyses to resolve the history and fate of nitrate contamination in two agricultural watersheds, Atlantic Coastal Plain, Maryland. Water Resources Research, 31(9), pp. 2319-2339, 1995. [7] Stoops, G., SEM and light microscopic observations of minerals in bog- ores of the Belgian Campine. Geoderma, 30, pp.179-186, 1983. [8] De Geyter, G., Vandenberghe, R.E., Verdonck, L., and Stoops, G., Mineralogy of Holocene bog-iron ore in northern Belgium. Neues Jahrbuch fuer Mineralogie. Abhandlungen, 153, pp. 1-17, 1985. [9] Langmuir, D., Aqueous Environmental Geochemistry, Prentice-Hall, Inc., 1997. [10] Crerar, D.A., Knox, G.W., and Means, J.L., Biogeochemistry of bog iron in the New Jersey Pine Barrens. Chemical Geology, 24, pp. 111-135, 1979.