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

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. 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 14 Geo-Environment 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]). 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 16 Geo-Environment 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). 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 17 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.

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