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Recent studies show that an industrial byproduct initially thought to contribute to the toxicity of Baltimore Harbor sediments is not to blame.
Because the studies established that chromium is not responsible for the harbor bottom being harmful to aquatic life, MDE is proposing to “delist” the pollutant as an impairing substance for the water body’s sediment. Meanwhile, the Department is working to determine whether other metals are responsible for the toxicity of the harbor sediment.
Chromium naturally exists as two species: hexavalent chromium (Cr (VI)) and trivalent chromium (Cr(III)). Cr (VI) is highly toxic and soluble, remaining in the water column and the water occuping the spaces between sediment particles. Cr (III) is relatively non-toxic and insoluble, capable of being absorbed into sediment where it is not available for ingestion or respiration by sediment-dwelling organisms.
Chrome ore processing residue (COPR), which was a by-product of the chrome processing and manufacturing plant operated by Mutual and Allied Chemical for more than 140 years in Baltimore City, is a significant source of chromium within the Baltimore Harbor. The plant ceased operation in 1985 and is currently being redeveloped as a commercial and residential property known as Harbor Point. The COPR material was used extensively as fill throughout the Baltimore Harbor watershed.
Waterbodies are listed as “impaired” by a specific pollutant in Maryland’s Integrated Report of Surface Water Quality. The Integrated Report is updated and submitted to the U.S. Environmental Protection Agency bi-annually. MDE assesses water quality data through a comparison with water quality standards to determine if a waterbody is impaired and then lists them in Category 5 of the Integrated Report. The federal Clean Water Act requires all states to develop TMDLs, which are often referred to as “pollution diets,” for Category 5 listings. The Northwest Branch, which includes Baltimore Harbor, and Bear Creek portions of the Patapsco River were added to Category 5 of MD’s Integrated Report as impaired for chromium in sediments in 1998.
A Water Quality Analysis (WQA) may be developed if new evidence, such as recently collected water quality data, demonstrates that an impairment no longer exists. Under this scenario, development of a TMDL, “pollution diet”, is no longer required and a WQA is used to support “delisting” – moving the waterbody from Category 5 ("impaired" list) to Category 2 ("meets standards" list) of MD’s Integrated Report for the specified pollutant. In 2004, MDE submitted to the EPA a WQA that established that Cr (III) is the predominant form of chromium in Harbor sediments and is therefore not a source of toxicity. EPA supported the findings of this WQA but chose to defer a delisting decision until the completion of an additional study to identify the chemical responsible for Harbor sediment toxicity. Because that study was inconclusive, EPA did not approve the delisting of chromium.
Recently, the Johns Hopkins University Center for Contaminant Transport, Fate, and Remediation conducted three studies to assist MDE in addressing the chromium listings. The studies found:
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Benthic organisms live and thrive in sediment composed of pure Cr (III) precipitate
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Amphipods – small, shrimp-like crustaceans – indigenous to Baltimore Harbor were not affected by high levels of Cr (III) in sediments
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Cr (III) in sediments does not have the potential to oxidize to the highly toxic, Cr (VI) form even when sediments are re-oxygenated from dredging or natural events.
The findings of these studies support the conclusions of the original WQA that chromium is not a source of toxicity within Baltimore Harbor sediments. As a result, MDE has determined that chromium is not an “impairing” substance and should be delisted for Northwest Branch and Bear Creek in MD’s Integrated Report.
Still, it is clear that Harbor sediments are toxic to aquatic life. MDE will continue to investigate what chemical stressors are responsible for impacting the health of sediment dwelling organisms. In addition to chromium, lead and zinc remain listed as impairing substances in the sediments of the Baltimore Harbor. MDE is currently working to determine whether they are responsible for the existing toxicity. If they are pollution diets will be developed for these contaminants.
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