Preliminary assessment of chromium mobility within the state line serpentine barrens of southeastern Pennsylvania
Date
2023
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Publisher
University of Delaware
Abstract
The state line serpentine barrens are a prominent geological feature of southeastern Pennsylvania. In the 19 th century, the extensive exposures of ultramafic bedrock that span ~25 miles along the Pennsylvania-Maryland border were the world’s primary source of chromium ore. Although the last major mine in the area closed ~75 years ago, the remnants of dozens of former mines and prospects are still evident. Given the region’s history of chromium production, it remains potential source of chromium in surface and ground water regionally. For this reason, an assessment of chromium mobility within the state line serpentine barrens is needed. Water, soil, and bedrock samples were collected from several serpentine barrens in Chester County, PA. The elemental and mineralogical compositions of the solid samples were characterized by polarized light microscopy, X-ray fluorescence, X-ray diffraction, and energy dispersive X-ray spectroscopy. The compositions of surface water samples were obtained by inductively coupled mass spectrometry, ion chromatography, and alkalinity titration. Thermodynamic modelling of these waters indicated that chromium bearing oxides are favored to dissolve. However, measured concentrations of chromium in the surface did not exceed 1.7 ppb, well below the U.S. Environmental Protection Agency’s maximum contaminant level of 1 ppm. High resolution elemental maps of individual oxide grains magnetically separated from serpentine soil samples suggest that chromian spinel may be the dominant chromium- bearing oxide, though chromite and magnetite are also represented in the sample. The same maps also revealed a silicon rich rim around the perimeter of chromian spinel grains, which suggests that these grains may be inoculated against dissolution by a passivating surface mineral coating
Description
Keywords
Chromium, Environmental contamination, Scanning electron microscopy, X-ray diffraction, Chromium-bearing oxide, Bedrock