Characterizing the geogenic background of the Midwest: a detailed mineralogical and geochemical investigation of a glacial till in southwestern Ohio

The state of Ohio has a long history of industrial pollution over numerous decades. Scientific investigations of the sources and impacts of this pollution are becoming increasingly common, particularly in the southwest region of the state, with a strong focus of recent studies being carried out within the context of air, urban soil and sediment quality, and aquatic environments. A key supporting issue in these, and future pollution investigations, is understanding the contribution from, and background concentrations of, natural geogenic materials. A glacial till sampled from Peffer Park, a publicly accessible site in Butler County southwest Ohio, was studied in detail to characterize its mineralogical and geochemical properties. The till sampled, a result of Wisconsin-aged glaciation, represents the regionally extensive glacial drift across the region and is the ideal candidate for targeting in efforts to quantify the background, geogenic environment. Samples were characterized for their elemental abundances using ICP-OES and ICP-MS, while mineralogy was investigated using powder X-ray diffraction, transmission electron microscopy, X-ray computed tomography, and reflective spectroscopy. Chemically, the till sampled is akin to the Earth’s bulk continental crust with characteristic light REE (LREE) enrichment and depleted middle/heavy REE signatures. Of the 39 minor and trace elements analyzed, 74% (n = 29) vary by less than 10 ppm between the 20 sites sampled, and of the ten major element oxides, there is a < 3 wt. % variation with a < 1 wt. % variation observed for seven of the major element oxides. All standard deviations (reported at 2σ) are < 1. Major minerals detected by powder XRD include illite, chlorite (ripidolite), quartz, calcite, dolomite and feldspar minerals with lesser amounts of amphibole, consistent with inferences from bulk chemistry. X-ray computed tomography on a selected sample highlights five orders of magnitude variation with respect to particle size, which is demonstrably skewed toward fine particle size. Accompanying reflective spectral features are dominated by signatures from the high clay mineral (e.g., illite) and carbonate contents, consistent with inferred source materials from the regionally extensive Ordovician stratigraphy associated with the Cincinnati Arch and the ancient crystalline continental basement of Canada. Through a wide variety of analytical techniques, it is demonstrated that the Peffer Park till of southwest Ohio has a remarkably low variation in composition and is highly representative of the geogenic background of this region of the USA. Its characteristics reported here therefore justify its use and application as a broad environmental reference material for the Midwest.