Abstract

Alluvial clay soil samples from six boreholes advanced to depths of 400–450 cm (top of limestone bedrock) from the Chattanooga Coke Plant (CCP) site were examined micromorphologically and geochemically in order to determine if pedogenic siderite (FeCO₃) was present and whether siderite occurrence was related to organic contaminant distribution. Samples from shallow depths were generally more heavily contaminated with polycyclic aromatic hydrocarbons (PAHs) than those at greater depth. The upper 1 m in most boreholes consisted of mixtures of anthropogenically remolded clay soil fill containing coal clinker, cinder grains, and limestone gravel; most layers of coarse fill were impregnated with creosote and coal tar. Most undisturbed soil (below 1 m depth) consisted of highly structured clays exhibiting fine subangular blocky ped structures, as well as redox-related features. Pedogenic siderite was abundant in the upper 2 m of most cores and in demonstrably historical (< 100 years old) soil matrices. Two morphologies were identified: (1) sphaerosiderite crystal spherulites ranging from 10 to 200 μm in diameter, and (2) coccoid siderite comprising grape-like “clusters” of crystals 5–20 μm in diameter. The siderite, formed in both macropores and within fine-grained clay matrices, indicates development of localized anaerobic, low-Eh conditions, possibly due to microbial degradation of organic contaminants. Stable-isotope compositions of the siderite have δ¹³C values spanning over 25‰ (+7 to −18‰ VPDB) indicating fractionation of DIC by multiple microbial metabolic pathways, but with relatively constant δ¹⁸O values (−4.8 ± 0.66‰ VPDB) defining a meteoric sphaerosiderite line (MSL). Calculated isotope equilibrium water δ¹⁸O values from pedogenic siderites at the CCP site are from 1 to 5 per mil lighter than the groundwater δ¹⁸O values that we estimate for the site. If confirmed by field studies in progress, this observation might call for a reevaluation of low-temperature siderite-water ¹⁸O fractionations. Investigations at the CCP site thus provide valuable information on the geochemical conditions under which siderite can form in modern soils, and thus insight on controls on siderite formation in ancient soils.