Flood-induced transport of PAHs from streambed coal tar deposits.
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 mm in diameter, and (2) coccoid siderite comprising grape-like ‘‘clusters’’ of crystals 5–20 mm 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 dC values spanning over 25% (+7 to218% VPDB) indicating fractionation of DIC by multiple microbial metabolic pathways, but with relatively constant dO values (24.8 ± 0.66% VPDB) defining a meteoric sphaerosiderite line (MSL). Calculated isotope equilibrium water dO values from pedogenic siderites at the CCP site are from 1 to 5 per mil lighter than the groundwater dO 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 O 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. INTRODUCTION AND OBJECTIVES Siderite (FeCO3) is a mineral that has become increasingly recognized in paleosol (fossil soil) deposits, where it has been used to reconstruct stable-isotope paleohydrology and paleolatitudinal gradients in the isotopic composition of rainfall, especially during the Cretaceous period (e.g., Ludvigson et al. 1998; Ludvigson et al. 2002; Ufnar et al. 2001, 2002, 2004a, 2004b, 2004c; Suarez et al. 2009; Suarez et al. 2010; Robinson et al. 2010). Pedogenic siderite has also been used to infer past reducing environments associated with persistent soil saturation (e.g., Driese and Ober 2005; Ufnar et al. 2005). Unfortunately there is still little understanding of the physical, chemical, and biological conditions under which siderite precipitates in surface soils, and the published interpretations of isotope ratios in ancient siderite samples rest on assumptions that have not been verified in modern systems. The present work provides observations, analyses, and important implications of siderite recently discovered in contaminated soil in Tennessee. Here we report, for the first time, unequivocal occurrences of historical (, 100 year old) pedogenic siderite crystals, ranging from 5 to 200 mm in diameter and formed in macropores and within fine-grained clay matrices. This study addresses the current need for empirical, actualistic studies of modern sites of pedogenic siderite formation that can test the simplifying assumptions that were used in the original research projects employing sphaerosiderite as a paleoclimate proxy. In this study of contaminated soils near Chattanooga, Tennessee, we seek to: (1) characterize the distribution of pedogenic siderite with depth, based on careful study of thin sections prepared from soil cores obtained from the former Chattanooga Coke Plant (CCP) site, (2) analyze pedogenic siderite in the contaminated soil material using X-ray diffraction and stableisotope geochemistry, and (3) interpret the possible mechanisms for historical (, 100 yr old) siderite precipitation at this site.