Abstract
The oxidation profile of a surficial clay aquitard was studied on a 35-meter borecore from the Albian Tégulines Clay near Brienne-le-Château (Paris Basin, France). Mineralogical, geochemical, and petrophysical data showed evidences of gradual oxidation taking place down to a depth of 20 m. Below 20 m, the clay material was nonplastic and nonfractured, and it inherited reduced redox conditions from bacterial sulfate reduction that occurred after sediment deposition. Above 20 m, the clay material was plastic. Up to a depth of 10-11 m, only rare yellowish aggregates of glauconite attested to limited oxidation, and pore water chemistry was unmodified. The 5–11 m depth interval was characterized by intensive pyrite oxidation, calcite dissolution, and formation of sulfate and iron hydroxide minerals. The upper 2-3 m was ochrous and entirely oxidized. These mineralogical changes were mirrored with pore water chemistry modifications such as an increase of alkalinity and sulfate concentration in the upper part of the profile. The presence of siderite at ~11 m evinced the reactivity of Fe(II) in the structure of clay minerals with dioxygen from meteoric waters that infiltrated into the Tégulines Clay through vertical fractures.
Highlights
Clay-rich geological formations act as natural low permeability protective barriers preventing contamination of groundwater resources due to extremely long time scales for groundwater flow and solute transport and the capacity of clay minerals to fix cationic contaminants [1,2,3,4,5,6]
Determining transport mechanisms is first important for evaluating and modelling how contaminants migrate through a geological formation
The Gault clay was only represented by the Tegulines Clay; the Brienne marls were absent
Summary
Clay-rich geological formations act as natural low permeability protective barriers preventing contamination of groundwater resources due to extremely long time scales for groundwater flow and solute transport and the capacity of clay minerals to fix cationic contaminants [1,2,3,4,5,6]. Weathering induces spatially extensive or discontinuous heterogeneities such as fractures [9,10,11] and uncompacted sand layers [9], or changes to pore water chemistry and alkalinity, pyrite oxidation, and sulfur state change [6], and changes in other redox proxies such as Ce, Mn, and U [12,13,14,15,16]. These can create perturbations in transport flow paths and rates. How far the oxidized layers extend vertically remains unclear
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