Natural arsenic and uranium accumulation and remobilization in different geological environments
- Natürliche Arsen- und Uran-Anreicherung und -Remobilisierung unter verschiedenen geologischen Bedingungen
Banning, Andre; Rüde, Thomas R. (Thesis advisor)
Aachen : Publikationsserver der RWTH Aachen University (2012)
Dissertation / PhD Thesis
Aachen, Techn. Hochsch., Diss., 2012
Despite the fact that both As and U represent geogenic trace elements potentially toxic to humans, little information is available on the development of their enrichments in German sediments and their potential impact on groundwater quality, let alone a systematic overview of the country´s natural occurrences. This work aims at characterizing accumulation processes in aquifers actually or potentially affected by elevated concentrations of As and/or U, and their timings in geological history. The five selected study areas provide different geological and stratigraphical backgrounds. Identification of As and U sources, and structural derivation of their environmental reservoirs as well as remobilization mechanisms potentially resulting in trace element release to groundwater were assessed. Drinking water supply in Franconia/Northern Bavaria is dependent on groundwater extraction from terrestrial Upper Triassic sandstones where elevated concentrations of geogenic U and As exceeding German drinking water limitations were identified. Characterization of aquifer material in terms of geochemical and mineralogical composition, trace elements distribution on a microscale and their mineralogical fractionation and mobilization behaviour showed that uraniferous francolite/hematite inclusions within the aquifer sandstones ("active arkoses") represent important sources for U and As in the study area. Francolite exhibits biologically, structurally and radiation-enhanced solubility; loss of both U and As during weathering was documented. Jurassic shallow marine Fe ores from the Upper Rhine Graben exhibit significant bulk As hosted in mainly goethite ooids slowly formed in times of condensed sedimentation. The study indicates that As accumulation was favoured over other potential contaminants, esp. heavy metals. Conditions for As accumulation varied during deposition, visible on a macro- (outcrop) as well as on a microscale (single Fe ooid). However, the risk of As release to groundwater of the region is considered rather insignificant. An oxidative terrestrial paleo redox process during the late Tertiary affected Santonian shallow marine sands in the western Münsterland Cretaceous Basin, resulting in a distinct sediment colour and geochemical boundary in several decametres below ground surface, and massive element redistribution. Arsenic resides in pyrite in the reduced section. Its behaviour changes from homogenous sulfide-control in the unaltered sediments to very heterogeneous Fe hydroxide-control above the paleo redox boundary. Early stages of hydroxidic Fe/As accumulations resulting from mobilization from the reduced sediments represent precursors of high-As goethite concretions, widespread in the near-surface oxidized facies. Widespread Oligocene marine sandy sediments from the Lower Rhine Embayment exhibit features of a paleo redox event: primary Fe(II) phases in a reduced facies, hydroxidic Fe mineralogy and significant major and trace element redistribution in near-surface sediments. Striking similarities to the postdepositional redox history of the Cretaceous sediments became obvious, also in terms of As control. Preferential As enrichment over heavy metals in Fe hydroxide concretions was detected. Uranium is rather homogeneously distributed in low concentrations in both redox facies and little affected by the redox event. The reduced deeper sediments are the more probable candidates for creating elevated As in groundwater. Uranium and As in deep groundwater of the volcano-sedimentary basin around San Luis Potosí/north-central Mexico partly exceed drinking water guidelines and thus endanger the most important drinking water source in the area. The As/U hydrogeochemical signatures, their behaviour during rock alteration and evidence from proxies like REE strongly argue for acid volcanic glass dissolution as the dominating process of U and As release to groundwater. The hydrogeochemical fingerprint is modified by additional mobilization from the sedimentary basin filling. Common behaviour of both incompatible elements during magmatic differentiation and growing drift-apart in sedimentary systems are discussed. Besides the importance of the obtained results for the studied regions, geochemical comparison of all study areas offers additional explanations for the large-scale As and U distribution in Germany. While Pleistocene geology explains their absence in sediments and related groundwater in northern Germany, their distribution in the central and southern parts is controlled by provenance geochemistry. Only highly felsic origin (Moldanubian Variscides) enables creation of elevated U in the systems while Rhenohercynian provenance allows for As presence only. Subsequent intrabasinal redistribution contributes to the present-day situation.