Groundwater uranium origin and fate control in a river valley aquifer.

Groundwater in a Quaternary gravel aquifer partly exhibits uranium (U) concentrations exceeding the new German drinking water limitation (22% of the samples >10 µg L(-1)). This study assesses relevant U reservoirs and hydrogeochemical processes responsible for U transfer between them. A large data set of solid materials (sediments and soils, 164 samples total) and groundwater (114 samples total) characteristics was created in terms of geo- and hydrochemistry, mineralogy, U microdistribution, and mobilization potential. Results show that U primarily derived from lignitic inclusions in Tertiary sediments is transported to and accumulated (complexation to organic substance and UO2 precipitation) in lowland moor peats of the river valley grown on the aquifer gravels. The alkaline character of the system predefines a hydrogeochemical framework fostering U mobility. Elevated concentrations (up to 96 µg L(-1) U) occur downstream of the moor areas and under Mn/NO3-reducing groundwater conditions. Oxic and stronger reduced settings are rather little affected. Supporting previous laboratory studies, this suggests enhanced U mobility in the presence of nitrate also in the field scale. While no anthropogenic U input was detected in the study area, agricultural usage of the moor areas triggers geogenic U release via nitrate fertilization, surface peat degradation, and erosion.

Crossing redox boundaries--aquifer redox history and effects on iron mineralogy and arsenic availability.

Cretaceous shallow marine sediments from northwestern Germany exhibit a distinct colour and geochemical boundary in a depth of several decametres, witnessing a terrestrial oxidative paleo redox process which resulted in cement loss and oxidation of Fe(II) phases. Sediment samples were obtained from boreholes drilled in near-coastal and further basinward paleo environments, including both reduced and oxidized redox facies, to characterize As and Fe occurrence in unaltered layers and redistributional consequences of the redox event. Geochemical and mineralogical composition and As fractionation were assessed. Arsenic resides in pyrite in the reduced section with a bulk rock maximum concentration of 39 µg g(-1), calculated Aspyrite is ~0.2 wt.%. Siderite concretions in the fine sands do not function as As sinks, neither does glauconite whose general As/Fe leaching behaviour was characterized. In the zone of redox transition, reduced and oxidized phases coexist and elevated As concentrations (up to 73 µg g(-1)) with high proportions of reactive As were detected. Arsenic behaviour changes from relatively homogeneous Fe sulphide-control in the unaltered sediments to very heterogeneous Fe hydroxide-control above the paleo redox boundary. The studied characteristics determine recent As availability in the subsurface and must be considered during groundwater extraction from this highly important aquifer.

Enrichment processes of arsenic in oxidic sedimentary rocks - from geochemical and genetic characterization to potential mobility.

Sedimentary marine iron ores of Jurassic age and Tertiary marine sandy sediments containing iron hydroxides concretions have been sampled from boreholes and outcrops in two study areas in Germany to examine iron and arsenic accumulation processes. Samples were analyzed for bulk rock geochemistry (INAA/ICP-OES), quantitative mineralogy (XRD with Rietveld analysis), element distribution (electron microprobe) and arsenic fractionation (sequential extraction). Bulk Jurassic ores contain an average arsenic content of 123 µg g(-1) hosted in mainly goethite ooids which slowly formed in times of condensed sedimentation. Enrichment occurred syndepositionally and is therefore characterized as primary. Iron concretions in Tertiary sediments mainly consist of goethite and yield arsenic up to 1860 µg g(-1). The accumulation process is secondary as it took place in the course of oxidation of the originally reduced marine sediments under terrestrial conditions, leading to element redistribution and local enrichment in the near-surface part. The scale of enrichment was assessed calculating Enrichment Factors, indicating that arsenic accumulation was favoured over other potential contaminants. In spite of higher bulk arsenic contents in the oxidic rocks, the mainly pyrite-hosted As pool within the reduced deeper part of the Tertiary sediments is shown to have a higher potential for remobilization and creation of elevated arsenic concentrations in groundwater.