Uraniferous dolomite: a natural source of high groundwater uranium concentrations in northern Bavaria, Germany?

Naturally high uranium (U) concentrations occur in the groundwater of northern Bavaria (southeastern Germany) although the source(s) and geochemical processes controlling its occurrence are poorly understood. An earlier study identified the weathering of uraniferous apatite as responsible for elevated groundwater U in a part of the region. This present study focuses on a uraniferous dolomite facies in the Triassic sandstone aquifer of northern Bavaria as a potential source of dissolved uranium in the regional groundwater. Hydrogeochemical and mineralogical analytical methods (INAA, ICP-OES, SEP, XRD, C/S measurements), in conjunction with existing hydro- and geochemical datasets, as well as hydrogeochemical modeling approaches indicate a strong connection between groundwater U and the dolomitic facies. Highest groundwater concentrations (max 58.3 µg L−1) occur under slightly alkaline and oxic to slightly reducing conditions. Uranium speciation is dominated by mobile U(VI), predominantly in the form of uranyl-carbonate complexes. Groundwater is undersaturated with respect to U mineral phases. In addition, high values in the dolomite extraction step (SEP) and a positive correlation of dolomite (XRD) and Ca with U (INAA) support the assumption of mobilization from the uraniferous dolomite as a potential source for elevated U concentrations, and hence one of the causes for the geogenic groundwater U problem in this region

Uranium in the Bunter Sandstone of Heligoland – impact on brackish water and drinking water quality

Das Brackwasser auf Helgoland, welches durch das Prinzip der Umkehrosmose zu Trinkwasser aufbereitet wird, weist zum Teil relativ hohe Urankonzentrationen auf. Aus mehreren Studien ist bekannt, dass die hier anstehenden Gesteine des Buntsandsteins uranführend sind. Die Uranmobilisierung aus dem Gestein in das Brackwasser ist jedoch nicht erforscht und gab Anlass zu dieser Arbeit. Für die geochemischen Analysen (INAA, XRD, SEP) wurden 19 Gesteinsproben verwendet. Zur Bestimmung der Wasserchemie (IC, ICP-MS & -OES) wurden Analysen von 16 Wasserproben durchgeführt. Diese zeigen Urankonzentrationen von bis zu 5,4 µg/l im Brackwasser, während Meerwasser durchschnittlich 3,3 µg/l Uran enthält. Die geochemischen Analysen ergeben Urangehalte von 2,1 µg/g im Buntsandstein und bis 65 µg/g in darin vorkommenden Vererzungen. Sequenzielle Extraktionsversuche lassen eine deutliche Mobilisierbarkeit des Urans gebunden an Karbonate erkennen. Die Ergebnisse der hydrogeochemischen Modellierung zeigen, dass das gelöste Uran überwiegend in Uranyl-Karbonatkomplexen vorliegt. Es wird auf eine Teilmobilisierung des karbonatisch gebundenen Uranpools im Buntsandstein durch zur Anreicherung der Brackwasserlinse verrieselten Wassers geschlossen.On Heligoland, brackish water is treated via reverse osmosis for use as drinking water. This brackish water often contains relatively high concentrations of uranium, assumed to originate from the uranium-bearing Bunter Sandstone that forms the island. However, mobilization processes are unknown which has given rise to this study. Nineteen rock samples were used for geochemical analysis (INAA, XRD, SEP) and 16 water samples for hydrochemical characterization (IC, ICP-MS & -OES). Results show U concentrations up to 5.4 µg/l in the brackish water whereas seawater shows an average concentration of 3.3 µg/l. Geochemical results reveal U contents of 2.1 µg/g in the Bunter Sandstone and up to 65 µg/g in geochemically reduced areas of the rock. Sequential extraction indicates a high mobilization of U bound to carbonates. Speciation modeling shows that U mostly occurs in the form of uranyl carbonate complexes. A partial mobilization of the U pool bound in carbonates by irrigated water used for recharge of the brackish water lens is deduced

Apatite weathering as a geological driver of high uranium concentrations in groundwater

Uranium is a heavy metal with potential adverse human health effects when consumed via drinking water. Although associated quality regulations have been implemented, geological sources and hydrogeochemical behavior of uranium in groundwater used for drinking water supply remain little understood. This study presents a hydrogeochemical and mineralogical characterization of a Triassic sandstone aquifer on a macro- and micro-scale, and an evaluation of uranium remobilization into groundwater, also considering the paleoenvironment and the distribution of the affected aquifer itself. Syndiagenetic uraniferous carbonate fluorapatite inclusions within the aquifer sandstones (“active arkoses”) were found to show structurally (chemical substitution in the crystal structure) and radiatively (α-recoil damage from uranium decay) enhanced mineral solubility. Extraction experiments indicated that these inclusions release uranium to groundwater during weathering. In conclusion, apatite alteration was identified as the responsible mechanism for widespread groundwater uranium concentrations >10 μg L−1 in the region representing Germany’s most significant problem area in this respect. Therefore, results indicate that the studied sedimentary apatite deposits cause the regional geogenic groundwater uranium problem, and must be considered as potential uranium sources in comparable areas worldwide

Denitrification in the vadose zone: Modelling with percolating water prognosis and denitrification potential.

Transport and transformation processes of nitrogen in the soil are an essential part of understanding the relationship between agricultural input and nitrate (NO3−) concentrations in groundwater. The presented study describes these transformation processes around NO3− degradation at a water catchment in the Lower Rhine Embayment, Germany. Despite intensive agriculture, extracted groundwater at a depth of 21 to 22 m shows unexpectedly very low NO3− levels, below 3 mg/L NO3− for all wells. The local water supplier therefore carried out investigations in this area and generated soil data from 22 representative areas (142 soil samples from 82 drilling meters from the surface to a max. depth of 5.5 m) and groundwater analyses from 17 groundwater monitoring wells (from 3 to 5 m below ground surface). Soil types are predominantly luvisol and gleysol. The substrate in the topsoil is mainly clayey silt; underneath there are mostly medium-grained sands with partial silt intercalations which appear as a separate layer. Based on this dataset, the percolating water residence times and the NO3− leaching potential were calculated in this study. Together with the nitrogen surplus and with the help of reactive transport modelling, the denitrification potential in the vadose zone was simulated. The comparison of simulation results with laboratory-measured data shows a high correlation. Substantial NO3− reduction in the vadose zone was observed: dependent on soil type, reduction capacity and water residence time, up to 25% of the NO3− was reduced here. The applied modelling is considered an improvement in NO3− degradation potential assessment because it considers many relevant variables such as precipitation, soil parameters (grain size, field capacity, available water capacity, coarse fragments) and nitrogen input. Therefore, a transfer to other sites with comparable hydro(geo)logical conditions is possible, also due to relatively easily determinable input data. This assessment of nitrogen degradation in the vadose zone will be a useful tool for NO3− levels forecast in groundwater

Trace element mobility during Corg-enhanced denitrification in two different aquifers

Nitrate (NO3−)-polluted groundwater treatment by enhanced denitrification is becoming increasingly important due to rising NO3− concentrations and decreasing degradation capacities in aquifers. Besides evaluating the efficacy of substrates added to trigger denitrification, secondary reactions must be closely monitored. Biodenitrification by applied organic carbon (Corg) can lead to considerable changes in redox potential (Eh) and pH, two decisive parameters for trace element mobility. In this study, two geologically and hydrogeochemically different groundwater catchments important for drinking water production were investigated and compared. Sediments were analyzed for trace elements as well as sulfur (S) and carbon (C) contents. Ongoing hydrogeochemical reactions were evaluated with depth-specific isotope characterization, and the potential for trace element mobilization by Corg addition was determined in column experiments. Results for enhanced denitrification showed up to 3.8 times lower reaction rates with respect to comparable studies, probably due to incomplete formation of the necessary denitrifying bacteria. Concentrations of trace elements such as nickel (Ni) must also be considered when evaluating enhanced denitrification, as these can negatively affect microorganisms. Added ethanol led to Ni concentrations dropping from 0.013 mg/L to below the detection limit. Thus, Corg addition may not only induce denitrification, but also lead to the immobilization of previously released trace elements

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

A procedure to identify natural arsenic sources, applied in an affected area in North Rhine-Westphalia, Germany

The aim of the study was to identify the geogenic source for elevated arsenic (As) concentrations recently discovered in soils of the Heubach plain, North Rhine-Westphalia, Germany. Therefore, a catalogue of conditions that an As source has to fulfil in order to be considered as geogenic was formulated, including the source’s linkage to the sink, its mineralogy, As total content, As mobilization potential, groundwater redox conditions and As output. Accordingly, mineralogical, geo- and hydrochemical investigations were carried out, including X-ray diffractometry, microscopy, ICP-OES and AAS analysis and a sequential extraction procedure. Paleo bog iron ores (PBIOs) of Tertiary age, occurring within unconsolidated sands (Haltern-layers, Santonian–Lower Campanian), and glauconitic marlstones (Dülmen-layers, Lower Campanian) were examined. Results indicate that output from the PBIOs is responsible for the elevated As levels. Accounting their diverse mineralogy, five types of PBIOs were defined. Type-dependent, they at least partly fulfil all of the formulated requirements. The relations and behaviour of As sources and sinks in space and time could be clarified for the area of interest. The approach presented in this paper may offer a tool for identifying natural As sources worldwide

Predisposing and triggering factors of large-scale landslides in Debre Sina area, central Ethiopian highlands

A large number of landslide events have repeatedly struck the border zone of the northwestern plateaus of Ethiopia. Debre Sina area is one of the most tectonically active areas located along the western margin of the Afar depression, which is frequently affected by landslides. Despite that, urban and rural development is currently active in almost the entire area. It is crucial, therefore, to understand the main causes and failure mechanisms of landslides in the Debre Sina area and its surroundings. The present study investigated landslides using field mapping of geological and geomorphological features, remote sensing, geo-morphometric analysis, structural analysis, rainfall data, landslide inventory, and earthquake data. The results of the study indicate that large-scale and deep-seated landslide problems appear to be caused by complex geological settings and rugged topography. In particular, the location and morphology of the Yizaba Wein and Shotel Amba landslides are strongly controlled by geological structures. Their flanks are bounded by high angle faults, and their main basal failure surfaces have developed within a W–E striking eastward-dipping normal fault zone. The complex litho-structural and morphologic settings play a vital role in controlling the geometry of the slip surfaces and the stability of the landslides

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