Project members: Andre Banning, Thomas R. Rüde

Introduction

Aquifers contaminated with naturally occurring arsenic (As) threaten millions of people worldwide. In major problem areas (e.g. Bangladesh, Vietnam, Latin America), As in groundwater originates from the weathering of As-bearing minerals of which Fe-oxides and -hydroxides play the most important role in many sedimentary systems. Also Germany is affected by regionally elevated As-concentrations in rocks, soils and groundwater (the German As-threshold value in drinking water is 10 µg/L). Known examples of this can be found in Franconia, Upper Bavaria, southern Lower Saxony, the Rhine area and the Münsterland. Stratigraphically, the affected sediments cover most of the Mesozoic and Cenozoic ages.

Conception/Aims

Studies of natural As in Germany carried out until today are restricted to local or regional scales. This project aims at a spatial and stratigraphical comparison of German aquifers dominated by oxidic Fe-minerals and affected by elevated As-concentrations. Results will allow a better understanding of natural As-solution and -accumulation processes and thus, contribute to the knowledge about the development of high As sedimentary rocks. Comparing different strata (Triassic to Tertiary) allows a general characterization of environments fostering geogenic enrichment. Arsenic bonding forms and chemical species are examined to estimate its mobility under natural conditions. Thus, risk assessments for natural As contamination of groundwater and soils might be derived which is of ecologic and economic relevance for water suppliers and others.

Investigation areas/Methods/Outlook

Samples taken so far derive from southwestern Germany (an Fe-ore containing strata series of Jurassic age, where elevated As-concentrations can be expected. Moreover, from several locations within the Lower Rhine Area, Oligocene sediments (Grafenberg-layers) were sampled. These layers are glauconitic in their lower part and altered (containing Fe-(hydr)oxides) in their upper part. The same is valid for the Haltern-layers (Upper Cretaceous) from the western Münsterland. In both cases, boreholes were sampled to analyse Fe and As horizontal distribution and enrichment processes. An additional investigation area will probably be located in Franconia.

In the laboratory, element concentrations and rock mineralogy (with a focus on Fe-phases) are analysed using a variety of methods. Sequential Extraction Procedures (SEP) are used to examine As-bonding forms and thus, its hydrogeochemical behaviour. The paleoenvironment and timing of As-enrichment will be characterized. Indicators assessing sedimentary rocks with respect to their As-accumulation and depletion potential will be established. Examples for that might be element distribution patterns (e.g. Rare Earth Elements), depositional facies or physicochemical groundwater parameters.
Arsenic enrichment and relocation in the geological past partly occurred under arid to semiarid climatic conditions. To study recent As-dynamics in such an environment, a research trip to Mexico is planned. The findings there might be transferred to the paleo situation in Germany and may help reconstruct millions of years old processes.