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Very high concentrations of uranium (up to 4000 ppm) were found in a natural soil in the Dischma valley, an alpine region in the Grisons canton in Switzerland. The goal of this study was to examine the redox state and the nature of uranium binding in the soil matrix in order to understand the accumulation mechanism. Pore water profiles collected from Dischma soil revealed the establishment of anoxic conditions with increasing soil depth.

* Manuscript solutions. This can be explained by the formation of Fe(II) carbonates on the mineral surfaces which represent an easily available Fe(II) pool for the U(VI) reduction. We also consider a facilitated U(VI) reduction as possible when uranium is present as a carbonate complex compared to non-complexed uranium (e.g. uranyl).

This study addresses the thermal-hydraulic-mechanical and chemical (THMC) behaviour of a research well doublet consisting of the injection well E GrSk 3/90 and the production well Gt GrSk 4/05 A(2) in the deep geothermal reservoir of Groß Schönebeck (north of Berlin, Germany). The reservoir is located between 3815 and 4247 m below sea level in the Lower Permian of the North German Basin (NGB). Both wells were hydraulically stimulated to enhance productivity. For the production well three stimulation treatments were performed in 2007: these three treatments result in a productivity increase from 2.4 m 3 /(hMPa) to

High-temperature geothermal systemsSystems with temperatures >225 °C. Here, it is denoted that geothermal resources are ones where fluids are present, allowing for power generation using flash and/or binary power plant technology.

Reasons for injectivity decline were investigated in a low-enthalpy geothermal aquifer in Klaipeda (Lithuania). It is one of the study sites within the DESTRESS project, which demonstrates different stimulation techniques in geothermal reservoirs. Due to low injectivity, production rates from the Lithuanian field are currently reduced, which lead to negative commercial implications for the site. Productivity from the same wells is measured to be 40 times higher. Injectivity decline in aquifers is often related to clogging processes in spatially correlated highly permeable structures, which control the main flow volume. We subdivided clogging processes into (1) physical, (2) chemical, and (3) biological processes and studied them by analyzing fluid and solid samples as well as operational data. The methods we used are fluid and solid analyses in situ, in the laboratory and in experimental setups, statistical interpretation, and numerical modeling. Our results show that the spatially correlating nature of permeable structures is responsible for exponentially decreasing injectivity because few highly permeable zones clog rapidly by intruded particles. In particular, field operations cause changes of the physical, chemical, and biological processes in the aquifer. Mineral precipitation and corrosion are the main chemical processes observed at our site. Microbial activity causes biofilm while fines migration is caused by changes in physical boundary conditions. Moreover, these processes can affect each other and generate further reactions, for example, microbial activity triggers corrosion in surface pipelines.

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