Distribution and Damage Potential of Surer and Sulfate-Rich Groundwaters in Germany with Special Consideration of Groundwater Level Rise - Part 1: Status Report

Project duration: 2009 - 2010

In various regions in Germany, elevated acid and sulfate concentrations occur in the groundwater. Such occurrences are often of anthropogenic origin and usually occur only locally and over short periods of time, so that as a rule there is no long-term and/or large-scale impairment of the durability of concrete structures. However, a high hazard potential due to acidic and sulfate-rich groundwater is to be expected primarily if the cause is due to the oxidation of iron disulfide in extensive areas. Large-scale iron disulfide oxidation occurs primarily in regions of open-pit lignite mines. On the one hand, the lowering of groundwater due to mining can initiate aeration and thus oxidation processes in deeper soil layers. On the other hand, oxidizing iron disulfide-containing overburden material is deposited on open pit dumps and over time releases considerable loads of sulfuric acid and iron sulfate into the groundwater via seepage water. If elevated carbonate occurrences are present in such areas at the same time, the released acid can be partially or completely buffered. While increased levels of sulfate and lime-dissolving carbonic acid occur with partial buffering, groundwater contamination is limited to increased sulfate concentrations with complete acid buffering. In some regions, a discharge of such oxidation products of iron disulfide into the groundwater surrounding opencast mines can already be observed. Initial forecast models suggest that a not inconsiderable discharge of substances can still be expected in such areas in the coming decades, not least as a result of the groundwater recharge following the cessation of mining pumping measures. The development of groundwater quality in the vicinity of opencast mines is explained using the examples of the Lusatian, Central German and Rhenish mining districts, although detailed knowledge of this is only beginning to be available. On the basis of characteristic properties of groundwaters contaminated by mining, their concrete aggressiveness is evaluated, taking into account significant influencing factors such as transport conditions, concrete composition, etc. The results of the study are presented in detail. Not least because of the lack of comprehensive knowledge about final groundwater levels (after completion of groundwater resurgence), flow conditions or orientation of material discharge in connection with the foundation depth of existing structures, the extent of damage due to concrete attack by groundwater contaminated by mining cannot be described precisely in quantitative terms at present. The effects of waters mineralized by iron disulfide oxidation on their attack potential on concrete structures are therefore derived using current research on combined acid-sulfate attack on concrete and results from investigations of acid- or sulfate-damaged structures. However, with the help of previous knowledge about the damage mechanisms when concrete is exposed to acidic and sulfate-rich waters, the corrosion progress for characteristic attack scenarios can be estimated approximately, taking into account essential boundary conditions. In order to effectively counteract the potential for attack on structures by groundwater contaminated by mining, measures must be taken to reduce the degree of attack by the attacking medium or to increase the resistance of affected structures. For this purpose, individual methods that have been proven in practice are listed as examples in this report.