Open-pit mining for tertiary brown coals characterizes western parts of Germany. The critical overburden is an unlithified fine to middle grained quartz-sand of marine origin, and contains 0.26 wt.% (average) pyrite-sulfur with little acid neutralizing capacity (ANC). Approx. 14% of the pyrite is oxidized during mining operations. A highly mineralized acidic dump water with up to ≤ 2000 mg L~(-1) iron and ≤ 5000mg L~(-1) sulfate will result without preventive measures. Fly-ash from lignite burning powerstations nearsite was envisaged as a low-contaminant, neutralizing additive. To predict the effect of fly-ash amendments to overburden material during mining operations on the dump-water chemistry in 45 years, experiments were set up. Sediment of pre-defined acidity (ACY) was mixed with fly-ash in ratios of ANC/ACY ranging from underdosing to overdosing under oxidizing and oxygen-free conditions. The mixture was saturated with water in closed containers. Subsequently the water chemistry was monitored for 200 days. Geochemical data of the runs and the fly-ash was taken to model the water chemistry using PHREEQE. Steady state was reached within days. Oxidizing starting conditions showed iron concentration below detection in accord with ironhydroxide equilibrium with appropriately and overdosed runs. The pH in overdosed runs was buffered to < pH 11 due to equilibrium with Mg(OH)_2 from fly-ash. As reducing conditions developed, reductive dissolution of iron(III)sulfates was observed with underdosed runs. Still mobilisation of heavy metals from fly-ash was insignificant. Oxygen-free starting conditions were obtained by keeping acidic sediment and additive apart, until oxygen was excluded by saturation, and flushing with CO_2 . With pumping, reaction started. These runs showed initially high iron concentrations. Precipitation of iron(II)carbonates from solution led to decreasing iron concentrations and ACY and will provide a solid speciati-on, thermodynamically stable under the reducing environment of a dump aquifer replete with pyrite. Redox properties of unoxidized fly-ash were observed.
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