We used a suite of techniques to characterize the mineralogy, geochemistry, and arsenic speciation in lagooned combustion waste (ash) from coal burning and ash buried under agricultural soil since 1965 when a dam of one of the ash ponds failed. Coal seams from Novaky (Slovakia) contain low-temperature hydrothermal mineralization with orpiment (As2S3). The lagooned ash waste has 1000-1400 ppm As and consists of vesicular and compact glasses (86.29% of the ash with an average of 0.13 wt.% As2O5), spheroidal glasses (2.53% of the ash; 1.35 wt.% As2O5), unburned coal particles (7.76% of the ash; 0.10 wt.% As2O5) with calcite veins (0.27% of the ash; 1.60 wt.% As2O5), as well as quartz, plagioclase, and traces of poorly crystalline mullite (3.15% of the ash; 0.01 wt.% As2O5). The major As carriers are the glasses and electron microprobe analyses document high affinity of As towards Ca-rich material in the glasses. Sequential extractions show massive release of Ca during the mildest leaching (water or MgCl2 solution); As is released especially under reducing conditions. The soil-ash mixtures from 1965 have 10781381 ppm As, almost the same As concentration as the lagooned ash waste. In addition to the constituents identified in the lagooned combustion waste, they contain an omnipresent fine-grained matrix (0.05 wt.% As2O5) between the grains, most likely made of poorly crystalline clays and iron oxides. More than 90% of the As in the lagooned ash waste and soil-ash mixtures occurs as As5+. Our results allow us to track the fate of As: during combustion, As is incorporated into glasses, especially in those rich in Ca; in the ash impoundment, calcium arsenates form (assumption based on literature data) and are slowly replaced by calcite with a concomitant release of As. In the ashes buried in soils, part of this As is retained on pre-existing or newly-formed iron oxides and clays. (C) 2011 Elsevier Ltd. All rights reserved.