Description : In order to mine lignite in open pits the aquifers have to be dewatered and the overburden must be excavated and
dumped. The overburden of the Rhineland mining area in Germany mostly contains pyrite, which is partly oxidized
due to mining activities. The master variable of all mobilizing reactions in dumps is the content of oxidized
pyrite–sulfur, which is formed under surface mining conditions (‘primary pyrite oxidation’). The value must be
known for a realistic prediction of the resulting groundwater quality in dumps and in order to determine the amount
of required additives to improve water quality. However, substantial methodical difficulties are associated with a
proper determination of the average oxidized pyrite–sulfur content. Apart from the discussion of acid mine drainage
problems, it is the aim of this article to present methods for determination of the average oxidized pyrite–sulfur
content in an unsaturated or a groundwater filled overburden dump (‘dump aquifer’), respectively. For the dumps of
the open pit at Garzweiler or the dump aquifer at Zukunft:West an oxidized pyrite–sulfur content of 0.036 or 0.026
wt.% (only about 10% of the average pyrite–sulfur content) has been determined. To improve the groundwater
quality in dumps an addition of crushed limestone or a mixture of limestone and fly ashes was considered. Beginning in October 1998, a part of the overburden of the surface mine Garzweiler was added with limestone prior to deposition. In order to prevent a secondary pyrite oxidation in the recultivated areas of the open pit at Garzweiler,
the upper part of the dump will contain only non-pyritic sediments in the future. © 2001 Elsevier Science B.V. All
rights reserved.