Sulfate and strontium water source identification by O, S and Sr isotopes and their temporal changes (1997-2008) in the region of Freiberg, central-eastern Germany

Various waters (bulk atmospheric precipitation, groundwater, river water, flowing mine water, mine drainage galleries and flooding mine water) were studied in the region of Freiberg to achieve an improved understanding of sources and mixing processes in the hydrological cycle and to specify the main sources that determine the isotope composition of dissolved sulfate and strontium (delta O-18(SO4), delta S-34(SO4) and Sr-87/Sr-86) in river water. The combined use of isotope and chemical data led to identification of three major sulfate sources: i) sulfate from groundwater, ii) sulfate from the oxidation of local ore sulfides that remained in unflooded und flooded parts after mine decommissioning, and iii) industrial sulfate released from abundant waste, slag and tailing deposits left behind after a long mining and industrial history of the region. The same three components (groundwater, pore water from oxidation of residual ores and industry source) were identified by analyzing Sr isotope ratios. However, the Sr isotopes pointed out a fourth, yet unrecognized source that contributed to water of the flooded mine section. The data show that the isotope composition of water of the river Freiberger Mulde was mainly determined by uncontaminated groundwater diluted with atmospheric precipitation, whereas the other two end-members played only a minor role for this river water. However, for the river Triebisch the isotope compositions of dissolved sulfate and strontium record a strong impact of end-members II and III after recharging the flooding water into the river. A twelve year monitoring (1997-2008) revealed a strong decrease of delta O-18(SO4) values of water from bulk atmospheric precipitation as of 2006-2008 (7 parts per thousand) compared to 1997 (14 parts per thousand), while delta S-34(SO4) values remained constant (5 parts per thousand). This isotope shift was probably caused by a lower contribution of primary sulfates to atmospheric precipitation in recent years. Groundwater and river water (Freiberger Mulde) also showed decreasing delta O-18(SO4) values albeit to a lower degree (by 4-5 parts per thousand). A decrease of delta O-18(SO4). values was again found in flowing mine water (by 5-6 parts per thousand) mainly affected by end-member I (uncontaminated groundwater). The flood event in August 2002 likely washed out large parts of older groundwater and thus enabled the relatively large oxygen isotope decrease (delta O-18(SO4)) of groundwater and river water within a few years. The previously high input from the industrial source (waste and slag deposits) decreased from 1997 to 2002 in some drainage galleries and flooding water that discharge their water into local rivers. This led to lower delta O-18 and delta S-34 values of dissolved sulfates in these waters accompanied by a concentration decrease of several elements (Cl, Na and Mg). However, no further decrease of the impact of this source could be detected after the storm and flood event in August 2002. (C) 2010 Elsevier B.V. All rights reserved.