Geochemistry and arsenic behaviour in groundwater resources of the Pannonian Basin (Hungary and Romania)

• Published in: Applied geochemistry Vol. 26; no. 1; pp. 1 - 17
• Main Authors: Rowland, Helen A.L., Omoregie, Enoma O., Millot, Romain, Jimenez, Cristina, Mertens, Jasmin, Baciu, Calin, Hug, Stephan J., Berg, Michael
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 2011; Elsevier
• Subjects: aquifers; arsenic; cluster analysis; drinking water; Earth Sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental Sciences; Exact sciences and technology; Geochemistry; Global Changes; groundwater; iron; iron oxides; methane production; methanogens; people; Pollution, environment geology; rain; Sciences of the Universe; sediments; sorption; South East Asia; Romania; Hungary; Serbia; Croatia; Eastern Europe; Asia; Europe; Southern Europe; Central Europe; Pannonian Basin; Europe, Pannonian Basin; Neogene; oxides; ground water; upper Tertiary; rain water; ice; upper Quaternary; methanogenesis; sorption; aquifers; solution; drinking water; Cenozoic; cluster analysis; retention; sulfides; Quaternary; Tertiary; dissolution; resources; concentration; arsenic; indicators; Pliocene; contamination; correlation; Phanerozoic; reduction; Holocene; Methane; Romania; Arsenic; Hungary; Groundwater; Reductive dissolution; Iron; Lithium isotopes; Sulphate reduction; Pannonian Basin
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2010.10.006

Elevated As levels in the Pannonian Basin are mainly present in very old (Palaeo) groundwater of methanogenic Pliocene/Quaternary aquifers, which is in contrast to Asian regions where arsenic-enriched groundwater is generally much younger. [Display omitted] ► Arsenic originates from Late Pliocene/Quaternary aquifers and some very old waters. ► Arsenic levels are controlled by both mobilisation and retention mechanisms. ► Mobilisation is caused by biogeochemical reductive dissolution. ► Sufficient sulfate supply triggers arsenic retention in sulfide precipitates. ► Nearly 500,000 people are exposed to elevated arsenic in their drinking water. Groundwater resources in the Pannonian Basin (Hungary, Romania, Croatia and Serbia) are known to contain elevated naturally occurring As. Published estimates suggest nearly 500,000 people are exposed to levels greater than the EU maximum admissible concentration of 10 μg/L in their drinking water, making it the largest area so affected in Europe. In this study, a variety of groundwaters were collected from Romania and Hungary to elucidate the general geochemistry and identify processes controlling As behaviour. Concentrations ranged from <0.5 to 240 μg/L As(tot), with As predominantly in the reduced As(III) form. Using cluster analysis, four main groups of water were identified. Two groups (1 and 2) showed characteristics of water originating from reducing aquifers of the area with both groups having similar ranges of Fe concentrations, indicating that Fe-reduction occurs in both groups. However, As levels and other redox characteristics were very different. Group 1, indicative of waters dominated by methanogenesis contained high As levels (23–208 μg/L, mean 123 μg/L), with group 2 indicative of waters dominated by SO 4 2 - -reduction containing low As levels (<0.5–58 μg/L, mean 11.5 μg/L). The remaining two groups were influenced either by (i) geothermal and saline or (ii) surface contamination and rain water inputs. Near absence of As in these groups, combined with positive correlations between δ 7Li (an indicator of geothermal inputs) and As(tot) in geothermal/saline influenced waters indicate that elevated As is not from an external input, but is released due to an in-aquifer process. Geochemical reasoning, therefore, implies As mobilisation is controlled by redox processes, most likely microbially mediated reductive dissolution of As bearing Fe-oxides, known to occur in sediments from the area. More important is an overlying retention mechanism determined by the presence or absence of SO 4 2 - . Ongoing SO 4 2 - -reduction will release S 2−, removing As from solution either by the formation of As-sulfides, or from sorption onto Fe-sulfide phases. In methanogenic waters, As released by reductive dissolution is not removed from solution and can rise to the high levels observed. Levels of organic C are thought to be the ultimate control on the redox conditions in these 2 groups. High levels of organic C (as found in group 1) would quickly exhaust any SO 4 2 - present in the waters, driving the system to methanogenesis and subsequent high levels of As. Group 2 has much lower concentrations of organic C and so SO 4 2 - is not exhausted. Therefore, As levels in waters of the Pannonian Basin are controlled not by release but by retention mechanisms, ultimately controlled by levels of TOC and SO 4 2 - in the waters. δD and δ 18O analysis showed that groundwaters containing elevated As dated mostly from the last ice-age, and are sourced from Late Pliocene to Quaternary aquifers. The importance of TOC and retention capabilities of SO 4 2 - -reduction have only previously been suggested for recent (Holocene) sediments and groundwater, most notably those in SE Asia as these are the most likely to contain the right combination of factors to drive the system to a redox situation leading to high aqueous As concentrations. In contrast, it is shown here that a much older system containing As bearing Fe-oxides, also has the potential to produce elevated levels of As if the TOC is suitable for the microbial population to drive the system to the correct redox situation and SO 4 2 - is either absent or wholly consumed.