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

Rowland, Helen A.L., Omoregie, Enoma O., Millot, Romain, Jimenez, Cristina, Mertens, Jasmin, Baciu, Calin, Hug, Stephan J., Berg, Michael
Applied geochemistry 2011 v.26 no.1 pp. 1-17
aquifers, arsenic, cluster analysis, drinking water, groundwater, iron, iron oxides, methane production, methanogens, people, rain, sediments, sorption, Croatia, Hungary, Romania, Serbia, South East Asia
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 10μg/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 240μg/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 (23–208μg/L, mean 123μg/L), with group 2 indicative of waters dominated by SO₄ ²⁻-reduction containing low As levels (<0.5–58μg/L, mean 11.5μg/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 δ⁷Li (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 SO₄ ²⁻. Ongoing SO₄ ²⁻-reduction will release S²⁻, 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 SO₄ 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 SO₄ 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 SO₄ ²⁻ in the waters. δD and δ¹⁸O 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 SO₄ 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 SO₄ ²⁻ is either absent or wholly consumed.