Influence of tectonics, sedimentation and aqueous flow cycles on the origin of global groundwater arsenic: Paradigms from three continents
Introduction
In general, arsenic (As) is present in trace concentrations (mean: ∼1.8 mg/kg [Matschullat, 2000, Bhattacharya et al., 2002]) in the Earth’s continental crust (e.g. Bhattacharya et al., 2002, Nordstrom, 2002, Smedley and Kinniburgh, 2002). However, hydrological systems, specifically groundwater systems, are often enriched in As (>0.01 mg/L). Over the past few decades, more than twenty geological provinces across the globe have been identified as having elevated, and sometimes toxic, concentrations of dissolved As in groundwater. The chemistry of groundwater is largely controlled by the composition of the recharging water, water-aquifer matrix interactions (e.g. cation exchange, mineral dissolution), availability of reactive organic matter and groundwater residence time within the aquifer (e.g. Edmunds et al., 1981, Stallard and Edmond, 1983, Chapelle, 1993, Faure, 1998, Drever, 1997, Mukherjee et al., 2009, Mukherjee et al., 2012). Accordingly, elevated concentrations of chalcophile As in contaminated aquifers may result from the dissolution of primary or secondary sulfidic minerals (such as pyrite), or desorption from secondary concentrations in hydrous metal oxides (Nordstrom, 2002), that are common mineral constituents of the aquifer matrix in many large, unconsolidated sedimentary systems. Some of the potential As source materials for water systems include mineralized areas (mostly sulfide), volcanogenic deposits and sediments, geothermal springs, organic-rich shales, authigenic sulfides in sediments and recent metal oxide/hydroxide-rich sediments in aquifers with low hydraulic gradients (Bhattacharya et al., 2004, Scanlon et al., 2009). However, it has been observed that As-rich groundwater are often separated from their sources, and it has been justifiably noted that As mobilization mechanisms might be a more critical issue than the source (e.g. Smedley and Kinniburgh, 2002, Nordstrom, 2002, Mukherjee and Fryar, 2008, Scanlon et al., 2009, Ravenscroft et al., 2009). Hence, while there is great emphasis on uncovering the hydrogeochemistry that controls mobilization processes in contaminated areas, studies delineating the source of the As have remained uncommon. Saunders et al. (2005) commented that although there is a consensus that mobilization processes lead to As enrichment, the ultimate source of As is not well understood. It is obvious that even the trace or normal concentrations of As in solid phase aquifer matrices (Korte and Fernando, 1991) is sufficient to produce toxic As concentrations in groundwater under conducive environments (Stüben et al., 2003, Guilliot and Charlet, 2007). However, it has been also repeatedly observed that certain portions of the geosphere are more enriched in As than others, and these enriched portions may, in part, the primary source for groundwater As.
Van Geen (2011) suggested that the two most important aspects of understanding the causes of high As groundwater is to identify whether the elevated As is reactively mobilized in the aquifer in situ, or if originally low-As areas have been contaminated by advectively transported As along flow paths from up-stream enriched sources (ex situ). Van Geen (2011) hypothesized that the spatial distribution of natural As in aquifers is largely controlled by mobilization processes with the possibility of local geologic influences, where the hydrochemical evolution leading to As enrichment in groundwater might take only a few decades to thousands of years. The importance of ex situ sources, in addition to the in situ mobilization mechanisms for As have been recognized in most previous global reviews (e.g. Smedley and Kinniburgh, 2002, Nordstrom, 2002, Mukherjee et al., 2008, Ravenscroft et al., 2009). Smedley and Kinniburgh (2002) noted that typical environments containing high As groundwater include (1) low temperature (including mining and non-mining) and (2) high temperature (hydrothermal) settings. High As groundwater in low temperature, non-mining areas are generally characteristic of young, sedimentary aquifers (Bhattacharya et al., 2004, Scanlon et al., 2009). This observation suggests that the cause of As in groundwater might not be linked to just “local” geology but may be a reflection of larger, regional-scale geodynamic processes. Ravenscroft et al. (2009) observed that many of the As-enriched areas are located in the proximity of young fold mountain belts, but did not elaborate on the relationship. Saunders et al. (2005) proposed that As-enriched groundwater is a product of interactions between tectonic, geochemical and biological processes. The geochemical processes are linked to chemical weathering of a variety of bedrock types, as noted by analyses of surficial material in major parts of continental crust (e.g. Ahmed et al., 2004, Grosz et al., 2004, Saunders et al., 2005, Mukherjee et al., 2012), which re-emphasizes the existence of some regional-scale geochemical process. In this context, a major debate concerns the source of the As-enriched aquifers of the major fluvial systems of Southeast Asia (e.g. Ganges–Brahmaputra, Indus, Mekong, Red, Irrawady, etc.), for many of which the Himalayas act as the sediment provenance and headwater (Fendorf et al., 2010). Notwithstanding these observations, only a few studies or rather hypotheses, have been proposed yet, to elucidate the Southeast Asian scenario (e.g. Stanger, 2005, Hattori and Guillot, 2003, Guilliot and Charlet, 2007). Knowledge of the cause of As-enrichment in most other parts of the world is very limited.
Of the known global-scale, near-surface geological processes, tectonics associated with crustal plate movements are most important. Zheng, 2006, Nordstrom, 2009 alluded to a link between As enrichment and crustal evolution. In spite of the heterogeneity of As concentrations based on lithotype and geography (e.g. Mukherjee et al., 2011), the known natural As enriched groundwater localities of the world (following Smedley and Kinniburgh, 2002, Mukherjee et al., 2008, Ravenscroft et al., 2009) show a striking systematic relationship when plotted on the global tectonic map. Most groundwater As enriched areas exist in or near the sedimentary basins adjacent to orogenic belts (Fig. 1). This observation signifies a large-scale cause-effect relationship between global orogensis and the concentration of arsenic in groundwater. The objective of this study is therefore to establish a hypothesis that identifies a common primary source-aggradation mechanism for the formation of different globally dispersed geogenic As-enriched aquifers that have been so far mostly studied and explained in terms of individual occurrences.
In geodynamics, the term “orogenesis” refers to the formation of long, mountain belts called orogenic belts or orogens. In plate tectonic terms, orogenesis is a manifestation of the convergence of tectonic plates, resulting in subduction or obduction of lithospheric plates. Present-day or examples include the Alpine–Himalayan orogen (Alps and Himalayas) that stretches from western Europe to eastern China, through the Indian subcontinent, the Cordilleran orogens along western North America, the Andean orogen of western South America, and the Lachlan orogen of southeastern Australia, among others. Ancient equivalents are the Appalachian of eastern USA, and the Caledonian orogen of northern Europe and eastern Greenland. Associated with these orogenic belts are extensive foreland basins, which are depressions that develop during the mountain building process by lithospheric flexure (DeCelles and Giles, 1996). These foreland basins act as sinks for the sediments that are eroded from the rapidly rising, adjacent orogens. The basins are characterized by thick sedimentary successions, which thin away from the orogenic belts. Examples of such foreland basins include the Indus–Ganges–Brahmaputra, Northern Tarim, Southern Junggar basins of Asia, the Northern Alpine, Po, Ebro, Guadalquivir, Aquitaine basins of Europe, the Appalachian and Western Canada Sedimentary basins of North America. Based on the structures (DeCelles and Giles, 1996), foreland basins may be classified as peripheral (where the basins occur on the plate that is subducted i.e. the outer arc of the orogen, e.g. the Indus–Ganges–Brahmaputra basin) and retroarc (where the basins occur on the plate, which overrides during the tectonic plate convergence, and are situated behind the magmatic arcs, e.g. the basins of the Andean or Western Cordilleran orogen). We hypothesize that the primary source of the As in groundwater is ex situ to the affected aquifers and globally, the groundwater As in the aquifers of the foreland basins might be derived from tectonic cycles, sedimentation and regional-scale water flow, and mobilized on availability of conducive surface geochemical and biogeochemical environments as illustrated, in this article, by the examples from three continents.
Section snippets
Provenance
Arsenic has a tendency to be enriched in chalcophile mineralized zones in the Earth’s crust. However, on an average, the continental crust has a solid phase As concentration of 1.0–1.8 mg/kg (Krauskopf and Bird, 1995, Wedepohl, 1995, Faure, 1998, Henke, 2009). Of these, the upper crust has been studied by many workers, and is in general As-enriched with average concentrations varying from 2 mg/kg (Wedepohl, 1995) to 5 mg/kg (Sims et al., 1990), in comparison to the less studied, As-depleted lower
Probable examples of the hypothesis
The discussion so far has mostly concentrated on framing a feasible model for explaining the potential link between the systematic presence of high groundwater As in certain parts of the Earth’s surface that are or were tectonically active. Table 1 summarizes the geological aspects and tectonic framework of some of the well known worldwide As-enriched geographical provinces. This list is not exhaustive, and extensive future work is required to establish this hypothesized groundwater
Summary and conclusion
Although, As is a trace element in the Earth’s crust, elevated concentrations of As in groundwater occur throughout the world. In spite of numerous studies on the mobilization and fate of As in hydrologic systems, there has not been much emphasis on understanding the primary provenance of the As. When viewed on a global scale, there seems to be a systematic pattern for the occurrences of As-enriched aquifers, most of them being parts of large sedimentary basins adjoining mountain belts. This
Acknowledgements
The authors are thankful to D. Kirk Nordstrom (USGS), Alan E. Fryar (University of Kentucky), Yan Zheng (Queen’s College) and Laurent Charlet (University of Grenoble) for helpful comments and inspiration. Special thanks to Soumendra Nath Bhanja, Palash Debnath and Nilasree Raychowdhuri (IIT Kharagpur) for assistance with figures.
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