Summary of effects of chronic arsenic exposure on human health. (Data from Mandal and Suzuki, 2002, and Ng et al., 2003, and references therein.)
\r\n\t
\r\n\tThey are hypersensitive to chemical pollution, habitat degradation, a variation of river and groundwater quality, climate change and even the sun's ultraviolet radiation, amphibians are among the vertebrate groups most endangered by human activity, and their abundance in wetlands is always one of the best indicators of good environmental conservation.
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\r\n\tIn this book, we have considered all aspects of amphibians biology, diversity, conservation and potential use of amphibians as environmental indicators.
Arsenic (As) is a metalloid element (atomic number 33) with one naturally occurring isotope of atomic mass 75, and four oxidation states (-3, 0, +3, and +5) (Smedley and Kinniburgh, 2002). In the aqueous environment, the +3 and +5 oxidation states are most prevalent, as the oxyanions arsenite (H3AsO3 or H2AsO3- at pH ~9-11) and arsenate (H2AsO4- and HAsO42- at pH ~4-10) (Smedley and Kinniburgh, 2002). In soils, arsine gases (containing As3-) may be generated by fungi and other organisms (Woolson, 1977).
The different forms of As have different toxicities, with arsine gas being the most toxic form. Of the inorganic oxyanions, arsenite is considered more toxic than arsenate, and the organic (methylated) arsenic forms are considered least toxic (for a detailed discussion of toxicity issues, the reader is referred to Mandal and Suzuki (2002)). Arsenic is a global health concern due to its toxicity and the fact that it occurs at unhealthful levels in water supplies, particularly groundwater, in more than 70 countries (Ravenscroft et al., 2009) on six continents.
Despite its use in medicines for nearly 2,500 years (Mandal and Suzuki, 2002; Cullen, 2008) As has long been recognized as a toxic and often lethal substance. Chronic exposure to As can cause harm to the human cardiovascular, dermal, gastrointestinal, hepatic, neurological, pulmonary, renal and respiratory systems (ATSDR, 2000) and reproductive system (Mandal and Suzuki, 2002). Research on health effects is summarized and discussed by Mandal and Suzuki (2002) and Ng et al. (2003). A compilation of their reviews is found in Table 1.
\n\t\t\t\tSystem\n\t\t\t | \n\t\t\t\n\t\t\t\tHealth effects\n\t\t\t | \n\t\t
Cardiovascular | \n\t\t\tHeart attack, cardiac arrhythmias, thickening of blood vessels, loss of circulation leading to gangrene of extremities, hypertension | \n\t\t
Dermal | \n\t\t\tHyperpigmentation, abnormal skin thickening, narrowing of small arteries leading to numbness (Raynaud’s Disease), squamous and basal-cell cancer | \n\t\t
Gastrointestinal | \n\t\t\tHeartburn, nausea, abdominal pain | \n\t\t
Hematological | \n\t\t\tAnemia, low white-blood-cell count (leucopenia) | \n\t\t
Hepatic | \n\t\t\tCirrhosis, fatty degeneration, abnormal cell growth (neoplasia) | \n\t\t
Neurological | \n\t\t\tBrain malfunction, hallucinations, memory loss, seizures, coma, peripheral neuropathy | \n\t\t
Pulmonary | \n\t\t\tChronic cough, restrictive lung disease, cancer | \n\t\t
Respiratory | \n\t\t\tLaryngitis, tracheal bronchitis, rhinitis, pharyngitis, shortness of breath, perforation of nasal septum | \n\t\t
Renal | \n\t\t\tHematuria, proteinuria, shock, dehydration, cortical necrosis, cancer of kidneys and bladder | \n\t\t
Reproductive | \n\t\t\tSpontaneous abortions, still-births, congenital malformations of fetus, low birth weight | \n\t\t
Summary of effects of chronic arsenic exposure on human health. (Data from Mandal and Suzuki, 2002, and Ng et al., 2003, and references therein.)
The carcinogenic properties of As were suspected as early as the late 19th Century (Smith et al., 2002). Arsenic is now widely recognized and regulated as a carcinogen (ATSDR, 2000; National Research Council, 1999; USEPA, 2001). Consequently, the occurrence of As in waters at concentrations that exceed existing standards for drinking-water supplies has become of increasing concern, leading to recommended or legislated decreases in concentrations of As in drinking water in many countries. In 1993, the World Health Organization provisionally recommended a decrease from 50 μg/L to 10 μg/L (WHO, 1993). The United States (USA) federal standard, the European Union (EU) Drinking Water Directive (98/83/EC), the New Zealand Drinking Water Standard, the Japanese standard, and recent laws in many Latin American countries (Argentina, Bolivia, Brazil, Chile, Colombia, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, and Panama) now place 10 μg/L as the drinking water maximum contaminant level (MCL) (Bundschuh et al., 2012; Robinson et al., 2004; Rowland et al., 2011; Smedley and Kinniburgh, 2002; USEPA, 2001). Mexico has adopted 25 μg/L as a standard (Bundschuh et al., 2012), whereas Australia has instituted a standard of 7 μg/L (NHMRC, 1996) and the State of New Jersey in the USA adopted an As MCL of 5 μg/L in 2006 (NJDEP, 2009). Some developing countries (Bangladesh, for example) have maintained the earlier 50 μg/L MCL standard (Ng et al., 2003). Many instances of As concentrations in groundwater that far exceed standards have been reported throughout much of the world (Smedley, 2008) and the number of countries in which groundwater is found to be contaminated by As has increased substantially over the past 80 years. This chapter presents a brief overview of the history of groundwater As contamination and summarizes information about the sources, occurrence and mobility of As in groundwater. A compilation of worldwide hazardous waste sites is beyond the scope of the chapter, and only a few examples will be presented. Information on As occurrence reported in previous important summaries and discussions (e.g., Bhattacharya et al., (eds) 2007; Smedley and Kinniburgh, 2002; Welch and Stollenwerk, (eds. ) 2003) is presented in addition to recent findings from the past decade.
Currently (2012), As contamination of groundwater resources has been identified in many parts of the world, although recognition of the widespread nature of the problem has been advanced only relatively recently. Despite localized inputs of As from human activities, much of the contamination of groundwater with As is shown to arise from geogenic sources and affected groundwater has been found in countries on nearly every continent or major land mass. To date, none has been reported for Greenland, and Antarctica (Figure 1).
Countries, states, provinces, or areas mentioned in this chapter in which arsenic concentrations in groundwater, including geothermal waters and water contaminated by mining, exceed 10 micrograms per liter (the World Health Organization (WHO) recommended standard (WHO, 1993)). Arsenic concentrations at most locations shown exceed 50 micrograms per liter. Locations of dots are spatially generalized and do not indicate precise locations of arsenic-contaminated waters.
The discoveries of As contamination of groundwater have occurred over a span of nearly 100 years, the most recent within the last decade. Observations of health problems first led to the realization that As was being inadvertently ingested. Arsenic poisoning in humans in Argentina was recognized as early as 1913, and attributed to the drinking of groundwater (Bundschuh et al., 2012, and reference therein). A possible connection between skin cancers and drinking water was recognized in Taiwan during the 1930s (Chen et al., 1994). In the 1940s, As contamination of well water in the Pannonian Basin in Romania and adjacent Hungary was discovered (Gurzau and Pop, 2012; Mukherjee et al., 2006). Recognition of similar occurrences in other European countries such as southwestern England, Germany, Greece, and Spain followed, and groundwaters with As concentrations that exceed standards are now observed in more than 70 countries worldwide (Nordstrom, 2002; Ravenscroft et al., 2009).
During the mid-20th century, instances of As contamination of groundwater were reported for the western USA and Alaska (Welch et al., 1988; Mueller et al., 2001), but were not fully recognized in the states of Oklahoma, Texas, and Arkansas and in the Midwest and Northeastern parts of the country until the 1980s (Ayotte et al., 2003; Ayotte et al., 1999; Haque et al., 2008; Peters, 2008; Peters and Burkert, 2007; Scanlon et al., 2009; Sharif et al., 2008; Welch et al., 2000 and references therein). Most recently, groundwater containing As in excess of USA Federal and State MCLs was found in parts of the Atlantic Coastal Plain (Barringer et al., 2010; Drummond and Bolton, 2010; Haque et al., 2008; Mumford et al., 2012; Pearcy et al., 2011). In Canada, As-contaminated groundwaters in New Brunswick and Nova Scotia were noted in the 1970s (Bottomley, 1984), and instances in western Canada were noted in the 1960s and 1980s (Wang and Mulligan, 2006a).
In West Bengal, India, cases of arsenic poisoning were first noted in 1983-84, according to Rahman et al. (2005), although Mandal and Suzuki place 1978 as the time when arsenicosis (skin lesions) and groundwater contamination were first noticed in West Bengal. Since the 1980s, extensive sampling of well water in West Bengal has revealed levels of As that exceed 50 μg/L —concentrations in some samples exceeding 1000 μg/L (Rahman et al., 2005). In 1983-84, several patients treated for arsenicosis in West Bengal came from neighboring Bangladesh. Sampling in Bangladesh during the early 1990s of waters from tube wells (installed two decades earlier to provide what was thought to be safe, pathogen-free drinking water) revealed elevated concentrations of As (Smith et al., 2000).
Continued sampling throughout West Bengal showed concentrations ranging from <1 to 2,500 μg/L (Nordstrom, 2002). Although the population exposed to As-contaminated water in West Bengal was ultimately estimated to have been about 6,000,000, the exposed population in Bangladesh has been estimated to be about five times that number (Nordstrom, 2002), making the contamination in Bangladesh the arsenic-related public health problem of the greatest magnitude yet observed. Following the early discoveries in West Bengal and Bangladesh, well-water sampling in Vietnam, Cambodia, and Pakistan also identified contamination of groundwater with As (Agusa et al., 2006; Berg et al., 2001; Chanpiwat et al., 2011; Hoang et al., 2010; Luu et al., 2009; Nickson et al., 2005; Smedley, 2008; Sthiannopkao et al., 2008;), as well as in 1999, Nepal (Gurung et al., 2005), and in 2000, Myanmar (Tun, 2003).
Health problems attributed to As exposure were first noted within a district in a province of Thailand in 1987. Sampling indicated As at concentrations in groundwater that exceeded 5,000 μg/L with about 15,000 people thought to have been exposed (Nordstrom, 2002; Smedley, 2008). Sampling of well waters during the 1990s and later in northern parts of China, including Inner Mongolia, have shown groundwater As concentrations that range widely, from <1 to about 2,400 μg/L (Nordstrom, 2002; Smedley, 2008). In Japan, As levels were found to be high in geothermal waters and springs as early as the 1950s (Noguchi and Nakagawa, 1969), and As contamination of groundwater was noted in 1994. Exposure to As from industrial sources was noted as early as the 1950s (Mukherjee et al., 2006). In 1981 in Iran, chronic arsenic poisoning was noticed and subsequent well-water sampling revealed concentrations exceeding 1,000 μg/L (Mukherjee et al., 2006).
Arsenic contamination of both surface water and groundwater also is found in many Latin American countries, but the full extent of the problem is not yet clear (Bundschuh et al., 2012). Groundwater concentrations in Argentina are reported to range as high as about 15,000 μg/L (Bundschuh et al., and references therein). High levels of As were found in waters in Chile and Mexico in the 1950s and 60s (Bundschuh et al., 2012; Rosas et al., 1999). In northern Chile, As concentrations in groundwater ranged from 20 to 5,000 μg/L, and there is strong evidence from hair, skin, bones and funerary preparations (clay, paint) of Chinchorro culture mummies that the population there was exposed to high levels of As more than 7,000 years ago (Bundschuh et al., 2012), and, presumably, has been ever since. Concentrations of As in groundwater in Mexico generally are not reported to reach the higher levels found in Argentina except for a geothermal area in Michoacán, where 24,000 μg/L are reported (Bundschuh et al., 2012, and references therein). In mined areas of Mexico, leaching from tailings piles have contributed As to groundwater (Carillo-Chávez et al., 2000; Méndez and Armienta, 2003). Arsenic-contaminated surface water and groundwater subsequently was recognized in parts of Peru in the 1970s (Bundschuh et al., 2010). Although As contamination of waters in Argentina, Chile, Mexico and Peru was known from the early and mid-20th century, surface water and (or) groundwaters (and geothermal waters) containing geogenic As at contaminant levels were discovered in alluvial, metasedimentary, volcanic, and metavolcanic aquifers only since the late 1990s in Bolivia, Brazil, Columbia, Cuba, Ecuador, El Salvador, Guatemala, Honduras, Nicaragua, and Uruguay (Bundschuh et al., 2012).
In Africa, the occurrence of high concentrations of As in groundwater from wells in a village in Burkina Faso was first noticed in the 1970s, but more extensive sampling did not occur until several decades later (Smedley, 2008). Arsenic-related skin diseases were noted in the region, and, although many wells yielded water with As concentrations < 10 μg/L, concentrations as high as 1,600 μg/L were found (Smedley et al., 2007). Effects of mining on soils and waters in other African countries (Ghana and Zambia) have been studied only recently (e.g., Bowell et al., 1994; Nakayama et al., 2011; Smedley and Kinniburgh (2002); As contamination of groundwater has been reported in Ghana. Arsenic contamination of well water was discovered during the drought in Perth, Australia in 2002 (Appleyard et al., 2006).
Sources of As that arise from human activities include mining and processing of ores and manufacturing using As-bearing sulfides. Smelters in numerous countries, including Canada, Chile, Italy, South Africa, the USA, and the former USSR have processed metal ores (mainly copper, but also zinc, gold, and tin) that contain As. The smelting process, both recent and ancient, has released As to the air and soils both locally and globally (Matschullat, 2000). Elevated As levels in precipitation and in soils surrounding smelters have frequently been documented (e.g., Ball et al., 1983; Beaulieu and Savage, 2005; Carpenter et al., 1978). Disposal of mining wastes has caused As contamination of groundwater in numerous places, including in Thailand, Ghana, and Turkey (Gunduz et al., 2010; Smedley and Kinniburgh, 2002; Smedley, 2008). In southwestern England, mineral deposits and mineral processing (tin, copper, with accessory As minerals) are recognized sources of As to soils and groundwater (Brunt et al., 2004; Camm et al., 2004; Palumbo-Roe et al., 2007). In southeastern Europe (Serbia, Bosnia, Poland), and in Spain, mining activities left a legacy of arsenic that has contaminated soils and waters (Dangic, 2007; Gomez et al., 2006; Karczewska et al., 2007). Groundwater contamination in mined areas is also found in parts of the western USA and Canada (Moncur et al., 2005; Welch et al., 2000).
Arsenic compounds have been used in the manufacture of numerous products. Arsenic has been used in glass production and by the wood-preservation industry. The latter industry has been, through the end of the 20th century, the most active user of such compounds in the USA (Welch et al., 2000), but the industry’s voluntary reduction of the use of chromated copper arsenate (CCA) since 2003 has resulted in a more limited availability of CCA treated wood and products (Brooks, 2008). Contamination of soils and surface-water bodies has resulted from use of CCA-treated wood (Khan et al., 2006; Rice et al., 2002).
The use of inorganic arsenical pesticides has waned in recent years owing to bans in the 1980s and 90s (Welch et al., 2000), but, in the past, manufacture and use of arsenical pesticides were important contributors of As to the environment (e.g., Barringer et al., 1998; Barringer et al., 2001; USEPA, 2011). Inorganic arsenicals have been used on a variety of crops (citrus, cotton, tobacco, and potatoes) and on fruit trees; (Walsh and Keeney, 1975; Welch et al., 2000). The use of lead-arsenate pesticides in orchards has prompted concern that additions of phosphate fertilizers could displace arsenate sorbed to soil particles, mobilizing As to groundwater (e.g. Davenport and Peryea, 1991). Manufacture of pesticides has been responsible for As contamination of soil, surface water, and groundwater. Examples of contamination caused by former pesticide manufacture are found in India and in the USA (Barringer et al., 1998; Mukherjee et al., 2006; USEPA, 2011). Soil contamination by As may also occur in areas where soils are amended with chicken and swine manure. Such fertilizers can contain As due to use of chicken and swine feeds containing the growth additive Roxarsone, an organic arsenical (4-hydroxy-3-nitrophenylarsonic acid) (Hileman, 2007; O’Day, 2006).
There have been military uses of As, and attempts to recycle As-bearing materials. Arsenic was used in chemical warfare agents, most recently in the first half of the 20th century (Krüger et al., 2007). In the 1930s, concern over disposal of stockpiles of As compounds led to experiments with incorporating As in cement which was then used to coat pilings and other wooden structures (van Siclen and Gerry, 1936). One ounce of white arsenic was added to 12 pounds of sand and 3 pounds of cement, water was added and the slurry was applied to wood pilings by air gun. To the authors’ knowledge, no studies of the effects of this practice on the environment have been published. Arsenic currently is used in various electronic devices, and improper disposal and lack of care in recycling these materials also can add As to the environment (Brooks, 2008).
Anthropogenic sources of As can affect the quality of surface water through groundwater discharge and runoff (Hemond, 1995; Martin and Pederson 2002). In the case of pesticides, the effect can be through direct applications to water bodies for control of nuisance vegetation (Kobayashi and Lee, 1978; Tanner and Clayton, 1990; Durant et al., 2004). Although groundwater contamination does exist at various sites affected by agricultural, industrial or military releases, for example (e.g., Hemond, 1995; Krüger et al. 2007; USEPA, 2011), contamination introduced at the land surface does not always move to groundwater. Owing to the affinity of As for soil constituents such as metal oxides and hydroxides (mainly iron (Fe), aluminum (Al), and manganese (Mn)) and clays (Goldberg and Glaubig, 1988; Manning and Goldberg, 1996), the As can be attenuated in the intervening soils by sorption to Fe hydroxides or clays, or by precipitation reactions, such as formation of As- or Fe-sulfides in anoxic soils (e.g., Brunt et al., 2004; Cancès et al, 2008).
For most known areally extensive instances of As contamination of groundwater, the sources of the As have been shown to be geogenic (Smedley and Kinniburgh, 2002). A summary table of the worldwide occurrence of As in groundwater (Nordstrom, 2002) indicates that mining of arsenic and metal ores and natural geologic sources of As dominate the environmental conditions listed for inputs of As to groundwater.
There are about 24 As-bearing minerals that are commonly found in hydrothermal veins, ore deposits, and rocks. Most primary As minerals are sulfides, of which arsenopyrite is the most common (Ehrlich and Newman, 2009). Secondary minerals tend to be less common arsenates and oxides. WHO (2001) provides a list of these minerals, which also can be found tabulated in Mandal and Suzuki (2002, p. 203). Arsenic in crustal rocks also has an affinity for, and is associated with, pyrite or Fe hydroxides and oxides (Nordstrom, 2002) for which chemical formulas are FeS2, FeOOH, Fe2O3, and Fe3O4, respectively. The As content of crustal rocks varies widely; Smedley and Kinniburgh, 2002, p. 531) compile and tabulate those results. Concentrations of As in water associated with crustal rocks are described below.
Geothermally active zones occur along plate boundaries, in tectonic rift areas such those in East Africa and at seafloor spreading centers, such as in Iceland, and at “hot spots” where mantle-derived plumes ascend, such as in Hawaii and Yellowstone National Park, USA. Arsenic is one of a suite of incompatible elements (these do not fit easily into the lattices of common rock-forming minerals), which include antimony (Sb), boron (B), fluoride (F), lithium (Li), mercury (Hg), selenium (Se) and thallium (Tl). Together with hydrogen sulfide, these elements are ubiquitous in high-temperature geothermal settings (Webster and Nordstrom, 2003). Concentrations of arsenic are high mainly in geothermal waters that leach continental rocks; geothermal waters in basaltic rocks, such as in Iceland, contain lesser amounts of As. Arsenic in hot geothermal fluids was shown to be derived mainly from leaching of host rocks at Yellowstone National Park, in Wyoming, USA (Stauffer and Thompson, 1984), rather than derived from magmas. The As in the hot fluid is present as As(III) in arsenious acid (H3AsO3); in low-sulfide fluids, the arsenite in the arsenious acid is oxidized to arsenate as the rising fluid mixes with cold oxygenated groundwater or encounters the atmosphere. In high sulfide solutions, As may be present as thioarsenate complexes (Webster and Nordstrom, 2003; Planer-Friedrich et al., 2007).
In the western USA, there are As inputs to groundwater and surface water from geothermal fluids in and near Yellowstone National Park (e.g., Ball et al., 1998, 2002; Nimick et al., 1998) and in other western mineralized areas (Welch et al., 1988). Groundwater associated with volcanics (tuffs and rhyolites) in California contain As at concentrations ranging up to 48,000 μg/L, with As-bearing sulfide minerals as the main source of As (Welch et al., 1988, and references therein). Geothermal waters on Dominica in the Lesser Antilles also contain concentrations of As >50 μg/L (McCarthy et al., 2005).
In general, because arsenic is an incompatible element, it accumulates in differentiated magmas, and is commonly found at higher concentrations in volcanic rocks of intermediate (andesites) to felsic (rhyolites) composition than in mafic (basaltic) rocks—as shown for the western USA (Welch et al., 1988; Welch et al., 2000). In Maine and New Hampshire, USA, where As-contaminated groundwater is present, pegmatites, granites and metamorphic rocks (granofels) were found to have substantial As contents—up to 60, 46 and 39 mg/kg, respectively (Peters, 2008; Peters et al., 1999; Peters and Blum, 2003). Weathering of pegmatite veins in Connecticut, USA, was thought to contribute As to groundwater (Brown and Chute, 2002). Although the As content of mafic rocks can be relatively low, fractured ultramafic rocks in Vermont, USA, contribute up to 327 μg/L of As to groundwater (Ryan et al., 2011).
Leached from surrounding rocks, As in hot springs from geothermal fields in New Zealand is found at concentrations that range to 4,800 μg/L (Brown and Simmons, 2003). Acidic (pH 1.2) geothermal springs in Japan contained As at 2,600 μg/L (Noguchi and Nakagawa, 1969). Arsenic in these springs precipitated out as As sulfides and lead (Pb) As sulfides in surface-water sediments. The As contents of the sediments ranged from about 5 to 56 wt. % (Noguchi and Nakagawa, 1969).
In Latin America, volcanic and geothermal activity along the Pacific tectonic plate boundaries produces As-rich waters and gases in springs and fumaroles. In sodium-chloride (Na-Cl)-rich waters, As concentrations can reach about 50,000 μg/L at the El Tatio geothermal field in Chile (Lopez et al, 2012). Arsenic concentrations in waters of geothermal fields in Mexico vary; 250 to 73,600 μg/L are reported, where the host rocks through which these waters have risen include sandstones and shales, lava flows and pyroclastics, and metamorphosed carbonate rocks, basalts and hornblende andesites (Lopez et al., 2012).
In the coastal volcanic areas of Central and South American countries, rocks are mainly andesitic or rhyolitic in composition. Arsenic concentrations in the geothermal springs throughout these areas vary widely—concentrations of several thousand micrograms per liter are reported, but none are reported as high as the highest concentration in a Mexican geothermal field (see above). Where springs and fumaroles discharge water and gases to a lake in the Bolivian Altiplano, however, As concentrations in the lake water are reported as high as 4,600,000 μg/L, the As apparently being contributed by oxidation of sulfide deposits (Lopez et al., 2012).
Dissolution of volcanic glasses in ash layers and leaching of loess-type deposits in the Chaco-Pampean plain of Argentina have resulted in groundwater As concentrations that range from <10 to 5,300 μg/L (Nicolli, et al., 2012) where the potentially affected rural population numbers several million people. Mining of various metals (gold, copper, silver) in Latin America has played an important role in mobilizing As from the geologic materials and exacerbating contamination of groundwater resources (McClintock et al., 2012).
In Europe, groundwaters containing As at concentrations that exceed 50 μg/L are found in geothermal fields of the Massif Central in France, and in Greece, (Brunt et al., 2004; Karydakis et al., 2005). Iceland, where As concentrations in groundwater can exceed 10 μg/L (Arnórsson, 2003) sits astride the Mid-Atlantic Ridge, and is subject to outpourings of basaltic lava that typically contain less As than do more silicic lavas (Onishi and Sandell (1955; Baur and Onishi, 1969; Ure and Berrow, 1982). In geothermal systems of northern and northeastern Spain, the As concentrations are high and deposit As-rich minerals (Navarro et al., 2011); the Caldes de Malavella field in northern Spain contributes substantial As to groundwater—50-80 μg/L in springs, and from <1 to 200 μg/L in groundwater (Piqué et al., 2010). Groundwaters (including brines) contained As concentrations ranging from 1.6 to 6,900 μg/L in the Phlegraean Fields in southern Italy (Aiuppa et al., 2006). Quaternary volcanic rocks with hydrothermal activity on the island of Ischia (offshore from Naples, Italy) impart As to groundwater at concentrations that range up to 3,800 μg/L (Aiuppa, et al., 2006; Daniele, 2004).
In the Mid-East, in northwestern Iran, As concentrations in thermal waters and hot springs are as high as 3,500 and 890 μg/L, respectively, in the area of Mt. Sabalan, a stratavolcano (Haeri et al, 2011). In western Anatolia, in Turkey, natural leaching, aided by pumping and discharge of waste geothermal fluids from an active geothermal system, has mobilized As from metamorphic, igneous and sedimentary rocks to groundwater in a shallow alluvial aquifer, where highest As concentrations in groundwater and geothermal waters were 561 and 594 μg/L, respectively (Gunduz and Simsek (2008).
Groundwater in early Proterozoic silicic volcanics and granites of the Chhattisgarh Basin of India contains As at concentrations that exceed 10 μg/L. The As is perhaps emplaced there by hydrothermal fluids (Acharyya, 2002).
Arsenic is found in coals, with the content of some coals from southwestern China being highest—826 to 2,578 mg/kg is reported (Nriagu et al., 2007) and up to 32,000 mg/kg is listed by Wang et al., (2006). In Germany, the As content of bituminous shales ranges from 100 to 900 mg/kg (Smedley and Kinniburgh, 2002). The As contents of American coals are reported to range as high as 2,200 mg/kg (Wang et al., 2006), but the mean concentration for more than 7000 samples is about 24 mg/kg (Kolker et al., 2006). Pyrite is the main source of As in coals with high As content, whereas in lower As coals, the As tends to be associated with the organic material (Yudovich and Ketris, 2005). In Pennsylvania, USA, As concentrations in water discharging from abandoned anthracite mines ranged from <0.03 to 15 μg/L and from abandoned bituminous mines, from 0.10 to 64 μg/L, with 10% of samples exceeding the USEPA MCL of 10 μg/L (Cravotta, 2008).
In Wisconsin, USA, As concentrations of water in sandstone and dolomite aquifers were as high as 100 μg/L. Oxidation of pyrite hosted by these formations was the likely source of the As, the transport of which was, in some instances, retarded by its association with Fe oxyhydroxides (Burkel and Stoll, 1999; Thornburg and Sahai, 2004). In the adjacent State of Michigan, USA, As concentrations in groundwater reached 220 μg/L in another sandstone aquifer (Haack and Rachol, 2000). In Australia, a combination of increased water withdrawals during development and declining recharge due to drought caused oxidation of pyrite in sedimentary aquifers, resulting in As contamination of well water (Appleyard et al., 2006). In England, groundwater from a sandstone aquifer contained As at concentrations that spanned 10 to 50 μg/L; the As content of the sandstone ranged from 5 to 15 mg/kg. Desorption at pH of about 8 appeared to be the mechanism for As release to groundwater (Kinniburgh et al., 2006). Water from wells completed in a Mesozoic Era sandstone in northern Bavaria also contained As at concentrations from 10 to 150 μg/L (Heinrichs and Udluft, 1999), although the mineralogy contributing the As was not identified.
In the Piedmont of Pennsylvania and New Jersey, USA, groundwater in Mesozoic age aquifers of red and black shale, mudstone, and siltstone contains elevated levels of As—domestic well waters from Pennsylvania contained up to 65 μg/L (Peters and Burkert, 2007), whereas in New Jersey the highest concentration measured recently was 215 μg/L (Serfes, 2005). Pyrites in the reduced black shales and mudstones are a major source of As, with measured As contents of 3,000 and 40,000 mg/kg in some samples (Serfes, 2005). Arsenic (as arsenate) is also released from the red shales by desorption as pH rises above 6.5 (Serfes, 2005). These Piedmont rocks also contain hornfels along contacts with diabase intrusions, some of which contain mineralization by copper (Cu), As, and uranium (U). Typical As-bearing minerals in the Piedmont rocks include arsenopyrite (FeAsS), cobaltite ((Ni, Co, Fe)AsS), alloclasite ((Co, Fe)AsS), gersdorffite (NiAsS), erythrite (Co3(AsO4)2.8H2O), and safflorite ((Co, Fe, Ni)As2) (Senior and Sloto, 2006).
In Taiwan, groundwater from artesian wells completed in black shales, muds and fine sands is contaminated with As. In northern Bavaria, mineralized sandstone yields As-contaminated groundwater (Smedley and Kinniburgh, 2002).
Carbonate rocks typically contain low concentrations of As (Baur and Onishi, 1969; Smedley and Kinniburgh, 2002), although some limestones may contain As-bearing pyrite (e.g. Price and Pichler, 2006). Because arsenate can substitute for phosphate in minerals, phosphorite deposits can contain substantial amounts of As—up to about 400 mg/kg is reported (Smedley and Kinniburgh, 2002). Barringer et al. (2011) report 19.5 to 56.6 mg/kg of As in phosphorite deposits in the Coastal Plain of New Jersey, USA.
The As content of meta-sedimentary rocks varies widely, with the contents of gneisses and quartzites generally < 10 mg/kg, and higher As contents for slates and phyllites —up to 143 mg/kg (Boyle and Jonasson, 1973). In the New England states of northeastern USA, groundwater contamination with As was found to be prevalent in water from wells completed in formations containing metapelite rocks (schists, phyllites, slates), particularly those rocks adjacent to intrusive bodies. In New England, meta-shales contain As-bearing minerals pyrrhotite, cobaltite, and arsenopyrite, and supergene minerals include orpiment (AsS) and loellingite (FeAs2)(Foley et al., 2002). Mineralization in a Proterozoic marble in New Jersey, USA, has resulted in zinc ores and, in addition to arsenopyrite and loellingite, a variety of uncommon As-bearing minerals. Past mining activities have contributed to the release of As from the bedrock to shallow groundwater; concentrations ranged from 2.02 to 22.0 μg/L in water discharging to the area’s major river, the Wallkill River (Barringer et al., 2007). In mineralized meta-sedimentary rocks containing sulfide minerals in Fairbanks, Alaska, USA, groundwater contains As at concentrations that range from <3 μg/L to 1,670 μg/L (Mueller et al., 2001).
The sediments shed from the Himalayas have formed the extensive alluvial plain and delta through which the Ganges, Brahmaputra and Meghna Rivers flow and which form aquifers in India (West Bengal Delta Region) and adjacent Bangladesh. To the east, the deltas of the Mekong and Red Rivers form aquifers in Vietnam and adjacent Cambodia and Laos. Arsenic released from these sediments has caused the most widespread contamination in the world, with populations of many millions affected by drinking As-laden well water. The largest number of people (about 35 million) exposed to As contamination is in Bangladesh, and about 6 million in neighboring West Bengal, India (Nordstrom, 2002). The As contents of these young sediments are not extremely high—about 1 to 15 mg/kg, but vary with depth (Smedley, 2008). The concentrations of As in groundwater range, however, from undetectable to several thousand μg/L, with a survey of about 3,200 wells in Bangladesh by researchers from the United Kingdom (UK) and Bangladesh finding that As concentrations in about 27% of samples exceeded 50 μg/L, the Bangladeshi Drinking Water Standard (Smedley, 2008). In local areas, the percentage of affected wells was higher; in central Bangladesh, As in water from about 75% of 6000 wells exceeded 50 μg/L (van Geen et al., 2003). The sediments of Holocene age are micaceous sands, silts, and clays. Reduced, gray sediments of the upper aquifer, where As concentrations in groundwater are high, are underlain by brown, oxidized sediments where As concentrations in groundwater are low. In some places the two layers are separated by a thick clay layer (Harvey et al., 2002). The mineralogy of both layers, where the clay was absent, was found to be similar for part of the Bengal Basin near Dhaka, Bangladesh. Minerals included quartz, plagioclase and potassium feldspar, micas (biotite, muscovite, and phlogopite), chlorite, and amphibole; carbonaceous material was sparse. Trace amounts of siderite were present in the reduced sediments, but not in the deeper, oxidized sediments (Stollenwerk et al., 2007). In the shallow aquifer in West Bengal, organic carbon is present as petroleum-related compounds (Rowland et al., 2006).
In Cambodia, Vietnam, and Laos, As in young deltaic sediments of the Mekong and Red River basins has also contaminated groundwater, again exposing several million people to unhealthy levels of As (> 1,000 μg/L in some cases) in drinking water (Agusa et al., 2006; Berg et al., 2001; Chanpiwat et al., 2011; Hoang et al., 2010; Luu et al., 2009; Sthiannopkao et al., 2008). The aquifers are composed of quartz sands and clays that host Fe oxide and hydroxide phases, also possibly manganese (Mn) oxides, and organic matter is present. Siderite, pyrite, and orpiment are also found in sediments of the Mekong delta (Quicksall et al., 2008) and siderite, ilmenite, vivianite, gibbsite and boehmite are reported for the Red River delta sediments (Eiche et al., 2008).
In the Pannonian Basin of Hungary and Romania, Quaternary sediments of fluvial and eolian origin have contributed As to groundwater; the sediments are composed of sands and loess. Quartz, feldspar, carbonates (calcite and dolomite, muscovite, chlorite, clays, and humic substances are reported and fine particles of Fe hydroxides are indicated (Varsányi and Kovács, 2006). Other Fe-bearing minerals, from which As may be released, are reported to include goethite, limonite, pyrite, and siderite (Rowland et al., 2011).
Alluvial and lacustrine sediments in the Huhhot Basin of Inner Mongolia form two aquifers, separated by a clay confining layer; some boreholes completed in the deeper aquifer are artesian. The sediments are more fine-grained in the low-lying parts of the basin and it is in these sediments that reducing conditions are present and As concentration in groundwater are highest (1,500 μg/L). Organic matter is found in the aquifers and dissolved organic carbon concentrations in groundwater are high (Smedley et al., 2003).
The Atlantic Coastal Plain is located along the east coast of the USA. In addition to quartz-rich deposits of near-shore origin, the Coastal Plain is composed partly of sediments of marine origin that contain the mineral glauconite, the As contents of which are high (up to 130 mg/kg) in some formations (Dooley, 2001). In the state of Maryland, USA, in an aquifer composed of these marine sediments, As concentrations in groundwater exceed 10 μg/L and have been found as high as 80 μg/L (Pearcy et al, 2011). Farther north, in the state of New Jersey, As concentrations in water from an observation well in a glauconite-bearing aquifer were 110 μg/L (dePaul and Szabo, 2007), and water from several domestic wells in similar aquifers in the same region has exceeded the state MCL of 5 μg/L. In shallow groundwater discharging to New Jersey Coastal Plain streams underlain by the glauconitic sediments, As concentrations have exceeded the MCL, ranging as high as 89.2 μg/L. The sediments below the streambeds also contain other phyllosilicates (illite, smectites, muscovite, biotite, chlorite) and quartz. In addition to the glauconite sands, associated phosphorite deposits were found to contain As up to 56.6 mg/kg and siderite that precipitated in sediments beneath a streambed contained 184 mg/kg of As (Barringer et al., 2011; Mumford et al., 2012).
Glauconite sands and clays of Pliocene and Miocene age, overlain by younger non-glauconitic sands, clays, thin coal beds and peats, are also present in the lowlands of xouth Sumatra, Indonesia. Arsenic concentrations exceeded the WHO guidelines in water from several wells completed in both the glauconitic formations and the younger sediments, with the higher concentrations found in water from the youngest (Holocene) sediments (Winkel et al., 2008). Glauconitic sediments are found on several continents (Barringer et al., 2010) and the recent findings in the Atlantic Coastal Plain and Sumatra indicate that these marine sediments, in addition to all the aquifers in other geologic settings, can now also be considered a potential source of As-contaminated groundwater.
Oxidation of As-bearing sulfides has been proposed as a mechanism for releasing As from geologic materials (Smedley, 2008). Although originally proposed as a mechanism for As release from the alluvial sediments of West Bengal and Bangladesh, the presence of sulfide minerals in those aquifers is rare and appears limited to biogenic framboidal pyrite, and pyrite in woody peat, and on magnetite (Acharyya, 2002). Oxidation of sulfides in mined areas throughout the world is a well-known phenomenon that has led to high concentrations of As in soils, surface water and groundwater; examples of such occurrences include western Canada, the western USA, and the Bolivian Altiplano (Lopez et al., 2012; Moncur et al., 2005; Welch et al., 2000). Sulfide oxidation has also resulted in high As concentrations (up to 215 μg/L) in groundwater in the eastern USA, in parts of the Piedmont rocks in Pennsylvania and New Jersey (Peters and Burkert, 2007; Serfes, 2005), where an As-oxidizing bacterium was involved in the mobilization of As (Rhine et al., 2008). A broad diversity of microorganisms oxidizes dissolved arsenic for different reasons including dissimilatory respiration, detoxification, and energy needs (Santini and Ward, 2012).
Because of the magnitude of the contamination in West Bengal and Bangladesh, these two regions have received substantial attention from the research community. In general, reductive dissolution of Fe hydroxides and release of sorbed As explains much of the observed mobilization of As from sediments to groundwater (e.g., Nickson et al., 2000; Zheng et al., 2004). Organic matter in the alluvial aquifers is likely an important component of the reduction process. Field and experimental studies have shown that metal-reducing microbes can enhance mobilization of As, and that oxidation of the organic matter drives the redox reactions whereby Fe hydroxides are reductively dissolved and sorbed As released (Islam et al., 2004; McArthur et al., 2004). Arsenic also is released from Mn oxides as they reductively dissolve, but may not remain in the groundwater, instead resorbing to Fe hydroxides (McArthur et al., 2004). Other studies indicate that As could also be released from biotite into Bangladeshi groundwater (Hopf et al., 2001; Seddique et al., 2008).
Reductive dissolution of Fe hydroxides has been proposed as a viable mechanism for As release in many other affected aquifers as well—for example, in Croatia, Inner Mongolia, northern China, and the eastern and southeastern USA (Barringer et al., 2010, Barringer et al., 2011; Guo et al., 2010; Haque et al., 2008; Mumford et al, 2012; Pearcy et al., 2011; Sharif et al., 2008; Ujevic et al., 2010; Xie et al, 2008). Although less intensely studied than Bengal Basin sediments, release of As from Fe oxides appears to be an important mechanism in the Inner Mongolian sediments (Smedley, 2008).
Given the similarities to the shallow aquifers in Cambodia, Vietnam, Laos, and Myanmar, much of what has been found in West Bengal and Bangladesh may apply to the aquifers in those countries as well. A study of indigenous bacteria in Cambodian sediments indicated arsenic-respiring bacteria that reduce As(V) (arsenate) to As(III) (arsenite) were fueled by inputs of organic carbon (Lear et al., 2007), similar to findings for bacteria in West Bengal sediments. Additionally, results of experiments showed microbially mediated reduction of As(V), and Fe, and release from minerals (clay, glauconite sands, oxides and hydroxides) (Campbell et al., 2006; Dong et al., 2003; Hopf et al., 2009; Kostka et al., 1999; McLean et al., 2006; Pearcy et al., 2011), demonstrating the involvement of microbes in the reduction-oxidation (redox) reactions.
The bacteria involved in the reactions involve several groups. Iron-reducing bacteria of the genus Geobacter can reduce Fe in minerals such as hydroxides (Lloyd and Oremland, 2006), thus leading to dissolution of the hydroxides and sorbed As release. Geobacter bacteria have been investigated as As reducers; although G. uraniumreducens contains genes for As respiration, it was not conclusively shown to respire As, and G. sulfurreducens did not reduce As enzymatically (Islam et al., 2005; Lear et al., 2007). Bacteria known as dissimilatory arsenate respiring prokaryotes (DARPs) are identified as arsenic reducers, which means that As(V) serves as the terminal electron acceptor in dissimilatory reduction of arsenate. In the glauconitic sediments of the New Jersey Coastal Plain, USA, amplification of the arsenic respiratory reductase gene (arrA) followed by alignment and gene sequencing revealed clones with close (99%) similarity to Alkaliphilus oremlandii (CP000453) (formerly Clostridium species strain OhlLAs)—a known arsenate-respiring bacterium (Mumford et al., 2012). Also using molecular techniques, an arsenate-respiring proteobacterium Sulfurospirillum sp. strain NP4 was identified by Lear et al. (2007) in Cambodian sediments. The bacterium Desulfotomaculum auripigmentum reduces As(V) to As(III) as well as sulfate to sulfide, and precipitates orpiment (Newman et al. 1997; Ehrlich and Newman, 2009)
It should be noted, however, that not all microbial reduction of arsenate is the result of bacterial respiration. Many bacteria detoxify arsenate by reducing it to arsenite and expelling it (Oremland and Stolz, 2005) and, in some cases, by methylating and expelling it (Bentley and Chasteen, 2002). Presence of dimethylarsinate and monomethylarsonate in groundwater, as was found in shallow groundwater in glauconitic sediments of the New Jersey Coastal Plain, probably was indicative of such microbial activity (Mumford, et al., 2012). In experiments, As was mobilized from apatite by the bacterium Burkholderia fungorum, which utilizes phosphorus from the apatite (Mailloux et al., 2009). There is apatite present in the sediments released from the Himalayas to the alluvial aquifers of South East Asia, and there are apatite-rich phosphorite beds in the glauconitic New Jersey Coastal Plain. In addition to reductive dissolution of iron hydroxides releasing sorbed As, arsenate reduction by As-respirers and other As-reducing bacteria, As may also be released by the mechanism suggested by Mailloux et al. (2009) in some aquifers.
Conditions that support microbial sulfate reduction are reported for the aquifers of the West Bengal, Mekong, and Red River deltas. Where such conditions exist, there is the potential for precipitation of sulfide minerals that could remove As as well as Fe from solution. Buschmann and Berg (2009) found As concentrations to be lower in groundwater from zones where sulfate (SO42-) and Fe reduction were occurring in the aquifers of the Bengal, Mekong, and Red River deltas. They suggest such conditions, which could result in precipitation of insoluble sulfides, are a control on As levels in groundwater. Some of the chemical reactions that likely affect As mobility are shown in Table 2.
\n\t\t\t\tReaction description\n\t\t\t | \n\t\t\t\n\t\t\t\tEquation\n\t\t\t | \n\t\t\t\n\t\t\t\tReference\n\t\t\t | \n\t\t
Oxidation of pyrite | \n\t\t\tFeS2 + 15/4 O2 + 7/2 H2O → Fe (OH)3 + 2H2SO4\n\t\t\t\t 10FeS2 + 30NO3\n\t\t\t\t- + 20 H2O → 10 Fe(OH)3 + 15 N2 + 15 SO4\n\t\t\t\t2- + 5 H2SO4\n\t\t\t | \n\t\t\tWelch et al., (2000) | \n\t\t
Oxidation of arsenopyrite | \n\t\t\tFeAsS(S) +11/4O2(aq) 3/2H2O (aq) →Fe2+(Aq) + SO4\n\t\t\t\t2-(aq) + H3AsO3(aq) | \n\t\t\tMorin & Calas, (2006) | \n\t\t
Oxidation of arsenite | \n\t\t\tH3AsO3 + ½ O2 (aq) →H2AsO4\n\t\t\t\t- + H+\n\t\t\t | \n\t\t\tMorin & Calas, (2006) | \n\t\t
Reductive dissolution of Fe hydroxides (release of sorbed arsenate not shown) | \n\t\t\t4FeIIIOOH + CH2O + 7H2CO3 →4FeII + 8HCO3 + 6H2O | \n\t\t\tNriagu et al. (2007) | \n\t\t
Reduction of sulfate, formation of sulfide | \n\t\t\t2CH2O + SO4\n\t\t\t\t2- → 2HCO3\n\t\t\t\t- + H2S | \n\t\t\tNriagu et al., (2007) | \n\t\t
Oxidation of organic carbon (lactate) and reduction of As(V) | \n\t\t\tCH3-CHOH-COO- + 2HAsO4\n\t\t\t\t2- + 3H+ → CH3COO- + 2HAsO2 + 2H2O + HCO3\n\t\t\t\t-\n\t\t\t | \n\t\t\tSaltikov et al., (2003) | \n\t\t
Microbially mediated precipitation of orpiment | \n\t\t\t2HAsO2 + 3HS- + 3H+ → As2S3 + 4H2O | \n\t\t\tEhrlich and Newman(2009) | \n\t\t
Incongruent dissolution of glauconite (with release of arsenic not shown) | \n\t\t\tK2(Fe1-x,MgX)2Al6(Si4O10)3(OH)12(s) + 3/2 O2 (g)+ 6H+ →K+ +2xMg+2\n\t\t\t\t(aq) + 6SiO2(aq) + 2(1-X).Fe(OH)3(s) + 3/2 Al4(Si4O10)(OH)8(s)\n\t\t\t | \n\t\t\tChapelle and Knobel (1983) | \n\t\t
Reactions involved in, or affecting, reduction, oxidation, and (or) precipitation of arsenic in water and sediments.
Aqueous sulfide was measured in As-rich groundwater discharging to a New Jersey Coastal Plain, USA, stream (Barringer et al., 2010), but sulfide minerals (pyrite) were rare or not present in cores of streambed sediments. Siderite (FeCO3) was found, however, more than a meter deep in the sediments and, judging from the As content (184 mg/kg), the siderite is an effective sink for As released to the shallow groundwater (Mumford et al, 2012). Siderite apparently forms when bacteria respire organic matter, creating bicarbonate (HCO3-), in an Fe-rich, reducing environment with circumneutral to alkaline pH (Fredrickson et al., 1998). Siderite is reported for parts of the Southeast Asian alluvial aquifers, and presumably acts as an As sink there as well, because Islam et al., (2005) found both arsenate (As(V)) and arsenite (As(III)) sorbed effectively to the siderite in their experiments. Jönsson and Sherman (2008), however, found that the binding of As(III) with siderite is weak. Clearly, however, siderite plays a role in removing one or both of the prevalent As species from solution. Whether siderite remains a permanent sink for As is not known; it would seem possible that biogeochemical conditions in an aquifer could change such that sorbed As would be released back into solution.
In the microbial release of As from geologic materials to groundwater, the presence of organic matter is seen as a critical factor, as it is an electron donor that fuels microbial activity. Organic acids also may compete with As species, along with oxyanions such as phosphate, molybdate, sulfate, and silicate, for binding sites on solids (Wang and Mulligan, 2006b). Nevertheless, the main role of organic matter in As release appears to be that it provides the necessary substrate to bacterial communities for growth and activity—as part of the process in which it is oxidized by bacteria, the organic matter also may produce quinone-like moities that act as electron shuttles in the resulting redox reactions (Mladenov, et al., 2010).
In the studies of the biogeochemistry of the Southeast Asian aquifers, the source of the organic matter was a matter of controversy. Buried peat was suggested as the source (McArthur et al., 2004), whereas Harvey et al. (2002) indicated that young carbon from the land surface moved to depth by irrigation pumping accounted for the organic matter in the redox reactions. (It may be that both sources are operative in different places.) Rowland et al., (2006, 2007) found naturally occurring hydrocarbons in West Bengal and Cambodian aquifers that could promote the microbial activity involved in arsenic release. Héry et al., (2010) point out that very low organic carbon contents (i.e. ≤1%) in sediments is sufficient to stimulate the microbially mediated reactions that result in metal and arsenic reduction in the aquifers.
Organic matter can come from anthropogenic sources as well as natural sources. Barringer et al. (2010) indicated that the likely source of organic matter in the glauconitic system they studied came from wastewater discharged for many years from farming and other subsequent activities. The issue of whether anthropogenic inputs from agricultural practices have contributed organic matter to shallow groundwater in West Bengal and Bangladesh has received much debate (Farooq et al., 2010; Neumann et al., 2009; Sengupta et al., 2008) and it is not entirely resolved. It has been noted, however, at various contamination sites, that petroleum leaks, organic-rich leachates from landfills, as well as inputs of organic carbon for remediation purposes has led to mobilization of As (Hering et al., 2009). Thus it is clear that inputs of organic carbon from both natural and anthropogenic activities can supply electrons and stimulate the microbially mediated processes that lead to As release from geologic materials into groundwater.
Several biogeochemical processes that release As from geologic materials have been identified, as presented in the previous section. Smedley and Kinniburgh (2002) point out that whether released As remains at problematic levels in groundwater depends not only on whether there are biogeochemical reactions that retard the transport of As, but also upon the hydrologic and hydrogeologic properties of the aquifer, such as flow velocity and dispersion. If the kinetics of As release are slow, and groundwater residence time is short, then As concentrations may not increase to the point where groundwater would be considered contaminated. Conversely, if reactions that mobilize As are rapid and residence time is long, then As can accumulate in groundwater such that concentrations become hazardous—as seen in Bangladesh, for example. Eventually, if the biogeochemical conditions that lead to release and mobilization of As continue to be present (within a geologic timeframe), then the source could become exhausted.
Natural fluctuations can affect the fate and transport of As within groundwater systems. Seasonal fluctuations in recharge could, during periods of high precipitation, bring dilution to shallow groundwater, but also transport surficially derived materials to the aquifer. As mentioned above, the transport of dissolved organic matter from agricultural land in the Bengal Delta has been suggested to fuel bacterial activity that releases As from the aquifer materials, and, in that region, seasonal (monsoon) rainfall has an important effect on recharge rates and the transmission of land-derived substances to depth in the aquifers. On a much smaller scale, As concentrations in shallow groundwater that discharges to Coastal Plain streams in New Jersey, USA, varies with season and hydrologic conditions. Increased recharge during springtime results in more diluted shallow groundwater and low As concentrations, whereas hot, dry weather results in decreased recharge and higher As concentrations. Where clay lenses underlie the stream channel shallow groundwater levels above the clay decline during warm, dry periods, and some stream segments may ultimately lose water to groundwater. Thus, in some stream segments seasonal hydrologic conditions control As-rich groundwater discharges to the stream. In other segments, As-rich groundwater may discharge on a relatively constant basis with higher concentrations of As being present during warm dry weather (Barringer et al., 2010).
Pumping-induced changes to hydraulic gradients can alter flow paths at regional and local scales and can lead to introduction of contaminants to otherwise potable water. For As-contaminated well water in Bangladesh, the contamination is found in the shallower of two aquifers. Although water of the deeper aquifer is generally free of As contamination, the replacement of shallow tube wells with wells in the deep aquifer was thought to have the potential to transport contamination downward to low-As ground water (Ravenscroft et al, 2001; Harvey et al., 2002; van Geen et al., 2003).
Pumping also can result in changes in redox conditions along a flow path. As suggested by Peters and Blum (2003), anoxic water can be drawn upward to oxic zones near the wellhead, resulting in disequilibrium between As species. The converse is also possible—anoxic waters could be introduced by pumping to oxic zones such that Fe(III) in ferric hydroxides is reduced, hydroxides are dissolved, and sorbed As is released. Or, in cases where As is released under anoxic conditions, introduction of oxic water through pumping could slow or terminate the reaction. An example of pumping-induced changes in redox conditions is found at the individual borehole scale in eastern Wisconsin, USA. In a field experiment, longer periods between pumping episodes allowed longer periods of anoxia, which resulted in higher concentrations of As in groundwater (Ayotte et al., 2011). Pumping also could move constituents such as organic carbon from the surface to depth, where the carbon could stimulate microbially mediated redox reactions such as Fe reduction that leads to As release from aquifer materials. Pumping could also move higher pH water into a zone of lower pH water, creating an environment in which As sorbed to aquifer materials can desorb.
Flow rates at the individual borehole scale can also be sufficiently rapid that contact time between groundwater and aquifer material is minimized, and reactions releasing As may be relatively slow. Limited contact time is thought to be the explanation for lower dissolved constituents, including As, in water from wells with high yield compared with those with low yield and higher dissolved constituents in the Hungarian Pannonian Basin ( Varsányi and Kovács, 2006). Similar results are reported for a well in a sandstone aquifer in Wisconsin, USA, where reducing conditions developed with no pumping and As concentrations increased (indicating dissolution of Fe hydroxides and As release), whereas rapid well purging introduced oxic conditions to water near the well bore (Gotkowitz et al., 2004).
Arsenic contamination of groundwater resources has been identified in many parts of the world. In some cases, as in geothermal fields, the impact of As on drinking-water supplies may not be great, but there are parts of the world where groundwater is a major drinking-water source for millions of people, many of which are in poverty and have limited ability to solve the problem of a contaminated water source. Since the discoveries of widespread groundwater contamination with As, considerable effort has been expended to find suitable, inexpensive methods for removing As. A discussion of those efforts, and the results, is beyond the scope of this chapter; the reader is directed to Feenstra et al. (2007) for an overview of As removal methods.
Geogenic sources of As are numerous—within the various geologic materials, the most common occurrences of As appear to be in sulfides (mainly pyrite) and as sorbed species on Fe hydroxides, although As also appears within some silicate and carbonate rocks. The main processes involved in As release to groundwater are reduction of Fe hydroxides, reduction of As within minerals and as a sorbed species, competitive sorption with other oxyanions, and sulfide oxidation. Increasingly, studies show that these processes can be mediated by microbes.
Although major geologic sources of arsenic include alluvial materials, mineralized sedimentary and metasedimentary rocks and volcanic rocks and related deposits, recent findings indicating that As also is released from glauconitic sediments suggests that not all geologic sources and conditions for the release of As to groundwater have yet been identified. Further, it is apparent from some studies that human activities can increase the rates and amounts of As mobilized and dissolved in groundwater through inputs of organic carbon, and from water withdrawals and other changes to natural hydrologic systems. Thus, while there are human efforts to mitigate the As contamination of drinking-water supplies, there are also human activities that exacerbate the problem. The more fully we understand how, when, and where As is mobilized from geologic materials, or from anthropogenic releases to the environment, the more effectively we can find solutions to this major contamination problem.
Investigation of rubberized concrete has received considerable attention since late twentieth century, when exploration of the idea of adding rubber particles to the concrete matrix began. The intriguing idea for many has been the combination of an ultra-flexible material to an ultra-rigid material to enhance ductile performance of the composite material. In addition, the idea of incorporating a waste material that may otherwise end up in a landfill is attractive for sustainable development. The main constituent of rubberized concrete is tire-derived aggregate (TDA), incorporated in a cementitious matrix through replacement of fine or coarse aggregate as a percentage of volume or weight. This application redirects a significant amount of waste rubber from landfills to infrastructure industries.
The idea of repurposing a waste material for use in concrete has roots in concerns regarding the amount of waste tires in landfills. The United States alone generates 289 million scrap tires on an annual basis as of 2006 [1]. The Environmental Protection Agency (EPA) identifies stockpiled tires as an “ideal incubator for mosquito larvae” and connects this to the spread of the West Nile Virus from 1999 to 2005 [1]. As of 2012, tires were being recycled at a rate of 44.6% with rubber and leather contributing 6.18 million tons of waste after accounting for recycling and recovery [2]. The idea of reducing the number of waste tires that accumulate in landfills through recycling rubber for use in concrete has continued to attract the attention of researchers.
The general focus of research on rubberized concrete is the evaluation of mechanical properties of the concrete. The basic properties include compressive, tensile, and flexural strengths. The performance of TDA concrete subject to dynamic loading is another essential property of TDA concrete. In addition, application of supplementary cementitious materials and admixtures, such as silica fume and fly ash, has potentials to enhance various characteristics of TDA concrete. Research seems to be in support of the fact that the lower strength and enhanced dynamic properties of the TDA concrete mixtures are valuable in certain practical applications such as traffic barriers and other impact-resistant systems.
Other than the inclusion of the rubber particles, the rubberized concrete mix is virtually the same as most other concrete including cement, fine aggregates, coarse aggregates, and water. Some researchers have incorporated super plasticizers, in order to achieve better workability. Others have experimented with the use of silica fume and fly ash in order to achieve enhanced strengths. Further, researchers have experimented with pretreating the rubber particles using chemical washes in attempts to develop better bonds between the rubber particles and the cementitious matrix. Following sections discuss individual components making up the rubberized concrete matrix.
TDA refers to the rubber particles, processed from multiple types of tires differing in composition and fiber type, used for replacing the mineral or rotary kiln expanded lightweight aggregates in many mixtures (Figure 1). Descriptive classification of rubber particles relies on the size and manufacturing processes of materials [3]. The first and largest classification of TDA is shredded tire chips, which are typically results of mechanical shredding. Resulting tire chips may be as large as 460 mm long by 230 mm wide to as small as 150 mm long. A combination of both primary and secondary shredding processes is also common to produce smaller shredded chips. The next classification is ground rubber; with a typical range of 19–0.1 mm in size. Ground rubber is subject to two stages of magnetic separation and screening to remove the steel fibers from the rubber particles. The smallest classification of TDA is crumb rubber, obtained through micro-milling, cracker-milling, and granular processes. Crumb rubber particle sizes range from 4.75 to 0.075 mm. Another method is a cryogenic method, in which the rubber is frozen using nitrogen and then shattered [3].
Crumb rubber manufactured through mechanical shredding of recycled tires.
The common fine aggregate used in most research studies on rubberized concrete is natural sand with a gravel coarse aggregate. The cement used is either Type 1 or Type 2 cement, with no significant evidence suggesting one type of cement performing better. Other admixtures incorporated into rubberized concrete include the addition of silica fume and fly ash by replacement of cement. This enhances the strength of rubberized concrete and bond between the rubber and cement. Further, rubber particles may also replace lightweight aggregates (LWA), such as rotary kiln expanded shale, clay, and slate, in various lightweight concrete materials, where, the similarity between the volume weights of TDA and LWA enhances the ease of mixing and placing operations [4].
Table 1 lists selected research projects and their characteristics. Existing literature indicates that an increase in rubber content results in a systematic decrease in the compression strength of the concrete material (Figure 2). The substitution of mineral aggregates with TDA is generally between 0 and 100% of the total aggregate volume in increments of 20%. The relationship between the compressive strength and rubber content in the mix is not linear [22]. Further, the size of the particles has an impact on this relationship. While, inclusion of 100% crumb rubber may reduce the compressive strength by more than 90% [8], substitution of fine aggregates by less than 25% appears to have no significant impact on this strength [13]. In particular, research has shown that application of fine crump rubbers has less negative impact on the compressive strength of the mix [5, 6, 15, 20]. A comparison between applications of coarse rubber particles versus fine rubber particles indicate that coarse and fine rubber particles are more effective at substitution ratios of less and more than 25%, respectively [14]. Using larger sizes of TDA, also known as tire chips, provides an opportunity to keep the steel belt wires after shredding in order to lower the costs, even though, they may not provide any specific advantage for the mix [25]. On the same line, application of fiber reinforcement by adding polypropylene fibers has shown to be effective in reducing crack propagation due to shrinkage [12].
Reference | Rubber aggregate | Replaced conventional aggregate | Specimen type/size | |
---|---|---|---|---|
Aiello and Leuzzi [5] | Tire shreds 20 mm w/ Steel Fibers Included | Coarse aggregate 12.5–20 mm | Cube 150 mm Beams 250 × 250 × 900 mm | |
Al-Tayeb et al. [6] | Crumb rubber 1 mm | Sand fine aggregate | Cylinder 100 Beams 100 × 50 × 400 mm | |
Atahan and Sevim [7] | Shredded tire chips 11–22 mm | Crushed limestone coarse aggregate 4–16 mm | Cylinder 150 Full Scale Barriers 1000 × 450 × 250 mm | |
Atahan and Yucel [8] | Large rubber 13 mm and crumb rubber #10-20 | Crushed Stone 19 mm and Sand | Cylinder 100 | |
Bignozzi and Sandrolini [9] | Scrap and crumb tires 0.05–2 mm | Fine aggregate sand 0–4 mm | Cube 150 mm | |
Ganjian et al. [10] | Chipped rubber 25 mm | Crushed siliceous coarse aggregate | Cube 150 mm Beams 100 × 100 × 500 mm | |
Guneyisi et al. [11] | Crumb rubber similarly graded to sand and tire chips 10–40 mm | Natural sand 4 mm and crushed limestone 20 mm replaced equally | Cube 150 mm Cylinder 150 (90-day strength) | |
Hernandez-Olivares et al. [12] | Rubber strip fibers 8.5–21.5 mm | No material removed | Cylinder 150 Beams 150 × 150 × 600 mm | |
Issa and Salem [13] | Crumb rubber (similar to sand used) | Crushed sand | Cylinder 150 | |
Khaloo et al. [14] | Coarse tire chips | Crushed stone gravel 20 mm | Cylinder (50-day strength) | |
Khatib and Bayomy [15] | Tire chips from mechanical shredding 10–50 mm | Coarse aggregate gravel | Cylinder 150 Beams 152 × 152 × 50 mm | |
Li et al. [16] | Truck & car tire chips and fibers with and w/o steel belt 25–51 mm | Coarse aggregate gravel | Cylinder 150 | |
Liu et al. [17] | Crumb tire rubber 0.178 mm | River sand fine aggregate 5 mm | Cubes 150 mm Cylinders 35 (SHPB Impact) | |
Miller and Tehrani [4] | Crumb rubber | Coarse lightweight expanded shale aggregate | Cylinder 150 Beams 152 × 152 × 50 mm | |
Mohammed et al. [18] | Crumb rubber 600 μm | River sand fine aggregate | Cube 100 mm | |
Son et al. [19] | Crumb rubber particles 1 mm | Total aggregate weight (coarse & fine) | Cylinder 100 | |
Topcu [20] | Rubber particles from mechanical grinding 6 mm | Crushed limestone coarse aggregate 4–16 mm | Cylinder 150 | |
Topcu and Avcular [21] | Large rubber particles 2.2 mm | Limestone coarse aggregate | Cylinder 150 | |
Toutanji [22] | Tire chips 12.7 mm | Crushed stone coarse aggregate 19 mm | Cylinder 100 Beams 100 × 100 × 350 mm | |
Xue and Shinozuka [23] | Crumb Rubber 6 mm | Gravel coarse aggregate 12 mm | Cylinder 100 Lumped Mass Columns | |
Zheng et al. [24] | Crushed rubber with steel belt wires 4–15 mm | Crushed stone coarse aggregate 31.5 mm | Cube 150 mm 60-day Beams 100 × 160 × 1000 mm | |
Zheng et al. [25] | Ground rubber 2.6 mm and crushed rubber with steel belt wires 4–15 mm | Crushed stone coarse aggregate 31.5 mm | Cylinder 150 |
Summary of selected research on rubberized concrete.
Selected reported relative changes in the compressive strength of rubberized concrete.
Similar to conventional concrete, application of supplementary cementitious materials such as silica fumes has shown to be effective on increasing the compressive strength of TDA concrete containing high water-to-cement ratios [11]. Replacing 7% of cement with silica fume has shown to increase the compressive strength between 3 and 7 MPa [23]. Tire-derived aggregates are also applicable to self-compacting concrete, which utilizes fine filler materials, admixtures such as superplasticizers, and viscosity modifying agents. Combination of shredded tire and crumb rubber has the potential to replace nearly 20–30% of the sand with a similar grain size [9]. Further, it is also possible to replace cement with ground rubber. Substitution of 5% of cement has shown to reduce the compressive strength by 5% [10]. Tire-derived aggregate concrete with enhanced characteristics due to admixtures and supplementary cementitious materials has applications in hollow concrete blocks. The recommended rubber contents for loadbearing and non-loadbearing systems are 6.5 and 40.7%, respectively [18].
Research shows that increasing the rubber content in concrete decreases the static modulus of elasticity [6, 7, 8, 19]. However, there is not much agreement on the amount of reduction at high rubber contents (Figure 3). Generally, Tire-derived aggregates influence the stress-strain relationship and enhance the ductility of the concrete [14]. Some comparative studies on the size of rubber aggregates suggest that using tire chips have more impact on the reduction of elastic modulus than using waste tire fibers do [16]. However, there are also evidences that the reduction of modulus of elasticity is only a function of the rubber content [10].
Selected reported relative changes in the static modulus of elasticity of rubberized concrete.
Further, using tire chips containing steel belts increases the stiffness of TDA concrete [16, 24, 25]. In addition, application of fly ash and silica fume can also enhance the modulus of elasticity [11, 23].
Figure 4 indicates how increasing the rubber content reduces the splitting tensile strength. However, existing research agrees that capacity of rubber in absorbing energy enhances the toughness of the TDA concrete [6, 16, 20]. Application of fiber reinforcement using polypropylene fibers has shown to improve the toughness further [12]. Comparison of results for compressive and tensile strengths suggests that the rate of reduction for split-tensile strength is less than the same rate for compressive strength [11]. Further, there are reports indicating that specimens with ground tires perform better in tension than specimens containing large tire chips [10].
Selected reported relative changes in the split-tensile strength of rubberized concrete.
The relationship between flexural strength and TDA content is similar to other mechanical properties (Figure 5). However, there are variations in this relationship. Studies generally indicate that reduction of flexural strength parallels an increase in the ductility of specimens [22]. Some research indicates that the rate of reduction for the flexural strength is much steeper than other mechanical properties, particularly at lower rubber contents [15]. Application of smaller rubber particles improves the observed flexural strength [5, 10]. Adding polypropylene fibers has also shown to be effective in crack control, but not necessarily in enhancement of the strength [12].
Selected reported relative changes in the flexural strength of rubberized concrete.
Rubberized concrete has also applications in composite floors. Research shows that floors with 10% rubber content have smaller failure load due to flexure in large spans, but nearly the same failure load caused by shear in short spans. The capability of TDA composite floors in withstanding larger deformations has a significant impact on the ductility of the system [26].
Figure 6 contains selected reported data on the relationship between toughness and rubber content in TDA concrete. Toughness is generally a measure based on the area covered by the load-deflection diagrams, thus, it relates to both ductility and strength. As a result, reported data points on toughness are scattered, as rubber contents increases the ductility, but reduces the strength. This explains how some studies have shown a reduction of toughness because of increasing the rubber content, even though subjected specimens have shown significant increase in ductility [20]. Nevertheless, most reported data show a general increase in toughness with the increase in rubber content [5, 22]. Some research studies have also suggested that a maximum toughness value exist for the optimum rubber content and proper TDA gradation [14]. Measuring toughness from the brittleness index using the stress-strain hysteresis loops also confirms the positive effect of rubber on the toughness [25]. In addition, the toughness of TDA concrete subject to impact load also increases with the increase in strain rate [17]. Furthermore, application of fibers, particularly in tensile specimens, significantly enhances the toughness [16].
Selected reported relative changes in the toughness of rubberized concrete.
Some of the most beneficial properties of rubberized concrete include its behavior under dynamic loading, making the enhancement of these properties desirable in comparison with the brittle and rigid behavior of plain concrete. Research suggests that rubberized concrete may have practical applications as traffic barriers, vibration mitigation, and seismic force mitigation among others. There are various techniques for investigation of these behaviors. Figures 7 and 8 show two significant parameters, energy absorption and damping, respectively, measured for TDA concrete at different rubber contents. As shown in these figures, there are limited studies as the basis for each of these relationships.
Selected reported relative changes in the damping ratio of rubberized concrete.
Selected reported relative changes in the absorbed energy of rubberized concrete.
The full-scale New Jersey-shaped safety barriers subject to non-severe impact loads indicate a gradual increase in the energy absorption when rubber content changes from 0 to 100% by volume [7]. These results are qualitatively comparable with similar collision testing studies [21]. However, impact testing on hybrid beams, containing a layer of TDA concrete on top of plain concrete has resulted in significant increase in energy absorption for only 20% rubber content [6]. Similar tests using falling weights confirms the effectiveness of TDA concrete in reducing the severity of the impact at only 20–40% rubber contents, even though, the pest performance is achieved at rubber contents larger than 60–80% [8].
Similarly, the damping ratio of TDA concrete measured by elastic wave method showed a moderate increase because of the increase in the rubber content in concrete [24]. However, the shake-table studies on TDA concrete columns indicated much higher damping ratios [23]. These comparisons indicate how TDA becomes more effective in post-peak performance of concrete specimens. Further, same shake table studies have shown that increasing rubber content in the TDA concrete reduces the natural frequencies and the response acceleration by 28 and 27%, respectively [23].
Non-destructive tests using ultrasonic pulses confirm that rubber reduces the velocity of waves, which is in correlation with lower dynamic modulus of elasticity [14]. Dynamic compression tests confirm the capability of TDA concrete in dissipating energy and show that increasing the load frequency or strain rate increases the dynamic modulus of elasticity [12, 17]. However, the age of the specimens has an adverse impact on the energy dissipation [12].
Nonstructural mechanical properties of TDA concrete have been subject to studies for specific applications. Non-loadbearing wall elements often require proper thermal and electrical insulation as well as sound absorption. Studies indicate that TDA improves the sound absorption of concrete, reduces the thermal conductivity coefficient, and increases the electrical resistivity [13, 18, 27].
A typical model for the behavior of TDA concrete is a modification of Holmquist-Johnson-Cook (H-J-C) constitutive model. The original H-J-C model contains 21 modifiable parameters to present characteristics of TDA concrete, which was simplified to ten parameters in the modified form [17].
Numerical simulations using finite element analysis are also available to model mechanical properties of TDA concrete. The basis for these simulations is generally an elastoplastic model for the behavior of materials. Successful modeling of beam specimens has been reported using hexahedron elements with standard shape functions [6]. Using a two-phase composite material helps to define the dispersion of TDA in the cementitious matrix. This model utilized three-node triangular elements to simulate split-tensile tests [16].
Figure 9 shows a comparative view of the relationships between mechanical properties of TDA concrete and the TDA content volume. This figure suggests that developing a simple model for practical design of TDA concrete elements may be possible, as various strengths follow similar trends in respect to TDA content.
Selected reported relative changes in the dynamic properties of rubberized concrete (marked data) and their general trends (trend-lines).
Eq. (1) presents a proposed model to find the strength reduction factor [15]:
In this model, SRF is the “strength reduction factor”; R is the rubber content as a volumetric ratio by the total volume of aggregates; and a, b, and m are modeling parameters. This equation is equal to unity at a rubber content of 0% and reaches an asymptote at higher rubber content values. The m parameter indicates the degree of curvature of the reduction and is a function of the particle size. In addition, parameters a and b must satisfy the relationship
Another important aspect of concrete design is the handling procedures used when placing the concrete while in the workable state. Care needs to be taken to ensure that segregation of materials does not occur and the mix remains as homogenous as possible while being placed and cured. An uneven distribution of rubber particles has been observed in concrete mixes, particularly when specimens are vibrated during placement causing light TDA to surface [10, 23].
The state of the research on TDA concrete warrants further studies on analytical, experimental, and practical areas. On material properties, the environmental impacts on durability of rubber and long-term properties of TDA concrete is an area of interest for future research. Further, investigating the micromechanical characteristics of the bond between rubber particles and the cement paste is essential to understand the mechanical properties of TDA concrete, including toughness and tensile strength, better. Constitutive modeling and numerical simulation of the behavior of rubberized concrete are other areas of interest that can benefit from recent advancements in computational engineering and mechanics.
Application of TDA concrete requires development of design guidelines and specifications. Defining strength reduction factors is an area that requires further development. Current research studies are scattered in respect to parametric modeling and can benefit from additional experimental results. Further, these parametric analyses are essential for optimization of TDA concrete mix design to obtain proper rubber content for specific objectives. In addition, practical issues in mixing and placing concrete require development of proper specifications for handling TDA materials in concrete.
Mechanical properties of TDA concrete have shown to be desirable for many applications, such as traffic and sound barriers. Toughness and ductility of TDA concrete can be also effective in concrete elements subject to dynamic loads caused by earthquake and wind. Large-scale experimental studies are required for investigating these applications.
There has been limited studies on alternative TDA concrete products with application of fiber reinforcement, lightweight aggregate, fiber-reinforced polymers, admixtures, and supplementary cementitious materials. Enhancing the properties of TDA concrete using these methods require further research.
The California State University, Fresno Foundation has partially supported this work.
The authors declare that there is no conflict of interest regarding the publication of this work.
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