Naturally Occurring Contaminants Arsenic

5.3.3 Naturally Occurring Contaminants Arsenic

Since 1993, the World Health Organization (WHO) lowered the drinking water standard for arsenic from 0.05 to 0.01 mg/L (10 ppb). Others followed, including the United States and the European Union. This naturally occurring element has become the most notorious and widely recognized groundwater contaminant. The following excerpts from a press release illustrate the point:

Arsenic in drinking water is a global threat to health affecting more than 70 countries and 137million people, according to new research presented to the annual conference at the Royal Geographical Society with IBG (The Institute of British Geographers) in London today (Wednesday 29 August 2007).

Large numbers of people are being unknowingly exposed to unsafe levels of arsenic in their drink- ing water, Peter Ravenscroft from the department of geography at the University of Cambridge told the geographers’ conference. At present, Bangladesh is the country worst affected, where hundreds of thousands of people are likely to die from arsenic causing fatal cancers of the lung, bladder and skin.

Arsenic poses long-term health risks “exceeding every other potential water contaminant”, ac- cording to research presented by Dr Allan Smith, of the University of California, Berkeley and adviser to the WHO on arsenic. Dr Smith added: “Most countries have some water sources with dangerous levels of arsenic, but only now are we beginning to recognise the magnitude of the problem. It is the most dangerous contaminant of drinking water in terms of long term health risks and we must test all water sources worldwide as soon as possible (RGS, 2007).

The most serious damage to health from drinking arsenic-contaminated water has occurred in Bangladesh and West Bengal, India. In the 1970s and 1980s, UNICEF and other international agencies helped to install more than 4 million hand-pumped wells in Bangladesh to give communities access to clean drinking water and to reduce diarrhea and infant mortality. Cases of arsenic-related diseases (generally referred to as arseni- cosis) were seen in West Bengal and then in Bangladesh in the 1980s. By 1993, arsenic from the water in wells was discovered to be responsible. In 2000, a WHO report (Smith et al., 2000) described the situation in Bangladesh as: “the largest mass poisoning of a population in history . . . beyond the accidents at Bhopal, India, in 1984, and Chernobyl, Ukraine, in 1986.”

In 2006, UNICEF reported that 4.7 million (55 percent) of the 8.6 million wells in Bangladesh had been tested for arsenic of which 1.4 million (30 percent of those tested) had been painted red, showing them to be unsafe for drinking water: defined in this case as more than 50 ppb (UNICEF, 2006). Although many people have switched to using arsenic-free water, in a third of cases where arsenic had been identified, no action had yet been taken. UNICEF estimates that 12 million people in Bangladesh were drinking arsenic-contaminated water in 2006, and the number of people showing symptoms of arsenicosis was 40,000 but could rise to 1 million (UNICEF, 2006). Other estimates are higher still (Petrusevski et al., 2007).

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Implementation of the WHO guideline value of 10 μg/L is not currently feasible for a number of countries strongly affected by the arsenic problem, including Bangladesh and India, which retain the 50 μg/L limit. Other countries have not updated their drinking water standards recently and retain the older WHO guideline of 50 μg/L. These include Bahrain, Bolivia, China, Egypt, Indonesia, Oman, Philippines, Saudi Arabia, Sri Lanka, Vietnam, and Zimbabwe. The most stringent standard currently set for acceptable arsenic concentration in drinking water is by Australia, which has a national standard of 7 μg/L (Petrusevski et al., 2007).

The disease symptoms caused by chronic arsenic ingestion, arsenicosis, develop when arsenic-contaminated water is consumed for several years. However, there is no universal definition of the disease caused by arsenic, and it is currently not possible to differentiate which cases of cancer were caused by drinking arsenic-affected water. Estimates, there- fore, vary widely. Symptoms may develop only after more than 10 years of exposure to arsenic, while it may take 20 years of exposure for some cancers to develop. Long-term ingestion of arsenic in water can first lead to problems with kidney and liver function, and then to damage of the internal organs including lungs, kidney, liver, and bladder. Ar- senic can disrupt the peripheral vascular system leading to gangrene in the legs, known in some areas as black foot disease. This was one of the first reported symptoms of chronic arsenic poisoning observed in China (province of Taiwan) in the first half of twentieth century. A correlation between hypertension and arsenic in drinking water has also been established in a number of studies (Petrusevski et al., 2007).

Elemental arsenic is a steel-gray metal-like substance rarely found naturally. As a compound with other elements such as oxygen, chlorine, and sulfur, arsenic is widely distributed throughout the earth’s crust, especially in minerals and ores that contain copper or lead. Natural arsenic in groundwater is largely the result of dissolved minerals

from weathered rocks and soils. Principal ores of arsenic are sulfides (As 2 S 3 , As 4 S 4 , and FeAsS), which are almost invariably found with other metal sulfides. The hydrogen form of arsenic is arsine, a poisonous gas. Arsenic also forms oxide compounds. Arsenic

trioxide (As 2 O 3 ) is a transparent crystal or white powder that is slightly soluble in water and has a specific gravity of 3.74. Arsenic pentoxide (As 2 O 5 ) is a white amorphous solid that is very soluble in water, forming arsenic acid. It has a specific gravity of 4.32 (USEPA, 2005b).

Dissolved arsenic in groundwater exists primarily as oxy anions with formal oxida- tion states of III and V. Either arsenate [As(V)] or arsenite [As(III)] can be the dominant inorganic form in groundwater. Arsenate (H n AsO n−3 4 ) generally is the dominant form in oxic (aerobic, oxygenated) waters with dissolved oxygen >1 mg/L. Arsenite (H n AsO n−3 3 ) dominates in reducing conditions, such as sulfidic (dissolved oxygen <1 mg/L with sul- fide present) and methanic (methane present) waters. Aqueous and solid-water reactions, some of which are bacterially mediated, can oxidize or reduce aqueous arsenic. Both an- ions are capable of adsorbing to various subsurface materials, such as ferric oxides and clay particles. Ferric oxides are particularly important to arsenate fate and transport, as ferric oxides are abundant in the subsurface and arsenate strongly adsorbs to these sur- faces in acidic to neutral waters. An increase in the pH to an alkaline condition may cause both arsenite and arsenate to desorb, and they are usually mobile in an alkaline environ- ment (Dowdle et al., 1996; Harrington et al., 1998; Welch et al., 2000; USEPA, 2005b). The toxicity and mobility of arsenic vary with its valency state and chemical form. As(III) is generally more toxic to humans and four to 10 times more soluble in water than As(V) (USEPA, 1997).

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All arsenic compounds consumed in the United States are imported. Arsenic has been used primarily for the production of pesticides, insecticides, and chromated copper ar- senate (CCA), a preservative that renders wood resistant to rotting and decay. Increased environmental regulation, along with the decision of the wood-treatment industry to eliminate arsenical wood preservatives from residential application by the end of 2003, caused arsenic consumption in the United States to decline drastically in 2004. Other industrial products containing arsenic include lead-acid batteries, light-emitting diodes, paints, dyes, metals, pharmaceuticals, pesticides, herbicides, soaps, and semiconductors. Anthropogenic sources of arsenic in the environment include mining and smelting op- erations, agricultural applications, and disposal of wastes that contain arsenic (USEPA, 2005b). Arsenic is a contaminant of concern at many remediation sites. Because arsenic readily changes valence states and reacts to form species with varying toxicity and mo- bility, effective treatment of arsenic can be challenging.

A recent study of arsenic concentrations in major U.S. aquifers by the USGS (accessible at: http://water.usgs.gov/nawqa/trace/pubs/) shows wide regional variations of nat- urally occurring arsenic due to a combination of climate and geology. Although slightly less than half of 30,000 arsenic analyses of groundwater in the United States were equal or less than 1 μg/L, about 10 percent exceeded 10 μg/L. At a broad regional scale, arsenic concentrations exceeding 10 μg/L appear to be more frequently observed in the western United States than in the eastern half (USGS, 2004). Interestingly, more detailed recent investigations of groundwater in New England, Michigan, Minnesota, South Dakota, Ok- lahoma, and Wisconsin suggest that arsenic concentrations exceeding 10 μg/L are more widespread and common than previously recognized. Arsenic release from iron oxide appears to be the most common cause of widespread arsenic concentrations exceeding

10 μg/L in groundwater. This can occur in response to different geochemical conditions, including release of arsenic to groundwater through the reaction of iron oxide with either natural or anthropogenic (i.e., petroleum products) organic carbon. Iron oxide also can release arsenic to alkaline groundwater, such as that found in some felsic volcanic rocks and alkaline aquifers of the western United States. Sulfide minerals in rocks may act both as a source and as a sink for arsenic, depending on local geochemistry. In oxic (aerobic, oxygenated) water, dissolution of sulfide minerals, most notably pyrite and arsenopyrite, contributes arsenic to groundwater and surface water in many parts of the United States. Other common sulfide minerals, such as galena, sphalerite, marcasite, and chalcopyrite, can contain 1 percent or more arsenic as an impurity.

Radionuclides Radionuclides are naturally occurring elements that have unstable nuclei that sponta- neously break down to form more stable energy and particle configurations. Energy released during this process is called radioactive energy, and such elements are called radioactive elements or radionuclides. The most unstable configurations disintegrate very rapidly, and some of them do not exist in the earth’s crust anymore (e.g., chem- ical elements 85 and 87, astatine and francium). Other radioactive elements, such as rubidium-87, have a slow rate of decay and are still present in significant quantity (Hem, 1989). The decay of a radionuclide is a first-order kinetic process, usually expressed in terms of a rate constant (λ) given as

λ= ln 2

t 1/2

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where t 1/2 = half-life of the element, i.e., the length of time required for half the quantity present at time 0 to disintegrate.

Radioactive energy is released in various ways, with the following three types being of interest in water chemistry: (1) alpha radiation, consisting of positively charged helium nuclei; (2) beta radiation, consisting of electrons or positrons; and (3) gamma radiation, consisting of electromagnetic wave-type energy similar to X-rays (Hem, 1989). Potential effects from radionuclides depend on the number of radioactive particles or rays emitted (alpha, beta, or gamma) and not the mass of the radionuclides (USEPA, 1981). Becquerel (Bq) is the unit for radioactivity in the International System (SI) of units, defined as the radiation caused by one disintegration per second; this is equivalent to approximately 27.0270 picoCuries (pCi). The unit is named for a French physicist, Antoine-Henri Bec- querel, the discoverer of radioactivity. One Curie (Ci; named after Pierre and Marie

Curie, the discoverers of radium) is defined as 3.7 × 10 10 atomic disintegrations per sec- ond, which is the approximate specific activity of 1 g of radium in equilibrium with its disintegration products. Maximum contaminant load (MCL) for radium and alpha and beta radiation is expressed in pCi/L in the United States. Where possible, radioactivity is reported in terms of concentration of specific nuclides, as is commonly the case with ura- nium, which is conveniently analyzed by chemical means (MCL of uranium is expressed in μg/L). For some elements, radiochemical analytical techniques permit detection of concentrations much lower than what can be analyzed by any current chemical method. This fact is of special significance when performing tracing with radioactive isotopes, which can be introduced into the groundwater in very small quantities.

Exposure to radionuclides results in an increased risk of cancer. Certain elements accumulate in specific organs. For example, radium accumulates in the bones and iodine accumulates in the thyroid. For uranium, there is also the potential for kidney damage. Many water sources have very low levels of naturally occurring radioactivity, usually low enough not to be considered a public health concern. In some parts of the United States, however, the underlying geology causes elevated concentrations of some radionuclides in aquifers used for water supply.

Contamination of water from anthropogenic radioactive materials occurs primarily as the result of improper waste storage, leaks, or transportation accidents. These radioactive materials are used in various ways in the production of nuclear energy, commercial products (such as television and smoke detectors), electricity, and nuclear weapons and in nuclear medicine in therapy and diagnosis.

Anthropogenic radionuclides have also been released into the atmosphere as the re- sult of atmospheric testing of nuclear weapons and, in rare cases, accidents at nuclear fuel stations and discharge of radiopharmaceuticals. The two types of radioactive decay that carry the most health risks due to ingestion of water are alpha emitters and beta/photon emitters. Many radionuclides are mixed emitters, with each radionuclide having a pri- mary mode of disintegration. The naturally-occurring radionuclides are largely alpha emitters, although many of the short-lived daughter products emit beta particles. An- thropogenic radionuclides are predominantly beta/photon emitters and include those that are released to the environment as the result of activities of the nuclear industry but also include releases of alpha-emitting plutonium from nuclear weapon and nuclear reac- tor facilities (USEPA, 2000b). The natural radionuclides involve three decay series, which start with uranium-238, thorium-232, and uranium-235, and are known collectively as the uranium, thorium, and actinium series. Each series decays through stages of various nuclides, which emit either an alpha or a beta particle as they decay and terminates with

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a stable isotope of lead. Some of the radionuclides also emit gamma radiation, which accompany the alpha or beta decay. The uranium series contains uranium-238 and -234, radium-226, lead-210, and polonium-210. The thorium series contains radium-228 and radium-224. The actinium series contains uranium-235 (USEPA, 2000b).

As part of the new MCL standard promulgation for the radionuclides, the USEPA, in cooperation with the USGS, issued a technical document (USEPA, 2000b), which includes sections on the fundamentals of radioactivity in drinking water, an overview of natural occurrence of major radionuclides in groundwater, and the results of a nationwide survey of selected wells in all hydrostratigraphic provinces in the United States performed by the USGS (Focazio et al., 2001).