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Metals and metalloids (an element such as arsenic or antimony with properties in between those of metals and nonmetals are common groundwater contaminants that present a risk to users of groundwater if concentrations exceed acceptable risk-based concentrations. Contamination of groundwater by metals and metalloids is most often related to industrial sources or mining and mineral processing. Their acute and chronic toxicity as well as their common occurrence, make metal and metalloid contamination environmentally significant. The behavior and toxicity of metals and metalloids can vary with the chemical composition of the groundwater and the minerals present in the aquifer. Under some conditions, a given metal or metalloid may pose little risk because it is adsorbed to the aquifer solids or precipitated from groundwater before it reaches any exposure point. In other cases these contaminants may be quite mobile, forming groundwater plumes that can pose a risk to groundwater users.

Related Articles:

• Mobility of Metals and Metalloid Contaminants in Groundwater
• Monitored Natural Attenuation (MNA) of Metals and Metalloids
• Remediation of Metals and Metalloids

CONTRIBUTOR(S): Dr. Miles Denham

Key Resource(s):

Introduction

Groundwater contaminated with metals and metalloids (collectively referred to as “metals” in this article for simplicity; Table 1) can be a major problem because of the broad spectrum of sources, the toxicity of many metals, and the difficulty in remediating metal-contaminated sites. Common metal contaminants, their primary sources, and their potential health effects, especially those that occur at U.S. Department of Defense, U.S. Department of Energy, and private sites are quite variable (e.g., Table 1). The behavior of metal contaminants in groundwater depends on the nature of the source, the chemistry of the metal, and the mineralogy of the aquifer. Regulation and remediation of metal contaminants is further complicated by the fact that metals occur naturally in aquifers, sometimes at levels that are safe for ingestion, but at other times at concentrations that naturally exceed acceptable risk-based concentrations such as Maximum Concentration Levels (MCLs).

Metal or Metalloid*	Potential Natural and Man-Made Sources to Groundwater	Potential Health and Other Effects If Concentrations are Above Risk-Based Levels	MCL(mg/L)
Antimony*	Enters environment from natural weathering, industrial production, municipal waste disposal, and manufacturing of flame retardants, ceramics, glass, batteries, fireworks, explosives, and ammunition.	Decreases longevity, alters blood levels of glucose and cholesterol in laboratory animals exposed at high levels over their lifetime.	0.006
Arsenic*	Enters environment from natural processes, industrial activities, pesticides, industrial waste, smelting of copper, lead, and zinc.	Causes acute and chronic toxicity, liver and kidney damage, decreases hemoglobin, carcinogenic.	0.010
Cadmium	May enter the environment from industrial discharge, mining waste, metal/metalloid plating, water pipes, batteries, paints and pigments, plastic stabilizers, and landfill leachate.	Replaces zinc biochemically in the body and causes high blood pressure, liver and kidney damage. Destroys testicular tissue and red blood cells. Toxic to aquatic biota.	0.005
Chromium	Used in metal/metalloid plating and as a cooling water additive. Also may enter environment from old mining operations runoff and leaching into groundwater.	Chromium (III) is a nutritionally essential element. Chromium (VI) is more toxic, causing liver and kidney damage, internal hemorrhaging, respiratory damage, dermatitis, and ulcers on the skin at high concentrations.	0.1 (total)
Copper	Enters environment from metal/metalloid plating, industrial and domestic waste, mining, and mineral leaching.	An essential nutrient in low doses. High doses can cause stomach and intestinal distress, liver and kidney damage, anemia. Essential trace element but toxic to plants and algae at moderate doses.	1.3
Lead	Enters environment from industry, mining, plumbing, leaded-gasoline, and recycling of lead-acid batteries.	Affects red blood cell chemistry; delays normal physical and mental development in babies and young children. Can cause deficits in attention span, hearing, and learning in children.	0.015
Mercury	Enters environment from industrial waste (e.g., chloralkali process), mining, pesticides, coal, electrical equipment e.g., batteries, lamps, switches), smelting, and fossil-fuel combustion.	Causes acute and chronic toxicity. Targets the kidneys and can cause nervous system disorders.	0.002
Uranium	Enters groundwater from uranium mining and ore processing operations, nuclear fuel reprocessing facilities, and potentially from exploded depleted uranium munitions.	Depresses renal function, can cause kidney damage and failure in extreme cases. More of a toxicological problem than a radiation problem, though long-term exposure can increase risk of cancer.	0.030
Zinc	Enters environment from industrial waste, metal/metalloid plating, plumbing, and is a major component of various industrial sludges.	Essential nutrient. Causes detrimental effects in humans at high doses. Can be toxic to aquatic organisms.	5(secondary)
Table 1: Several metal/metalloid contaminants of concern in groundwater. (Revised from).

Behavior in Groundwater

Dissolved metal behavior in groundwater is controlled by reactions within the aqueous phase and reactions between the contaminant metal and the solid phase (such as clay particles or sand grains in the aquifer). Hence, the chemical composition of the water and the aquifer mineralogy are important to metal contaminant behavior. In turn, the water chemistry can be controlled by the aquifer mineralogy, the chemical composition of the infiltrating plume, or reactions between the infiltrating plume and aquifer minerals. If carbon and other nutrients are available, reactions controlling metal behavior can be influenced by microbial activity. All metal contaminants in groundwater are distributed among dissolved species in which the metal ion (an ion is a dissolved species with a positive or negative charge) is associated with one or more ions, producing a different dissolved species with different chemical behavior. For example, the mercury ion, Hg⁺², readily associates with the chloride ion, Cl^-, to produce a series of species (HgCl⁺, HgCl2°, HgCl₃^-) with concentrations dependent on the chemical composition of the groundwater. The collection of species is called the aqueous speciation of the contaminant metal. Some contaminants have a simple aqueous speciation and few species dominate over a broad range of groundwater conditions. Others have a complicated aqueous speciation that changes with pH, redox potential, and/or the presence or absence of certain ions. Uranium is an example of a complicated aqueous speciation in common aquifer conditions (Fig. 1). The aqueous speciation strongly influences sorption and precipitation reactions involving the contaminant metal, and thus influences the mobility of the metal in the aquifer and the methods of effective remediation (see article on Mobility of Metals and Metalloids).

Figure 1: Uranium’s complicated aqueous speciation (diagram produced with The Geochemist’s Workbench®, PCO₂ = 0.01 atm., [Ca] = 10 (mg/L).

The most important aspect of a metal contaminants aqueous speciation is whether multiple oxidation states can occur over the range of conditions found in groundwater. Contaminants that have multiple oxidation states that are stable in groundwater are considered “redox sensitive”. Different oxidation states of a metal can have very different behaviors and toxicity in groundwater. For example, chromium in a plus six oxidation state (chromate) is more mobile and toxic than chromium in a plus three oxidation state. Table 2 lists whether several common metal contaminants are redox sensitive and the relative complexity of their aqueous speciation. Here, a complicated aqueous speciation is defined as having 4 or more species that dominate various portions of the field defined by a pH range of 4 to 10 and an Eh (Eh is a measure of the tendency of electrons to flow from one ion to another that covers the stability field of water (as determined using the thermodynamic database “thermo_minteq” and The Geochemist’s Workbench® with [Cl^-] and [SO₄^-2] = 10 mg/L). The complexity of aqueous speciation is not determined by redox sensitivity because many metals that do not change oxidation states with varying redox conditions may still form aqueous sulfide or bisulfide species under sufficiently reducing conditions.

The importance of aqueous speciation of a contaminant metal to environmental remediation can be illustrated by the behavior of uranium. Many groundwaters contaminated by uranium contain dissolved oxygen and moderate concentrations of dissolved carbon dioxide. Under these conditions, the dominant aqueous species of oxidized uranium [U(VI)] are neutrally or negatively charged carbonate species (Fig. 1). These species tend to keep oxidized uranium in the dissolved state, making it difficult to limit its mobility by causing it to adsorb or precipitate in a solid phase. The challenge is to find some method that either overcomes the effects of the carbonate species or changes the speciation of the uranium.

One method that has been studied extensively is to change the speciation by converting the uranium to the reduced form, U(IV), causing the uranium to precipitate as a low solubility U(IV) oxide. This can be done with chemicals (abiotic reduction) or by stimulating indigenous microbes to consume all of the oxygen in the groundwater (bioreduction). A common problem with this approach is that the groundwater naturally contains oxygen, and oxygenated groundwater will quickly flow into the treated zone re-oxidizing and remobilizing the uranium.

At some sites, oxidized uranium has been successfully treated with phosphate minerals because, under the conditions of the site groundwater, U(VI) is bound more strongly in the phosphate minerals than it is in the aqueous carbonate complexes. There are other ways to overcome the effects of the carbonate complexes and limit the mobility of uranium. The key idea to selecting a uranium remediation method that is likely to be successful, is understanding how the aqueous speciation of uranium will affect the treatment.

Contaminant	Redox Sensitive?	Complicated Aqueous Speciation?	References
Antimony	Yes	No	Filella, et al., 2002; Wilson et al., 2010; Ilgen et al., 2014
Arsenic	Yes	Yes	Smedley and Kinniburgh, 2002; Ford et al., 2007
Cadmium	No	No	Wilkin, 2007a; McLean and Bledsoe, 1992
Chromium	Yes	Yes	Kent et al., 2007; Palmer and Puls,1994; Richard and Bourg, 1991
Copper	Yes	No	Wilkin, 2007b Wilkin, 2007a
Lead	No	No	Wilkin et al., 2007; McLean and Bledsoe,1992
Mercury	Yes	Yes	Barringer et al., 2013; Hem, 1970
Uranium	Yes	Yes	Langmuir, 1997; Amonette, et al., 2010
Zinc	No	Yes	McLean and Bledsoe, 1992.
Table 2: Redox sensitivity and complexity of speciation of some metal contaminants.

All metal contaminants other than some man-made radionuclides exist naturally in subsurface aquifers. In some cases their natural concentrations in groundwater can exceed the drinking water standards such as MCLs . This can present a challenge to remediation, as well as regulation.

Building on the fundamental concepts described above, the Department of Energy developed a framework that relates metals mobility to redox sensitivity and other factors and is presented in a “scenarios approach” guidance document. With the tools in the scenarios document, groundwater professionals can use their knowledge of site data to quickly evaluate the mobility of several metals and other inorganics in groundwater (see Fig. 2 in MNA of Metal and Metalloid Contaminants).

Summary and Conclusions

Metals and metalloids (compounds such as arsenic and antimony) are commonly found in groundwater, either as naturally occurring elements or as contaminants from man-made sources. The pH and Eh (redox potential) of groundwater, and other constituents present, determine the aqueous speciation of contaminant metals and greatly influences their mobility. Under some conditions, aqueous complexes may be dominant that promote partitioning of the metal/metalloid into the dissolved state, enhancing mobility of the contaminant. Under other conditions, aqueous speciation may promote adsorption or precipitation of the metal/metalloid, limiting its mobility. Some metals/metalloids are redox sensitive, meaning they can exist in groundwater in different oxidation states, determined by the conditions of the groundwater. The different oxidation states for a particular metal/metalloid can have vastly different mobility in groundwater. Therefore, understanding the speciation of a contaminant metal/metalloid is critical to determine if there is environmental risk associated with these metals in groundwater. Compounds that are adsorbed or precipitated may not pose any risk to the users of groundwater, while compounds that are present in the aqueous phase can pose a threat to those ingesting the groundwater if the concentrations are above risk-based levels. Design of effective remediation must also consider how speciation will affect different remedial options.