Metals and Other Trace Elements
Groundwater + Rocks and Sediment = Trace Elements in Groundwater
Climate, geology, groundwater age, and human actions all contribute
Science Center Objects
Metals, metalloids, and radionuclides all are trace elements that occur naturally in the Earth's crust. In small quantities many trace elements are essential for health in all living organisms, but some trace elements can be toxic or cause cancer, and some can bioaccumulate. The USGS investigates where and how trace elements make their way into our Nation's surface water and groundwater.
Trace elements are simply elements present in minute amounts in the environment. Trace elements include metals, such as lead and iron; metalloids, such as arsenic; and radionclides (radioactive elements), such as radium and radon. Trace elements in our Nation's streams, rivers, and groundwater have natural and manmade sources. Rock weathering, soil erosion, and dissolution of water-soluble salts are examples of natural sources of trace elements. Many human activities also contribute trace elements to the environment—mining, urban runoff, industrial emissions, and nuclear reactions are just some of the many manmade sources. Trace elements tend to concentrate in sediment, but also can dissolve to some degree in water, and can present a risk to human and aquatic health.
► Learn about trace elements in groundwater in Principal Aquifers of the United States, our invisible, vital resource.
Many people might not realize that most elements are metals. Metals tend to be shiny, they make good conductors, and they're malleable and ductile. Most corrode when exposed to sea water or air, and lose electrons during reactions. We're familiar with many metals, for example gold, silver, lead, zinc, chromium, cadmium, and mercury. It's less obvious that other elements—beryllium, sodium, and lithium, for example—are metals too. Although manmade metal objects surround us each day, metals represent only a minute proportion of the elements in the Earth's crust.
There is no agreed-upon definition of "heavy metals," but heavy metals generally are considered to be those metals with a high density. Gold, silver, tin, copper, zinc, and iron are well-known examples of heavy metals. Some heavy metals, like iron and zinc, are essential nutrients at low concentrations but toxic at high concentrations. Other non-essential heavy metals, like cadmium, mercury, and lead, are toxic even at relatively low concentrations.
A "metalloid" has properties intermediate between metals and non-metals. From a water-quality point of view, arsenic is perhaps the metalloid of most concern. Other metalloids include boron and silicon, and carbon and some other trace elements are sometimes classified as metalloids.
Metals in water used for drinking and in sediment can present a risk to human and aquatic health. Various concentration benchmarks have been developed that indicate the concentration above which a metal is a health concern.
Radionuclides (radiocative elements) also are trace elementa. Radionuclides in our environment are produced by minerals in the Earth’s crust, by cosmic rays hitting atoms in the Earth’s atmosphere, and by human activities. Radionuclides occur naturally in many rocks and minerals and therefore occur frequently groundwater. The most common examples of radionuclides in groundwater are uranium, radium, and radon.
► Learn more about radionuclides and water quality.
OTHER TRACE ELEMENTS
A small number of trace elements, such as selenium, are neither metals nor radionuclides. Selenium occurs naturally in sedimentary rocks, shales, coal and phosphate deposits, and soils. Application of irrigation water, which contains dissolved oxygen, can caused selenium to be released from sediment into groundwater, particularly in arid areas. This processes has been documented in the shallow Denver Basin aquifer in Colorado and in parts of the West where selenium occurs in rocks and sediments. Selenium in groundwater can discharge into streams, where it can bioaccumulate in the aquatic food chain. Chronic exposure in fish and aquatic invertebrates can cause reproductive impairments.
TRACE ELEMENTS AND DRINKING WATER
Concentrations of trace elements are more likely to be a problem in groundwater than in surface water, unless the area is impacted by mining. That’s because when groundwater moves through the rocks and sediments that make up an aquifer, some of the minerals in or adhered to those rocks and sediment are released into the water. Groundwater that has been in an aquifer a long time has had more time to interact with aquifer materials than groundwater that has recharged recently. Additionally, geochemical conditions, such as pH and redox, change as groundwater slowly moves along a flowpath from recharge to discharge—those geochemical conditions can affect whether metals are released into the groundwater.
Groundwater age is just one of the factors that can affect the concentration of trace elements. Other factors include climate and, geology, and human actions. Climate4 plays a role because in regions where precipitation is low and evaporation rates are high, there is less water to dilute the products of rock weathering. Geology plays a role because the metals available for leaching into groundwater depend on types of minerals present in the rocks and sediment. Finally, human actions such as irrigation and pumping can affect concentrations of trace elements in groundwater, often by changing the geochemical conditions, such as pH and redox conditions, within the aquifer.
Metals reported to widely occur at concentrations above drinking-water benchmarks in untreated groundwater from some aquifers include manganese and the metalloid arsenic. Other metals, like iron, might not be present at levels that are a health risk, but can be a nuisance by making water unpleasant to drink or by staining fixtures. Levels of metals can be lowered through treatment. Water from public-supply wells is required to be tested by the well operator on a routine basis to help assure that the water provided to consumers meets Federal and State water-quality standards, which exist for many but not all metals. Routine testing of water from domestic (private) wells is not required, and it is up to the homeowner or private-well owner to test, maintain, and treat the water from their well. The best way to know the water quality of a domestic well is to have it tested.
In areas impacted by mining, acid runoff dissolves heavy metals, such as copper, lead, and mercury, into groundwater or surface water. Acidic, metal-laden drainage from abandoned coal mines can have substantial effects on aquatic resources. Problems that can be associated with mine drainage include contaminated drinking water, disrupted growth and reproduction of aquatic plants and animals, and the corroding effects of the acid on parts of infrastructures such as bridges.
Corrosive water can contribute to elevated concentrations of metals in drinking water, but in this case the metals come from within the water distribution system, such as pipes used for plumbing. Naturally corrosive water is not dangerous to consume in itself, but if plumbing materials contain lead or copper, corrosive water can cause these metals to leach into the water supply. Both surface water and groundwater can be corrosive. Many factors contribute to corrosivity, including elevated concentrations of chloride and other dissolved solids, pH out of neutral range, elevated concentrations of suspended solids, and low alkalinity.
METALS IN LAKE SEDIMENTS—RECONSTRUCTING CONTAMINANT TRENDS
Metals tend to adhere to sediment; they can be carried by suspended sediment in streams and rivers to lakes and reservoirs, where the sediment and metals settle to the bottom. The history of metal contamination in a watershed is recorded in the lake sediment, and by collecting and analyzing cores of that sediment the watershed's contamination history can be reconstructed.
Trends in metals, as recorded in sediment cores, reflect legislation, regulation, and changing demographics and industrial practices in the United States. For example, sediment cores clearly indicate the peak in the use of leaded gasoline in the late 1960s and early 1970s. A study of metals trends in 35 reservoirs and lakes across the U.S. found decreasing trends in both lead and chromium in most lakes, and increasing trends in few or no lakes. Sediment cores can also record trends in metals associated with local sources such as mining and smelters. In urban areas, fluvial sources (urban runoff and streams) contribute far greater fluxes of metals than do atmospheric sources.
► Learn more about metals and lake sediment cores.