Well Water Trace Metals
The water that people drink is supplied from several possible sources such as rainwater collection, ponded surface water, streams, desalinized seawater and well or ground water. Wells can vary greatly in depth, water volume and water quality. Well water typically contains more minerals in solution than surface water and may require treatment to soften the water by removing minerals such as arsenic, iron and manganese. About 20% of untreated water samples from public, private, and monitoring wells across the nation contain concentrations of at least one trace element, such as arsenic, manganese and uranium, at levels of potential health concern, according to a new study by the U.S. Geological Survey.
Shallow pumping wells can often supply drinking water at a very low cost, but because impurities from the surface easily reach shallow sources, a greater risk of contamination occurs for these wells when they are compared to deeper wells. Dug and driven wells are relatively easy to contaminate, and dug wells are unreliable in most of the U.S.
Public well water supply are controlled and monitored under a variety of standards and policies. Private wells may have little or no water quality evaluation.
Trace elements in groundwater exceed human health benchmarks at a rate that far outpaces most other groundwater contaminants, such as nitrate, pesticides, and volatile organic compounds (VOCs). Most trace elements, including manganese and arsenic, get into the water through the natural process of rock weathering. Radon, derived from naturally occurring uranium in aquifers, also occurs frequently at high levels in groundwater.
Arsenic, uranium, and manganese, were the trace elements in groundwater that most frequently exceeded USEPA human-health benchmarks. Arsenic was found above the USEPA human health benchmark in 7% of wells. Uranium was found in 4% above the human health benchmark, and manganese was found in 12%.
Long-term exposure to arsenic can lead to several types of cancer, and high levels of uranium can cause kidney disease. In doses similar to some of those found in this study, manganese can adversely affect child intellectual function and, in large doses, acts as a neurotoxin, causing symptoms similar to those experienced by sufferers of Parkinson’s disease. Radon, a product of the decay of natural uranium, also exceeded its proposed EPA maximum contaminant level in 65% of wells tested (300 Picocuries per liter).
Climate and land use are important factors in trace element distribution. Differences in the concentration of trace elements are related to the climatic conditions and land use of the area. Drier areas of the United States saw higher concentrations of trace elements in groundwater than humid regions. Meanwhile, wells in agricultural areas more often contained trace elements than those in urban areas. However, wells in urban areas contained concentrations of trace elements that more often exceeded human health benchmarks.
Basic geology and geochemistry of water samples helps to predict risk of trace elements exceeding human-health benchmarks. The acidity and amount of dissolved oxygen in the water affect which trace elements persist in groundwater. For example, aluminum, lead, and manganese more often exceeded human health benchmarks in samples that were slightly acidic and had high dissolved oxygen concentrations. Slightly alkaline samples with low dissolved oxygen concentrations more often had exceedances of arsenic, molybdenum, and uranium. Additionally, glacial and non-glacial sand and gravel aquifers consistently had more potential for human health hazards.
The effects of mixtures of trace elements are poorly understood and could cause further health concerns. Further analysis of the data showed that about one-fifth of wells had exceedances of human health benchmarks and that, of those, about 10 percent actually contained two or more trace elements exceeding human health benchmarks. This raises additional concerns because contaminants can act together to be more toxic than each individual contaminant.
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