Trace Elements and Radon in Groundwater Across the United States: U.S. Geological Survey Scientific Investigations Report 2011–5059 by Joseph D. Ayotte, Jo Ann M. Gronberg, and Lori E. Apodaca (review)

The US Geological Survey recently published a comprehensive report reviewing trace elements and radon in groundwater collected from 1992 to 2003 across the United States. A trace element is a metal or semimetallic element that is usually found at a concentration of less than 1 milligram per liter in natural groundwater. Most trace elements—such as selenium, uranium, manganese, arsenic, and others, along with radon—have a Human Health Benchmark (hhb), set by the US Environmental Protection Agency, due to the potential for significant human health impacts over certain levels. Because over 46% of the freshwater used across the United States comes from groundwater, it is important to thoroughly understand trace element occurrences in groundwater, and the mechanisms causing them to occur above acceptable limits. Within the Great Plains states, trace element contamination is particularly relevant as they primarily overlie the High Plains (or Ogallala) aquifer system, arguably the most productive aquifer in the world. The High Plains aquifer system—an unconsolidated aquifer within a dry region—is more vulnerable to well contamination by trace elements.

The usgs report further indicates that nearly one in five (19%) wells across the United States have trace element concentrations that exceed the hhb for at least one parameter. Most commonly these exceedances included manganese, arsenic, uranium, and strontium. In addition, radon, a product of natural uranium decay, exceeded the new proposed hhb in 65% of all wells. It is also noted that in many cases co-contamination by multiple trace elements occurs. In unconsolidated sand and gravel aquifers such as the High Plains aquifer system, the most prevalent co-occurrences were combinations of uranium, arsenic, and manganese.

The occurrences and co-occurrences in groundwater of this aquifer type are controlled primarily by pH (acidity) and redox (reduction-oxidation reaction) conditions. However, trace elements have naturally occurred in groundwater to some extent across the Great Plains throughout geological time, possibly only now being discovered in some regions. Anthropogenic activities, such as the addition of oxidants to aquifers and an increase in groundwater withdrawals by 5% in the last decade in the Great Plains, are both major factors that can influence pH and reduction-oxidation conditions. Thus, the existence of trace metals in many wells may be due in part to increased testing activities, or it may be attributed to increased aquifer stress from anthropogenic additions or from pumping. In turn this stress has disrupted the balance between aquifer waters and the underlying mineralogical materials.

One trace metal of note, uranium, a naturally occurring radionuclide, is particularly prevalent in the High Plains aquifer within the Platte River Basin. The presence of uranium can be attributed to high levels of radon and radium, which are decay products, in groundwater wells. The mechanisms contributing to or controlling geogenic uranium contamination of groundwater wells are not fully understood. Further scientific investigation and enhanced well-sampling programs are both important to our understanding of well contamination. It is also clear that the testing of private (domestic) water supplies and those of small communities (serving under 25 persons) for trace elements and radon, while not required under law, is important to the health of the citizens of the Great Plains. Then the Great Plains as a whole can answer the question, What is in your water?