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The U.S. Geological Survey (USGS) released a report in August revealing that 20% of untreated water samples from wells across the U.S. contain concentrations of trace elements exceeding human health benchmarks. Raissa Rocha, editorial intern for Water Quality Products, spoke with Joe Ayotte, USGS hydrologist and lead author of the study, about the report and the occurrence of trace elements in groundwater.
Raissa Rocha: What was the purpose of this study?
Joe Ayotte: The main goal was not necessarily to report on private wells in particular, but more to look at the occurrence of trace elements in various types of groundwater across the U.S. So we looked at trace element concentrations in private wells, public wells, monitoring wells and even in a few other types.
These samples were collected prior to any form of treatment of the water. For public wells, water that is delivered to customers has to meet standards established by the Safe Drinking Water Act. For private wells, it does not have to meet standards necessarily. We don’t know whether they treat or don’t treat water.
Rocha: What factors contribute to these trace elements being in the water?
Ayotte: Those trace elements get into groundwater in different ways. It really all comes down to the geochemistry in the water sample and in the aquifer. Often times there are geochemical conditions that make one [element] soluble and [able to] get into the groundwater [that] inhibit the solubility of other [elements] to varying degrees.
An example is manganese, which we tend to see in humid climate areas (the eastern half of the U.S. primarily) and where there is not much dissolved oxygen in the sample water. Basically, either through metabolism of bacteria or water-rock reactions, the oxygen has been consumed in the sample, and those conditions favor manganese mobility. [Manganese] is also consistent with low pH.
Arsenic is similar. It is more mobile under low oxygen conditions, but it is definitely more mobile under high pH than low pH. So you can start to see these competing geochemical effects work differently depending on the trace element. Uranium, by contrast, is less mobile when there is low dissolved oxygen, so we find it mostly where water has plentiful oxygen in it.
These were the kinds of things we were able to, with a large sample, find good information on.
Rocha: Were there any trends between which trace elements occurred in private, public and monitoring wells?
Ayotte: Yes. Trace elements that exceeded benchmarks occurred in general at similar rates for public and private wells. The ones that occurred most frequently were manganese, arsenic and uranium.