USGS profiles private wells
As many as 43 million people in the U.S. get their drinking water from
private wells, yet the quality of the water from those wells is largely
unknown. A new report from the U.S. Geological Survey (USGS) provides a
comprehensive survey of these private wells. The report also documents areas
where residents may be at risk of exposure to both natural and human-made
compounds and elements in groundwater. The 214-contaminant assessment
includes dozens of metals; radionuclides such as radon; organic compounds,
including pesticides such as atrazine; and other compounds of concern.
Leslie DeSimone and her USGS colleagues analyzed water from more than 2000
wells sampled from 1991 to 2004. Groundwater generally movesand therefore
changesslowly, so levels of natural compounds should remain fairly constant,
DeSimone explains. The team added to existing data for areas long known to
be of high concern for arsenic, such as New England. The researchers also
found in some areas detectable levels of newer human-made compounds such as
methyl-tert-butyl ether (MTBE) and currently used pesticides in
drinking-water wells.
"We identified microbial contaminants, which are possible indicators of sewage contamination, as one of the most frequently occurring contaminants of potential health concern," notes DeSimone. Her team detected total coliform in about one-third of 400 sampled wells, an indicator of bacteria present and possible contamination from human or animal waste from septic tanks or agricultural waste ponds.
Unregulated wells serve about 15% of the U.S. population, according to
USGS estimates based on state and other local databases. Overall, the
contaminants found in the groundwater were generally at lower levels than
the maximum contaminant levels (MCLs) set by the U.S. EPA for known
human-health hazards. If no MCLs had been set for certain contaminants, USGS
used its own human-health benchmarks, known as health-based screening levels
(HBSLs). These levels were determined in partnership with EPA. For example,
USGS uses an HBSL of 1000 micrograms per liter (μg/L) for boron. There is no
EPA MCL for boron, but the agency has a health advisory level of 600 μg/L.
The great unknowns, says DeSimone, are the human-health impacts from
exposure to mixtures of multiple compounds over a long period of time. Some
pairings, such as nitrate and atrazine, could react in the human body to
form suspected carcinogens, and these particular compounds were detected
together in a small fraction of the wells tested, as were other potentially
hazardous combinations.
Nearly three-quarters of the wells contained two or more contaminants at
levels greater than 1/10 their health benchmarks. Mixtures that frequently
occur at these levels provide a starting point, say DeSimone and her
colleagues, for evaluating which mixtures may merit investigation for
potential human-health impacts.
Jrg Drewes of the Colorado School of Mines says that although the new
approach to assessing mixtures is “questionable,” the reported data on
pathogen exposures are important.
Possible exposure to bacterial and other sewage-related contaminants from
these wells is worrisome, agrees Ben Stanford, a postdoctoral researcher at
the Southern Nevada Water Authority, a regional public water agency and
drinking-water research facility. But in the context of past reports on
arsenic, pesticides, and other compounds in groundwater from USGS and other
researchers, the study’s broader findings about contaminant levels are “not
that surprising,” he says. However, this USGS survey provides huge data
sets, and its value lies in its breadth, Stanford adds. Further research
should include “emerging” contaminants, such as pharmaceuticals,
personal-care products, and especially endocrine-disrupting compounds,
Stanford suggests.
Each region studied by USGS may also face different groundwater problems,
Stanford notes, such as high salinity in the Las Vegas basin. Prior land use
is also a concern, he says; for example, past contamination from
agricultural pesticides or manufacturing activities might have left
contaminants in groundwater.
The report highlights the need to educate homeowners about point-of-use
treatment and other issues, Drewes says. The National Ground Water
Association (NGWA) has backed proposals for tax credits for homeowners who
test their wells. “Owners are responsible for their own treatment devices,”
says NGWA director Kevin McCray. NGWA offers best-practice guidelines for
well owners and is working with the Centers for Disease Control and
Prevention to evaluate owners’ drinking-water habits.
Researchers sampled more than 2000 private drinking-water wells in rural
and urban areas. The map (above) shows the geographic distribution of fecal
indicator bacteria detected in the researcher’s samples. Green shows total
coliform bacteria detected; brown shows E. coli bacteria; red shows both
total coliform and Escherichia coli bacteria; and clear shows no detections
of either.
USGS