More than 20 percent of untreated water samples from 932 public wells across the nation contained at least one contaminant at levels of potential health concern, according to a new study by the U.S. Geological Survey (USGS).
About 105 million people – or more than one-third of the
nation's population – receive their drinking water from one of the 140,000
public water systems across the United States that rely on ground water pumped
from public wells.
The USGS study focused primarily on source (untreated) water
collected from public wells before treatment or blending rather than the
finished (treated) drinking water that water utilities deliver to their
"By focusing primarily on source-water quality, and by
testing for many contaminants that are not regulated in drinking water, this
USGS study complements the extensive monitoring of public water systems that is
routinely conducted for regulatory and compliance purposes by federal, state and
local drinking-water programs," says Matthew Larsen, USGS Associate
Director for Water. "Findings assist water utility managers and regulators
in making decisions about future monitoring needs and drinking-water
Findings showed that naturally occurring contaminants, such
as radon and arsenic, accounted for about three-quarters of contaminant
concentrations greater than human-health benchmarks in untreated source water.
Naturally occurring contaminants are mostly derived from the natural geologic
materials that make up the aquifers from which well water is withdrawn.
Man-made contaminants also were found in untreated water
sampled from the public wells, including herbicides, insecticides, solvents,
disinfection by-products, nitrate and gasoline chemicals. Man-made contaminants
accounted for about one-quarter of contaminant concentrations greater than
human-health benchmarks, but were detected in 64 percent of the samples,
predominantly in samples from unconfined aquifers.
"Detections of contaminants do not necessarily indicate
a concern for human health because USGS analytical methods can detect many
contaminants at concentrations that are 100-fold to 1,000-fold lower than
human-health benchmarks," says lead scientist Patricia Toccalino.
"Assessing contaminants in these small amounts helps to track emerging
issues in our water resources and to identify contaminants that may warrant
inclusion in future monitoring."
Scientists tested water samples for 337 properties and
chemical contaminants, including nutrients, radionuclides, trace elements,
pesticides, solvents, gasoline hydrocarbons, disinfection by-products and
manufacturing additives. This study did not assess pharmaceuticals or hormones.
Most (279) of the contaminants analyzed in this study are
not federally regulated in finished drinking water under the Safe Drinking
The USGS also sampled paired source and finished (treated)
water from a smaller subset of 94 public wells. Findings showed that many
man-made organic contaminants detected in source water generally were detected
in finished water at similar concentrations. Organic contaminants detected in
both treated and source water typically were detected at concentrations well
below human-health benchmarks, however.
Additionally, the study shows that contaminants found in
public wells usually co-occurred with other contaminants as mixtures. Mixtures
can be a concern because the total combined toxicity of contaminants in water
may be greater than that of any single contaminant. Mixtures of contaminants
with concentrations approaching benchmarks were found in 84 percent of wells,
but mixtures of contaminants above health benchmarks were found less
frequently, in 4 percent of wells.
This USGS study identifies which contaminant mixtures may be
of most concern in ground water used for public-water supply and can help
human-health researchers to target and prioritize toxicity assessments of
contaminant mixtures. The USGS report identifies the need for continued
research because relatively little is known about the potential health effects
of most mixtures of contaminants.
Wells included in this study are located in 41 states and
withdraw water from parts of 30 regionally extensive aquifers, which constitute
about one-half of the principal aquifers used for water supply in the United States.
Human-health benchmarks used in this study include U.S.
Environmental Protection Agency (EPA) Maximum Contaminant Levels for regulated
contaminants and USGS Health-Based Screening Levels for unregulated contaminants,
which are non-enforceable guidelines developed by the USGS in collaboration
with the EPA and other water partners.
Treated drinking water from public wells is regulated under
the Safe Drinking Water Act. Water utilities, however, are not required to treat
water for unregulated contaminants. The EPA uses USGS information on the
occurrence of unregulated contaminants to identify contaminants that may
require drinking-water regulation in the future.
Contaminants in Ground Water Used for Public Supply
May 25, 2010