More than 20 percent of private domestic water wells sampled
nationwide contain at least one contaminant at levels of
potential health concern, according to a study by the
About 43 million people - or 15 percent of the
Nation's population - use drinking water from private wells, which are not
regulated by the Federal Safe Drinking Water Act.
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USGS scientists sampled about 2,100 private wells in 48
states and found that the contaminants most frequently measured at
concentrations of potential health concern were inorganic contaminants,
arsenic. These contaminants are mostly derived from the
natural geologic materials that make up the aquifers from which well water
Nitrate was the most common inorganic contaminant derived
from man made sources such as from fertilizer applications and
septic tanks that was found at concentrations greater than the Federal
drinking water standard for public water supplies (10 parts per million).
Nitrate was greater than the standard in about 4 percent of sampled wells.
The study shows that the occurrence of selected contaminants varies across
the country, often following distinct geographic patterns related to
geology, geochemical conditions, and land use. For example, elevated
concentrations of nitrate were largely associated with intensively farmed
land, such as in parts of the Midwest Corn Belt and the Central Valley of
California. Radon was found at relatively high concentrations in
crystalline rock aquifers in the Northeast, in the central and southern
Appalachians, and in central Colorado.
"The results of this study are important
because they show that a large number of people may be unknowingly
affected," said Matt Larsen, USGS Associate Director for Water.
"Greater attention to the quality of drinking water from private wells and
continued public education are important steps toward the goal of protecting
The USGS sampled private wells from 1991 to
2004 in 30 of the Nation's principal aquifers used for water supply. As many
as 219 properties and contaminants, including pH, major ions, nutrients,
radionuclides, trace elements, pesticides,
arsenic, volatile organic compounds, and
microbial contaminants, were measured. Sampled water was taken from private
water wells before any
whole house filtration home treatment. Other contaminants found in the private
water wells were man made organics, including herbicides, insecticides, solvents,
disinfection by-products, and gasoline chemicals. Few organic contaminants
(7 out of 168) exceeded health benchmarks, and were found above health
benchmarks in less than 1 percent of sampled wells. Organic
contaminants were detected at lower concentrations in more than half (60
percent) of sampled wells, indicating that a variety of contaminant sources including
agricultural, residential, and industrial that can affect the quality of water
from private water wells.
Contaminants found in private water wells usually co-occurred
with other contaminants as mixtures rather than alone, which can be a
concern because the total combined toxicity of contaminant mixtures can be
greater than that of any single contaminant. Mixtures of contaminants
at relatively low concentrations were found in the majority of well water.
The USGS report identifies the need for continued research because
relatively little is known about the potential health effects of most
mixtures of contaminants, and the additive or synergistic effects on human
health of mixtures of man made chemicals even at low levels are not well
Bacteria, including total coliform
Escherichia coli (e coli), were found in as many as one third of a subset of 400
water wells. These bacteria are typically not harmful but can be an indicator of
About half of the 2,100 sampled water wells had at
least one property or contaminant outside recommended ranges for cosmetic or
aesthetic purposes, such as total dissolved solids, pH,
manganese. Human health benchmarks used in the study included
drinking water standards for contaminants regulated under the Federal Safe
Drinking Water Act and non-enforceable USGS Health Based Screening Levels
(HBSLs) for unregulated contaminants, developed by USGS in collaboration
About half of the wells deemed to have
potential health concerns had concentrations greater than Maximum
Contaminant Levels specified by the Safe Drinking Water Act for public water
supplies. In relating measured concentrations to health benchmarks, this
study offers a preliminary assessment of potential health concerns that
identifies conditions that may require further investigation. The research
is not a substitute for comprehensive risk and toxicity assessments.
Private water well owners, who generally are responsible for
testing the quality
of their well water and treating, if necessary, can find more information
about well maintenance, testing and water treatment options including
Ground Well water quality
Just because you have a
yields plenty of water doesn't mean you can go ahead and just take a drink. Because water is
such an excellent solvent色戒无删除158分钟完整版
it can contain lots of dissolved chemicals. And since ground water moves through rocks and subsurface soil, it has a lot
of opportunity to dissolve substances as it moves. For that reason, ground
water will often have more dissolved substances than surface water will.
Even though the ground is an excellent mechanism for filtering out
particulate matter, such as leaves, soil, and bugs, dissolved chemicals and
gases can still occur in large enough concentrations in ground water to
cause problems. Underground water can get contaminated from industrial,
domestic, and agricultural chemicals from the surface. This includes
chemicals such as pesticides and herbicides that many homeowners apply to
their lawns, or farmers apply to their fields.
Contamination of ground water by road salt is of major concern in
northern areas of the United States. Salt is spread on roads to melt ice,
and, with salt being so soluble in water, excess sodium and chloride is
easily transported into the subsurface ground water. The most common
water quality problem in rural water supplies is bacterial contamination
septic tanks, which are often used in rural areas that don't have a
sewage treatment system. Effluent (overflow and leakage) from a septic tank
can percolate (seep) down to the
water table and
maybe into a homeowner's own well. Just as with urban water supplies,
chlorination may be necessary to kill the dangerous bacteria.
The U.S. Geological Survey is involved in monitoring the Nation's
ground water supplies. A national network of observation wells exists to
measure regularly the water levels in wells and to investigate water
Contaminants can be
natural or human induced
Naturally occurring contaminants are present in the rocks and sediments. As ground water flows through sediments, metals such as iron and manganese
are dissolved and may later be found in high concentrations in the water.
Industrial discharges, urban activities, agriculture, ground water pumpage,
and disposal of waste all can affect ground water and well water quality. Contaminants from
leaking fuel tanks or fuel or toxic chemical spills may enter the ground
water and contaminate the aquifer. Pesticides and fertilizers applied to
lawns and crops can accumulate and migrate to the water table.
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