Arsenic is not
uncommon in groundwater. The U.S. has a legal limit for arsenic in public water
systems at 10 parts per billion. It is estimated that 100-280 million Americans
drink water with arsenic, typically at low levels around 1 part per billion, but
sometimes at levels exceeding the legal limit by as much as 1000 times.
Groundwater and private water systems may contain high levels of arsenic in
various parts of the country, but it is more common in the West, particularly
the Southwest. It is also a concern with glacial aquifers in the Upper Midwest
and Northern Plains. In the U.S., about seven percent of
wells exceeded the federal safety standard for arsenic. High levels of arsenic
occur in 25 states, and it is estimated that 2.1 million Americans are exposed
to dangerous levels of arsenic from groundwater. People who own water wells, or
private water systems as they are often called, may not know their water
contains high levels of arsenic unless it is tested specifically for it.
The USGS gives more background
information about occurrences of arsenic in groundwater:
“Arsenic occurs naturally as a trace component in many
rocks and sediments. Whether the arsenic is released from these geologic
sources into groundwater depends on the chemical form of the arsenic, the
geochemical conditions in the aquifer, and the biogeochemical processes that
occur. Arsenic also can be released into groundwater as a result of human
activities, such as mining, and from its various uses in industry, in animal
feed, as a wood preservative, and as a pesticide. In drinking-water supplies, arsenic
poses a problem because it is toxic at low levels and is a known carcinogen. In
2001, the USEPA lowered the MCL for arsenic in public-water supplies to 10
micrograms per liter (µg/L) from 50 µg/L.”
Below are some maps that show
exposure to arsenic, the geographical distribution of high levels of arsenic in
groundwater, and the area of the Upper Midwest and Plains where glacial
aquifers are high in arsenic. The highest levels are generally found in the
Southwest basin-fill aquifers. Other areas with arsenic-rich groundwater
include the Glacial aquifer system (northern U.S.), the crystalline rock
aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Aquifers (northern U.S.),
and the Mississippi Embayment–Texas Coastal Uplands Aquifer System and
Mississippi River Valley Alluvial Aquifer (southeastern U.S.). The last figure shows some of the predictor variables for arsenic-rich groundwater in the Upper Midwest/Plains glacial aquifers. Arsenic-rich soil C-horizons, low recharge, glacial sediment thickness, and pH are some of the predictor variables.
Toxicity, Health Effects, and Past Events
One of the major events for
arsenic poisoning from groundwater is the collective poisoning of about 100
million people in Bangladesh with arsenic-tainted groundwater.
The CDC has some information about
arsenic, the different compounds it forms, the difference between inorganic and
organic arsenic, and the forms of it found in groundwater.
“Arsenic is a naturally occurring element that is widely
distributed in the Earth’s crust. Arsenic is classified chemically as a
metalloid, having both properties of a metal and a nonmetal; however, it is
frequently referred to as a metal. Elemental arsenic (sometimes referred to as
metallic arsenic) is a steel grey solid material. However, arsenic is usually
found in the environment combined with other elements such as oxygen, chlorine,
and sulfur. Arsenic combined with these elements is called inorganic arsenic.
Arsenic combined with carbon and hydrogen is referred to as organic arsenic.”
“Most inorganic and organic arsenic compounds are white
or colorless powders that do not evaporate. They have no smell, and most have
no special taste. Thus, you usually cannot tell if arsenic is present in your
food, water, or air.”
Mineral ores and smelting
emissions may contain high levels of arsenic. Currently, 90% of all arsenic
produced is used as a wood preservative. The preservative is copper chromated
arsenate (CCA), and wood treated with it is known as “pressure-treated.” It is
also used as a pesticide and was previously used as a rodenticide.
New Study Shows Removing the Arsenic Source Results in
Improved Health Outcomes
A new study published in the
Journal of the American Medical Association compared levels of arsenic in the
body with health outcomes. The results were dramatic health outcome
improvements with less arsenic in the body. Less arsenic is associated with
less overall chronic disease, less cardiovascular disease, and less cancer.
“To ensure an accurate measurement of arsenic exposure
and its concentration in the body, scientists report it as micrograms of
arsenic per gram of creatinine (μg/g creatinine).”
“This method corrects for urine dilution by using
creatinine, a natural waste product with a stable baseline, as a reference
point, thereby providing a reliable result regardless of the amount of water a
person has consumed.”
“Urine samples showed that, as mitigation measures were
implemented, arsenic concentrations fell from an average of 283 μg/g to 132
μg/g creatinine.”
“For each major decrease in urinary arsenic,
specifically, a reduction of 197 micrograms per gram of creatinine, the risk of
death fell by 22 percent from chronic diseases, 23 percent from cardiovascular
disease, and 20 percent from cancer.”
“The study also concluded that if everyone with high
arsenic exposure had reduced their levels, it would have prevented
approximately 5.1 chronic disease deaths per 1,000 people annually.”
The researchers collected water
samples from nearly 6,000 water wells within a 25km radius in Bangladesh. They
also studied people who had been living in this area for over five years and
took urine samples. They also conducted interviews and physical exams from 2000
to 2018. The main method for mitigating arsenic poisoning in groundwater was to
drill wells to a deeper aquifer to water with lower arsenic concentrations.
This made for a nice experiment to show conclusively that lowering exposure to
arsenic leads to better health outcomes, and this was found to be true
regardless of how long one had been exposed. That was a pleasant surprise and
shows that we should address this problem better where it might occur. The
following graphs show pretty conclusively that less arsenic = better health
outcomes in the form of less mortality from chronic disease, cardiovascular
disease (CVD), and cancer.
Perhaps it should be noted that
shallow groundwater is usually, but not always, more likely to be polluted than
deeper groundwater. One reason is that it is better connected to surface water
that can be easily polluted by human activities as well as natural activities.
However, the main reason some groundwater aquifers are high in arsenic is the
arsenic in the rocks and soils with which the water comes into contact.
References
Reducing
exposure to arsenic in household essential slashes cancer and heart disease
death by 50%. Cassidy Morrison. Daily Mail. November 17, 2025. Reducing
exposure to arsenic in household essential slashes cancer and heart disease
death by 50%
Drinking
Water Arsenic Rule History. U.S. Environmental Protection Agency. Drinking
Water Arsenic Rule History | US EPA
Public
Health Statement for Arsenic. Agency for Toxic Substances and Disease Registry.
Center for Disease Control. Arsenic | Public
Health Statement | ATSDR
Arsenic
and Drinking Water. U.S. Geological Survey. March 1, 2019. Arsenic
and Drinking Water | U.S. Geological Survey
Machine-Learning
Predictions of High Arsenic and High Manganese at Drinking Water Depths of the
Glacial Aquifer System, Northern Continental United States. Melinda L. Erickson,
Sarah M. Elliott, Craig J. Brown, Paul E. Stackelberg, Katherine M. Ransom, James
E. Reddy, and Charles A. Cravotta III. American Chemical Society
Publications. Environmental Science & Technology. Vol 55/Issue 9. April 6,
2021. Machine-Learning
Predictions of High Arsenic and High Manganese at Drinking Water Depths of the
Glacial Aquifer System, Northern Continental United States | Environmental
Science & Technology
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