Thanks for adding the clarification to your question.
EPA recently (in 2001) lowered the drinking water standard for arsenic
from 50 parts per bilion to 10 parts per billion, and the new standard
is gradually going into effect. EPA estimates that drinking water
supplies for 13 million Americans will be improved by the new standard
-- that is, 13 million Americans have been drinking water that
contains arsenic in excess of the new standard of 10 ppbs.
In particular, there are about 3,000 community water systems serving
11 million people that have levels above 10 ppbs, and another group of
about 1,100 water systems serving 2 million people that also have to
come into compliance.
NOTE that since the rule was promulgated in 2001, some -- or even many
-- of these water systems may have already come into compliance with
the new limits.
A fact sheet on the EPA rule can be found here:
Fact Sheet: Drinking Water Standard for Arsenic
and the most relevant excerpts are these:
--The Environmental Protection Agency (EPA) is finalizing a regulation
to reduce the public health risks from arsenic in drinking water. The
Agency is revising the current drinking water standard for arsenic
from 50 parts per billion (ppb) to 10 ppb. This revision will provide
additional protection for 13 million Americans against cancer and
other health problems, including cardiovascular disease and diabetes,
as well as neurological effects.
--Studies have linked long-term exposure to arsenic in drinking water
to cancer of the bladder, lungs, skin, kidney, nasal passages, liver,
and prostate. Non-cancer effects of ingesting arsenic include
cardiovascular, pulmonary, immunological, neurological, and endocrine
(e.g., diabetes) effects.
--EPA is setting the new arsenic standard for drinking water at 10 ppb
to protect consumers against the effects of long-term, chronic
exposure to arsenic in drinking water. EPA is using its discretionary
authority under the 1996 Amendments to the Safe Drinking Water Act to
set the standard at a level that "maximizes health risk reduction
benefits at a cost that is justified by the benefits."
--The new standard will apply to all 54,000 community water systems. A
community water system is a system that serves 15 locations or 25
residents year-round, including most cities and towns, apartments, and
mobile home parks with their own water supplies. EPA estimates that
roughly five percent, or 3,000, of community water systems, serving 11
million people, will have to take corrective action to lower the
current levels of arsenic in their drinking water.
--The new standard will also apply to 20,000 water systems that serve
at least 25 of the same people more than six months of the year, such
as schools, churches, nursing homes, and factories. EPA estimates that
five percent, or 1,100, of these water systems, serving approximately
2 million people, will need to take measures to meet the new arsenic
standard. Of all of the affected systems, 97 percent are small systems
that serve fewer than 10,000 people each.
--Arsenic occurs naturally in rocks and soil, water, air, and plants
and animals. It can be further released into the environment through
natural activities such as volcanic action, erosion of rocks, and
forest fires, or through human actions. Approximately 90 percent of
industrial arsenic in the U.S. is currently used as a wood
preservative, but arsenic is also used in paints, dyes, metals, drugs,
soaps, and semi-conductors. Agricultural applications, mining, and
smelting also contribute to arsenic releases in the environment.
--Higher levels of arsenic tend to be found more in ground water
sources than in surface water sources (i.e., lakes and rivers) of
drinking water. Compared to the rest of the United States, western
states have more systems with arsenic levels greater than 10 ppb.
Parts of the Midwest and New England have some systems whose current
arsenic levels are greater than 10 ppb, but more systems with arsenic
levels that range from 2-10 ppb. While many systems may not have
detected arsenic in their drinking water above 10 ppb, there may be
geographic "hot spots" with systems that may have higher levels of
arsenic than the predicted occurrence for that area.
I trust this information fully answers your question.
However, please don't rate this answer until you have everything you
need. If you would like any additional information, just post a
Request for Clarification to let me know how I can assist you further,
and I'm at your service.
All the best,
search strategy -- searched the EPA drinking water site for [ arsenic ]