Geology Reference
In-Depth Information
county, it helps researchers and policy-
makers identify areas of high concen-
tration so that they can make informed
decisions about water use. We should
point out, however, that a high degree
of local variability in the amount of ar-
senic in the groundwater can be caused
by local geology, type of aquifer, depth
of well, and other factors. The only way
to learn the arsenic concentration in
any well is to have it tested.
What is considered a safe level of ar-
senic in drinking water? In 2001, the U.S.
Environmental Protection Agency (U.S.
EPA, or EPA) lowered the maximum
level of arsenic permitted in drinking
water from 50 µg (50 micrograms) arse-
nic per liter to 10 µg of arsenic per liter.
This is the same fi gure used by the World
Health Organization.
From the data in Figure 1, additional
maps were created.
the new standard of 10 µg of arsenic per
liter of drinking water.
Public water supply systems that ex-
ceed the existing EPA arsenic standard
are required to either treat the water to
remove the arsenic or fi nd an alternative
supply. Although reducing the accept-
able level of arsenic in drinking water
will surely increase the cost of water to
consumers, it will also decrease their
exposure to arsenic and the possible ad-
verse health effects associated with this
toxic element.
Figure 2 shows
arsenic concentrations found in at least
25% of groundwater samples per county.
Based on these data, approximately 10%
of the samples in the USGS study exceed
Arsenic concentrations in at least
25% of samples exceed:
50
µ
g/L
Insufficient
data
10
5
3
1
Alaska
Puerto Rico
Hawaii
Figure 2 Arsenic concentrations found in at least 25% of groundwater samples per county. The map
is based on 31,350 groundwater samples collected between 1973 and 2001.
calcium (Ca +2 ) and magnesium (Mg +2 ) ions. Water contain-
ing less than 60 milligrams of Ca +2 and Mg +2 per liter (mg/L)
is considered soft, whereas 61-120 mg/L indicates moderately
hard water, values from 121 to 180 mg/L characterize hard wa-
ter, and any water with more than 180 mg/L is very hard.
One of the negative aspects of hard water is the pre-
cipitation of scale (Ca and Mg salts) in water pipes, water
heaters, dishwashers, and even on glasses and dinnerware.
To remedy this problem, many households have a water soft-
ener, whereby calcium and magnesium ions in the water are
 
 
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