139A change in the taste, odor or appearance of the water at customers’ taps may be the first indication
of a problem. Some water quality parameters to consider monitoring, depending on your arsenic
treatment technology, include iron, pH, manganese, alkalinity, and aluminum.
The current drinking water standard or Maximum Contaminant Level (MCL) set by the U.S.
Environmental Protection Agency (EPA) is 0.010 mg/L or parts per million (ppm). This is equivalent
to 10 ug/L (micrograms per liter) or 10 ppb. In 2001, the U.S. Environmental Protection Agency
(EPA) reduced the regulatory MCL from 50 ppb to 10 ppb on the basis on bladder and lung cancer
risks. The MCL is based on the average individual consuming 2 liters of water a day for a lifetime.
Long term exposure to drinking water containing arsenic at levels higher than 10 ppb increases the
chances of getting cancer, while for lower arsenic water levels the chances are less.
If your water has arsenic levels above 10 ppb, you should obtain drinking water from another source
or install a home treatment device. Concentrations above 10 ppb will increase the risk of long-term
or chronic health problems, the higher the level and length of exposure, the greater the risk. It is
especially important to reduce arsenic water concentrations if you have children or are pregnant.
Children are at greater risk (to any agent in water) because of their greater water consumption on
a per unit body weight basis.
Pregnant women may wish to reduce their arsenic exposures because arsenic has been found at
low levels in mother's milk and will cross the placenta, increasing exposures and risks for the fetus.
If your water has arsenic levels above 200 ppb, you should immediately stop drinking the water
until you can either obtain water from another source or install and maintain treatment.
Physical Characteristics
The three most common arsenic allotropes are metallic gray, yellow and black arsenic, with gray
being the most common. Gray arsenic (α-As, space group R3m No. 166) adopts a double-layered
structure consisting of many interlocked ruffled six-membered rings. Because of weak bonding
between the layers, gray arsenic is brittle and has a relatively low Mohs hardness of 3.5. Nearest
and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same
double-layer being slightly closer than the three atoms in the next. This relatively close packing
leads to a high density of 5.73 g/cm^3. Gray arsenic is a semimetal, but becomes a semiconductor
with a bandgap of 1.2–1.4 eV if amorphized. Yellow arsenic is soft and waxy, and somewhat similar
to tetraphosphorus (P 4 ). Both have four atoms arranged in a tetrahedral structure in which each
atom is bound to each of the other three atoms by a single bond. This unstable allotrope, being
molecular, is the most volatile, least dense and most toxic. Solid yellow arsenic is produced by
rapid cooling of arsenic vapor, As 4. It is rapidly transformed into the gray arsenic by light. The yellow
form has a density of 1.97 g/cm^3. Black arsenic is similar in structure to red phosphorus.
Isotopes
Naturally occurring arsenic is composed of one stable isotope,^75 As. As of 2003, at least 33
radioisotopes have also been synthesized, ranging in atomic mass from 60 to 92. The most stable
of these is^73 As with a half-life of 80.3 days. Isotopes that are lighter than the stable^75 As tend to
decay by β+ decay, and those that are heavier tend to decay by β- decay, with some exceptions.
At least 10 nuclear isomers have been described, ranging in atomic mass from 66 to 84. The most
stable of arsenic's isomers is 68mAs with a half-life of 111 seconds.
Chemistry
When heated in air, arsenic oxidizes to arsenic trioxide; the fumes from this reaction have an odor
resembling garlic. This odor can be detected on striking arsenide minerals such as arsenopyrite
with a hammer.