Apr 30, 2002

Arsenic in Drinking Water - Part 3

Occurrence of Arsenic in U.S. Waters

Editor’s Note: Part 1 of this series provided a
timeline for the development of a drinking water standard for arsenic. It also
summarized the political and public reactions to the U.S. EPA decision to delay
and withdraw the arsenic rule.


Part 2 dealt with human exposure and advances in knowledge
concerning human health effects of exposure to arsenic.


Only recently has a substantial amount of data become available on the concentrations of arsenic in United States drinking water supplies. Most of these data have been accumulated by the state regulatory agencies responsible for monitoring drinking waters. Since the arsenic standard has been 50 µg/L, some state agencies have recorded arsenic concentrations only in excess of that concentration. Others have been limited by the sensitivity of the analytical techniques and equipment used for the arsenic analysis. As a result, much of the available arsenic data are “below the limits of detection.”

The U.S. Environmental Protection Agency (EPA) has compiled
the available arsenic data from the 25 states that have conducted monitoring
programs. In turn, the Natural Resources Defense Council (NRDC) has utilized
the EPA data to calculate “best estimates” of the concentrations of
arsenic in finished drinking waters for communities in each of the 25 states.
In many instances, only a single analysis had been conducted.

Recognizing that these results are still tentative and very
limited, the graphs prepared for this review are an initial attempt to
visualize the impact of various arsenic standards on the proportion of
community water supplies that would be affected in a few of the states for
which arsenic data has been reported. The graphs present the array of estimated
arsenic concentrations, from lowest to highest, in the water systems or, in
some cases, in well waters surveyed.

These arrays allow visualization of the impact of state-

mandated arsenic limits of 3 µg/L, 5 µg/L and 10
µg/L on the proportion of water supplies potentially affected. For
example, assuming the data are representative and the NRDC best estimates are
an effective guide, about 40 percent of the 155 New Hampshire water supplies
surveyed would be affected by a 10 µg/L maximum contaminant level(MCL).

Alternately, the NRDC data indicate that fewer than 8 percent
of Missouri supplies, many of which are untreated groundwaters derived from
limestone strata, might be affected at the 10 µg/L level. A more recent
review of arsenic data for Missouri, conducted by the Missouri Department of
Natural Resources, indicates that few, if any, water supplies in the state will
require treatment to meet a 10 µg/L MCL.

It is important to note that the nationwide cost of arsenic
removal primarily will be a function of the number of water utilities that will
be required to take remedial action rather than the incremental cost of
removing arsenic to meet a lower limit such as 10 µg/L or even 3
µg/L. Currently, EPA estimates that 13 million people in the United
States drink water with more than 10 µg/L arsenic.

A comprehensive United States Geological Survey (USGS)
review (Water-Resources Investigations Report 99–4279) of a range of
existing water quality data indicates that most of the water utilities that are
affected by a lowered arsenic standard are in the western, midwestern and
northeastern states. USGS estimates that 1 percent of 54,000 U.S. public water
supplies currently exceed 50 µg/L; 3 percent exceed 20 µg/L; 8
percent exceed 10 µg/L; and 14 percent exceed 5 µg/L arsenic.

It is almost certain that more comprehensive arsenic data
will become widely available as communities and regulatory agencies attempt to
resolve MCL compliance issues. Only then will spatial and temporal variations
in arsenic concentrations, including seasonal effects and the effects of
antecedent rainfall, become evident. In many instances, source selection or
blending will permit utilities to avoid implementing treatment for the removal
of arsenic.


Part 4 of this series summarizes the methods available for
the removal of arsenic from drinking water sources.

About the author

John T. O’Connor, EngD, P.E., is CEO of H2O’C Engineering, Columbia, Mo. Phone 877-22-WATER; e-mail: [email protected].