Risk Assessment for Safe Drinking Water Supplies - Global Drinking Water Management and Conservation - page 130

6.3.5 Risk Assessment Application to Water Treatment Plants

It is clear that risk assessment methods should be integrated into the entire water
production process from source water protection to the delivery of potable water to
the consumer. In this section, we consider the application of risk at water treatment
plants. We relate risk to regulation and show that regulatory policy implies a
measure of risk aversion, but that still leaves some residual risk. In addition, there
must be some benchmark to establish the acceptable level of risk. As in other public
policy domains, there is no such thing as being completely risk free.
Most water treatment processes in developed countries are required to consider
risk, either implicitly or explicitly, in the regulations, such as 3-log reduction of
cryptosporidium (99.9 percent reduction) and 4-log reduction of viruses
(99.99 percent reduction). In addition to this regulation, the US now goes further in
the direction of taking risk into account. For the US, under the 1996 Amendments
to the Safe Drinking Water Act (

300j-18(a)(1)), the U.S.
EPA must consider susceptible subpopulations in its health risk assessments. The
amendments mention speci

§ 1458(a)(1), 42 U.S.C. §

c groups, including young children, the elderly,
pregnant women, and people who are immunocompromised by disease or treatment
for diseases. The concept of susceptibility to adverse health outcomes from envi-
ronmental exposures can be extended to other groups as well. But the statute
expressly limits this undertaking to subpopulations that are

“ identi ed and char-

acterized. first report to Congress on susceptible subpopulations under the

Safe Drinking Water Act, in December 2000, EPA concluded that because

” In its

“ genetic

in is unclear to what extent

individuals with heightened sensitivities due to genetic factors meet the statutory
criterion of

uences are complex and still poorly understood, ” it “

' subpopulations that can be identi ed and characterized '” (USEPA

2000 ). While directing the EPA to study and report to Congress its findings on

susceptible subpopulations, the Safe Drinking Water Act is silent on whether and
how EPA should apply these data on susceptible subpopulations in regulatory
decisions for drinking water contaminants. However, the new amendments make it
clear that MCLs for new contaminants are to be based on risk assessment and a
cost

bene t analysis, i.e. the risk-based bene ts must outweigh the costs of stip-

-

ulating an MCL for a candidate contaminant.
It should be apparent that some degree of risk averseness is implied in the
requirement of 3-log reduction of cryptosporidium and 4-log reduction of all
viruses. But this is the

granted to the water treatment plant; it does not by
itself minimize the risk. The residual risk depends on the quality of source water; if
the source water is very poor, the 3 and 4-log credits may not be suf

“ credit ”

cient to

eliminate all risks of infection.
Of course the requirement of a multibarrier approach, i.e. the removal of
pathogens at multiple stages between the source waters and the consumer, can be
indicative of risk avoidance, but it may not be enough. Regulation reduces the
probability of infection but it is not eliminated; depending on the quality of the
source water, the treatment plant may need additional log credits.