None of the existing
guidelines
or standards for
lead in drinking
water adequately specifies
the
method of sampling that should 'be used for
monitoring
the concentration
of lead at the
consumer's tap. A similar,
though less important,
question of sampling arises for other water
contaminants derived from pipe systems.
Project 4156 had the objective
of developing a
reliable,
practicable
and economical. method of
assessing the mean intake of metals derived from
plumbing systems by typical
inhabitants
of a
particular
house or water supply zone. This
objective
embraced several metals
emphasis was to be given to lead.
The work undertaken
comprised
but particular
the gathering
of
information
to provide the justification
for
recommendations.
Surveys were undertaken of
water-use patterns and of stagnation
curves for
lead, which have been separately
reported
elsewhere.
The present report welds this
information
together in order to demonstrate the
operating
characteristics
of different
types of
sample. The main method considered is that of
sampling after a fixed time of stagnation
(FTS).
FTS is recommended as the most suitable
method of
assessing the mean concentration
of lead in water
that would be consumed by members of a typical
household.
A stagnation
time of 30 minutes would
be a reasonable time to choose and would enable the
concentration
in the sample to be directly
interpreted
as an estimate
of the mean lead
concentration
in water used for drinking
and
cooking.
The reasons and assumptions on which this
advice is based are set out and discussed.