Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 51

Moreover, the general belief that measuring gives a more reliable value than a
calculation is often true, but not always. In some cases, measuring a representative
value is extremely difficult for technical reasons and due to spatial and/or temporal
variation. Moreover, measurements are strongly related to the present situation and
may not represent the long-term conditions for which it is stated, for example, that
'the site is safe to live on'. The risk assessor needs to find a smart balance between
the determination of a more reliable measured input parameter that might be less
representative for the long-term risk, or a less reliable calculated input parameter
that does represent long-term site conditions. Obviously, the risk assessor also has to
account for the additional costs of measurements when formulating the involvement
of measurements.
An example of such a dilemma is the Human Health Risk Assessment for a well-
maintained (manuring, liming) vegetable garden. The risk assessor has to decide
if the Risk Assessment benefits from measured concentrations in the vegetables
present at the site, which are relatively accurate for the present situation, but may
give underestimations for future situations in which another owner neglects soil lim-
ing. Alternatively, the risk assessor could perform relatively unreliable calculations
of the concentrations of a representative combination of vegetables of choice, on
the basis of total soil concentrations and soil properties belonging to an appropriate,
that is, average or neglected liming conditions.
Generally speaking, measurements of direct input parameters are most valuable
for Risk Assessments that relate to, let's say, the first few years (maybe one to three
years), while measurements of basic input parameters are useful for the subsequent
few years, see Fig. 1.9 . As illustrated in this figure, supportive input parameters
often represent time spans up to decades.
This picture only gives a general insight into the time spans for which measure-
ments are useful, mainly for the purpose of illustrating the importance of the time
frame when deciding on measurements. Direct measurements for Human Health
Risk Assessment, such as measurements of the cadmium levels in blood, are often

Fig. 1.9 Significance of
measurements as a function
of the time span for which the
Risk Assessment applies
( straight line ). Indication of
the time span, for which
different type of
measurements are useful
( dark shaded arches )

5 years 50 years

Time span (years)