Ecological Risk Assessment of Diffuse and Local Soil Contamination Using Species Sensitivity Distributions - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 657

The techniques that can be of help to discriminate natural background concentra-
tions from man-made enrichments is a subject beyond the scope of this chapter. See
Chapter 1 by Swartjes, (this topic) or Vijver et al. ( 2008 ) or Spijker et al. ( 2008 )for
more detailed information. However, handling knowledge on natural background
concentrations in combination with SSDs can be done with the so-called added-risk
approach, which enables the determination of the added risk due to anthropogenic
enrichment, given a local natural background concentration. For technical details
see Struijs et al. ( 1997 ).
14.10.3 The Influence of Soil Type and Soil Properties
The differences in soil types and soil properties among sites influence sorption
of contaminants to the soil matrix, and - hence - bioavailability (see Chapter 16
by Hodson et al., this topic). When acknowledging this phenomenon, regulatory
agencies may need a system for soil-type dependent correction of soil quality stan-
dards, so that the standards calculated for a soil with a certain total contaminant
concentration are different when soil composition (and thus sorption strengths) vary.
A system that addresses soil type differences has been in use for a long time in
the Netherlands (VROM 1987 ), using soil-type and compound specific formulae. In
practice, a formal national soil quality standard value of (e.g.) 10mg/kg dw for con-
taminant X can, by applying these formulae, be recalculated into soil-type specific
values like 8.3mg/kg dw for soils with a high sand content, and 12.9mg/kg dw for
soils with a high organic matter content. The latter (soil-type corrected) standards
are then used in practical assessments like a Hazard Index evaluation. In practice,
this means that the first soil would be considered “slightly polluted” when contain-
ing 8.4mg/kg dw , and the latter “non-polluted” when containing 12.8mg/kg dw .
The corrections known so far have been described as formulae that are based
on the observation that different soils contain different background levels of met-
als, which likely is a consequence of different geochemical sorption properties.
However, the formulae have been implicitly used as if the corrections made by these
formulae also imply quantitatively similar differences in biological availability (see
Chapter 16 by Hodson et al., this topic). The latter would imply differences in actual
exposures and impacts of the same total concentration. Though the quantitative
corrections work out in this way (lower sorption implies a lower local soil qual-
ity standard), the system was not built as a bioavailability correction system. The
system of formulae currently used in the Netherlands, developed in the 1980 s, is
rather out-of-date. A modernization could result from recent scientific investigations
(Spijker et al. 2008 ).
14.10.4 When Soil Concentrations are Very High
It is relevant to consider the level of contamination at a contaminated site, and
the context of the assessment, before one decides to use NOECs, EC50s, or other
measures of effect as SSD input data.