The key to utilising combustion residues as well as other mineral wastes in civil works and keeping them out of landfills is the assessment of the environmental impact in the permitting procedures. This was one of the main reasons for creating the Ash Programme: a method of computing impact had to be developed and numerical values for the properties have to be fed into the method. This work was performed in conjunction with environmental authorities, but this does not mean that these authorities reach the same conclusion in their assessment.

Initially, concepts similar to the “end of waste” presently discussed in the EU were considered, but as an assessment still has to be done, the method of the Swedish Environmental Protection Agency for assessing the impact of contami­nated soil on health and the environment was chosen as a starting point and adapted to the pilot cases (Bendz et al. 2006):

• A non-surfaced road in a forest, with a comparatively thin layer of ash

• A surfaced road with MSWI bottom ash in the subbase, with a comparatively thick ash layer

The purpose of the assessment is to define the boundary between a low risk level and a not low risk level (in legalese terms, as “no risk” does not exist and “high risk” will not be allowed). Below the boundary, a simplified procedure could be defended, for example only giving notice to the environmental authorities. Above the boundary, a full permitting procedure would of course be necessary, with a detailed analysis of the expected local environmental impact.

All mechanisms for dispersal from the body of the road and for human exposure were described in the model. The model is conservative: a plausible worst-case
scenario is assumed, yielding rather large safety margins. For example, the most exposed person is assumed to live all his or her life within 20 m from the road, 30% of this person’s intake of vegetables is home-grown close to the road (hardly washing them) and when the road is disused after some 60 years, it is used as a recreation area by adults and children, assuming 40 windy days per year and person.

The result of these first computations is that dispersal of dust yields the dom­inating health risk (Bendz et al. 2006). Criteria based only on leaching to ground­water yield significantly larger limit values for, for example, heavy metals in the combustion residues. In the simulations, even with MSWI bottom ash, leaching from several roads yields insignificant increases in heavy metal content in the recipients of two catchment areas (Wik 2009).

To put this result in perspective, the composition and leaching properties of most combustion residues are such that, if correctly used, these residues present a “low risk” to health and the environment. The only ones that cannot satisfy the upper limit values for low risk are APC residues from MSWI and fly ash or APC residues from combustion of impregnated wood.

The low risk limit values for some of the trace elements are shown in Table 11.7. Two references are given in Table 11.7 for the sake of comparison. The first reference is a set of limit values derived by the Swedish Environmental Protection Agency after the Ash Programme had published its proposal (Swedish Environ­mental Protection Agency 2010). The purpose of this set is to define a boundary below which the user of a waste material does not even need to give notice of his or her use of these materials. These values are substantially lower as they represent the 90th percentile of concentrations in soil. The boundary defined by the Swedish Environmental Protection Agency and that proposed by Bendz et al. (2006) and later updated (Bendz et al. 2009) are not the same. The second reference is the recommended limit values for ash spread to forest soils as nutrient compensation (Swedish Forest Agency 2008).

Note the comparatively low concentration found for arsenic, 15 mg/kg dry substances, the immediate reason for which is that arsenic is genotoxic. One should keep in mind though that the results in Table 11.7 are the first results based on conservative models, conservative assumptions and uncertain data. For the time being, it is recommended that ash with an arsenic content in excess of this value should not be left on the surface when the road is abandoned.