How Dangerous Are Contaminated Military Sites?

Soils at contaminated military sites contain numerous chemicals of ecotoxicological concern. A case study shows that an assessment based solely on chemical analyses falls short and cannot adequately predict toxicity. Additional bioassays and bioavailability measurements are necessary to comprehensively assess soil quality.

Military activities and accidents involving ammunition or explosives leave residues in the environment worldwide, endangering soil, water bodies and groundwater. According to the Federal Department of Defence, Civil Protection and Sport, there are more than 2,000 sites in Switzerland contaminated by the army. In addition to metals from bullet casings and detonators, such sites contain explosive residues, perchlorate and polycyclic aromatic hydrocarbons (PAHs), which are produced by incomplete combustion.

Numerous toxic substances

Elevated concentrations of heavy metals can have toxic effects on numerous soil organisms, for example by reducing plant growth and the reproduction of springtails. PAHs are also a cause for concern, as they are persistent in the soil, have genotoxic effects and accumulate in organisms. Ecotoxicological guideline values apply to metals and PAHs, and they are regulated at both national and international levels. In Switzerland, soils are regulated under three different ordinances: the Ordinance on Soil Contamination (VBBo), the Ordinance on the Remediation of Contaminated Sites (Ordinance on Contaminated Sites, AltlV) and the Ordinance on the Prevention and Disposal of Waste (VVEA), which set different limit values for these classes of substances. Despite their widespread use, there are no such limit values for explosives. Perchlorate is mainly regulated in relation to drinking water and can act as an endocrine disruptor.

But how dangerous are soils contaminated with these substances for soil organisms and plants? And do the chemical limit values adequately reflect the actual ecological risk? Researchers at the Ecotox Centre have investigated excavated material from a Swiss military site as a case study, using an integrated approach combining chemical and ecotoxicological methods to answer these questions. The study was commissioned by the Federal Office for Defence Procurement (armasuisse).

More than just exceedances of limit values

First, the soil samples were extracted and chemically analysed. The analyses revealed high levels of metal contamination: concentrations of copper, zinc, lead, cadmium and antimony were significantly above Swiss and international guideline values. Various PAHs were also detected at critical concentrations. Under current legislation – such as the Swiss Ordinance on Soil Contamination – such exceedances of guideline values often lead to remediation measures such as excavation and landfill. “These guideline values are generally based on the assessment of individual substances under standardised laboratory conditions,” explains project leader Mathieu Renaud. “They take into account neither complex mixtures of pollutants nor the actual bioavailability in the field.”

To realistically assess the ecological relevance of the contamination, the researchers investigated the biological effects on various soil organisms using several standardised bioassays. The tests involved springtails, enchytraeids, plants such as garden cress and onions, and nitrifying bacteria. The contaminated excavated material was gradually diluted with uncontaminated reference soil to identify thresholds of effect.

Hardly any effects despite high contamination

“Given the high overall levels of metals and PAHs, we had expected significant ecotoxicological effects,” says project leader Mathieu Renaud. “But the results surprised us: the reproduction of springtails and enchytraeids was inhibited only in the undiluted excavated material, and significant negative effects on plants were also observed only in the undiluted material.” The nitrifying bacteria reacted somewhat more sensitively. “However, part of their inhibition could also be due to indirect effects of dilution and not necessarily to chemical toxicity,” says Renaud. “This reveals a clear discrepancy between the total concentration of pollutants and the observed ecotoxicological effects.”

Bioavailability as the key

The most likely explanation for this discrepancy is the low bioavailability of the pollutants. In aged contaminated soils, metals and organic compounds are often strongly bound to soil particles. Through sorption, incorporation into organic matter or ageing, the biologically available fraction is significantly reduced. “Particularly with metals, it is not the total content but the bioavailable fraction that determines the actual toxicity,” says Mathieu Renaud. “It is also known for PAHs that strong binding to organic matter limits their uptake by organisms.” However, bioavailability was not directly determined in the study – it was indirectly inferred from the low biological effect. Future work should therefore incorporate suitable methods for determining the bioavailable concentration of the substances, such as measuring internal concentrations in organisms.

Another finding was the high variability between replicates. The variation in the chemical measurements points to local ‘hotspots’ with elevated metal or explosive content – presumably due to fragments of munitions remnants. Such small-scale differences can lead to significant effects in individual samples without providing a consistent overall picture.

Implications for risk assessment and remediation

“The results show that it does not make sense to use chemical limit values alone as the basis for deciding on remediation measures,” says Mathieu Renaud. “These do not provide a reliable indication of the ecological impact. Ecotoxicological tests are an important complement, as they can capture mixtures of pollutants and ageing processes.” Particularly in the case of large-scale military contamination sites, where complete excavation is difficult to implement, an integrated approach can help to make realistic and site-specific decisions. A sustainable assessment of military contaminated sites should combine chemical analysis, bioavailability measurements and standardised ecotoxicity tests. Only in this way can the actual ecological risk be properly assessed – and a distinction made between theoretical risk and real environmental impact.

Photo: VBS/DDPS - Nicola Pitaro