Shooting ranges - how can we manage contaminating trace elements?
Switzerland is a highly militarised nation with an annual defence budget of US$ 2.5 billion. The maintenance of the small-arms capability requires readily available shooting ranges. An estimated 400 tons of Pb enter Swiss soils each year at some 2000 shooting ranges scattered throughout the country. Lead (Pb) and to a lesser extent antimony (Sb) are common Trace Elements (TE)s in areas adjacent to the bunds of shooting ranges. On average, new bullets and pellets consist of over 90% Pb, 1-7% Sb, <2% arsenic (As) and <0.5% nickel (Ni). Low-quality lead may also contain Bi and Ag. Zinc and Cu covers improve the ballistic properties of high velocity rounds. Tracer and incendiary bullets contain Sr, Ba and Zn. The total concentrations of these toxic TEs in the soils increase over time upon the deposition of more bullets. Once in the soil, the bullets and bullet fragments gradually oxidise through the weathering actions of air, water, organic acids and microbial activity.
All trace elements associated with shooting ranges are toxic to humans and other organisms when present at high concentrations. TEs in shooting ranges cause a general decrease in soil biological activity. Vegetation has failed to establish on the bunds of most disused shooting ranges, indicating that the trace elements contained therein are phytotoxic. Such localised environmental effects aside, the risk posed by trace elements in shooting ranges greatly increases if they become mobile. The migration of toxic trace elements to the food chain or to drinking water can reduce ecosystem functioning on a large scale and may present a human health risk. Although insoluble trace elements in shooting ranges may migrate with surface runoff and erosion, the soluble trace elements have the greatest mobility and present the largest environmental risk.
How can we manage the soil of disused shooting ranges? Conceivably, one could remove the stop-butts and reprocess the soil to recover the lead. However, it is impractical to remove of all the contaminated soil, or cap entire shooting ranges with non-contaminated material. Nor is there any technology that would permit the safe in situ decontamination of such sites. The only viable option is to use vegetation and soil conditioners (phytomanagement) to reduce the mobility of the contaminating trace elements, thereby reducing the exposure pathways to humans and ecosystems.
Since vegetation will eventually cover most disused shooting ranges, land managers could select plants that do not accumulate toxic TEs in their aboveground portions and reduce the leaching of these TEs into receiving waters. Intuitively, deep-rooted phreatophytic tree species may seem like a logical choice, since they would maximise evapotranspiration from the site, thus minimising drainage. However, roots of such deep-rooted trees may create preferential flow pathways that actually exacerbate TE leaching. We aim to improve the phyto-management of contaminated sites by elucidating the mechanisms of TE-root interactions.
Similar to other polluted sites, TEs in shooting ranges occur heterogeneously. The interactions of roots with hotspots are of overriding importance on the TE fate. Root avoidance of hotspots will reduce TE mobility because of reduced plant-uptake. Conversely, when roots penetrate a hotspot, the plant may accumulate higher amounts of the TE and the TEs may have a preferential flow conduit into receiving waters.
Our research focused on unravelling the effect of roots on TEs in shooting ranges with a view to developing land management strategies that minimise the negative environmental effects of TEs in shooting range soils.
