Biogeochemical study of spent gunshot fate in the environment: Influence of aggressive bacterial strain

The hunting of waterfowl may contribute significantly to environmental contamination through the leaching of highly toxic elements (As, Pb, Sb) from spent gunshot deposited on hunting grounds. It is therefore clearly necessary to develop a biogeochemical protocol that might decipher the fate of spent gunshot in the environment. In that context, we present a study that follows the laboratory simulation approach and discusses the methodical pros and cons of the protocol. We performed an experimentally induced 60-day shot bio-weathering under three conditions: 1) neutral control conditions (WAT; ultrapure water), 2) control conditions (MED; sterile water with growth medium), and 3) biotic conditions (BAC; presence of Acidithiobacillus thiooxidans). We measured the elements leached from the shot (based on ICP-MS analysis), solution pH, and shot surface changes (using SEM-EDS observations). Lastly, we checked the efficiency with which plants bio-accumulate elements from MED and BAC leachates. We observed severe gunshot weathering and leaching of all the elements (reaching the maximum for Pb - 1120 μg/g in BAC treatment, which was 0.11% of the Pb burden of the shot). Among other elements studied (As, Cu, Sb, and Zn) the highest relative leaching (also in BAC) was observed for Zn and Cu, 83.6% and 61.0% respectively, a phenomenon that may be linked to the non-homogenous distribution of elements in the gunshot due to the potential existence of a Cu-Zn coating or a consequence of different solidification temperatures of the elements, which are important in the casting of gunshot. BAC treatment was also associated with the greatest surface alterations (seen by SEM-EDS), which were probably stimulated by the bacteria lowering of the pH. Plant response to metals released from shot was comparable for studied Brasica juncea and Festuca rubra, suggesting that similar bio-accumulation trends may take place in a field-weathering scenario.