Metal contamination and retention of the former mining site Schwarzwand (Salzburg,Austria)

The Schwarzwand is a unique hygric, Cu-contaminated habitat formed by mining activities from the 16th to 18th century. Today, a large spoil heap and several creeks fed by Cu-rich mine drainage are present. The vegetation of the Schwarzwand differs clearly from the surrounding subalpine forests. It is by no means impoverished but rather a hotspot of biodiversity. Interestingly, most of the Cu precipitates within the Schwarzwand and the creeks leave the Schwarzwand virtually clean. This study maps the distribution of Cu within the Schwarzwand and within selected vascular plants, moss and microorganisms and correlates them with water and soil chemistry in order to identify the sinks of Cu and to elucidate the remarkable capability of the Schwarzwand for natural attenuation.Two types of water could be distinguished, one acidic precipitating limonite with a constant Cu content of about 0.6 mg L⁻¹, and one circumneutral, which decreases far more rapidly in Cu content than would be expected due to chemical considerations. A dense microbial mat covering most of the bed of the circumneutral creeks could be identified as the main sink. It consists of the cyanobacterium Phormidium sp. and retains Cu both by adsorption to mucilaginous sheaths and by precipitation as secondary minerals such as sampleite. Layers of dead biofilm can be found covered by a few centimetres of soil at the banks of the circumneutral creeks; the extremely high concentration and the low solubility of Cu in this soil indicates permanent immobilisation of the metals. High concentrations of Cu were also found in mosses of the family Bryaceae which, however, play a negligible role for the metal retention of the habitat due to their low biomass.The retention of Cu within the Schwarzwand is a remarkable example of the sustainable self-cleaning of a contaminated habitat which takes place without any human intervention. The artificial establishment of microbial communities similar to the Schwarzwand could result in cheap and sustainable strategies for the remediation of suitable metal-contaminated waters.