The overall load of Zn along the Naracauli stream increases with water discharge ranging from a few kilograms per day upstream to about 35 kg/day in the Zn silicate precipitation area (Wanty et al. ). Apart from the difference in Zn concentration, water chemistry along the river always has the same Ca-Mg-SO42− character (Medas et al. ), while pH slightly changes and others parameters do not show any significant change. Water discharge shows a significant increase along the river from few liters per second upstream to tens of liters per second downstream. This hydrologic condition has continued for at least 40 years, and should imply some microbiological adaptation to heavy metals among the biota present.
X-ray diffraction pattern of the biomineral collected at station 2,840 m, along the Naracauli stream. Qz quartz, Ph phyllosicate are the only crystalline phases detected
Taking into account chemical variations in the part of the Naracauli stream where biologically mediated Zn silicate precipitates, from upstream (station 2,660 m) to downstream (station 3,040 m) a decrease in dissolved Zn concentration up to 20 % due to the bioprecipitation process was observed. The decrease in Si is about 9 % and falls close to the uncertainty due to analytical error.
The Ingurtosu area is characterized by Palaeozoic rocks forming a landscape of moderate relief and gentle slope. The Naracauli stream flows from Punta Tintillonis at 552 m a.s.l. westward to the Mediterranean Sea. The stream first receives water from an adit, then from some tributaries, which increase its water flow. The flow regime is typically torrential, with large flow variations between wet (from October to April) and dry (from May to September) seasons (Medas et al. ).