Our research goal was to investigate geological and biological effects of acid mine drainage. For this purpose we have used different types of analytical methods, such as electric conductivity and pH measurements, optical microscopy (on thin sections, TS), X-Ray Powder Diffraction (XRD), electron microprobe analysis (EPMA) and Mössbauer Spectroscopy (MSP) and inductively coupled plasma optical emission spectroscopy (ICP-OES) for solved, fine-grained and coarser solid components of the rivulet water. The biological investigations consisted of ecotoxicological algal tests and biodiversity studies. The geological methods evidenced the presence of poorly crystallized Fe-phase(es), like akageneite, ferrihydrite and schwertmannite, with heavy metal content (Cu and Zn). Our work provides the first description of such geological materials from the study area. The acid mine drainage contaminates the creek excessively by high amounts of solved metals (e.g.: Cd, Zn, Cu, Pb and SO42-), while the low pH of the water causes extensive leaching of silicate minerals of the bottom rock and soil, resulting in a very high solved concentration of elements such as Al and Si, unusual in natural waters. The results of the phytotoxicological tests are in good agreement with the geochemical data, and reveal several inhibitory effects of the mine water leakage on the development of aquatic microalgae. The high concentrations of Al, Cd, Pb, As, Cr and Zn inhibit the growth, the metabolism, the nitrate up-take and are responsible for oxidative stress.