This chapter defines and discusses the leaching and bioleaching process in the context of mining and soil remediation. It focuses on the prevention, mitigation, and technological utilization of the leaching process and on the treatment of the acidic leachate.

The topic is discussed in three sections:

(i) The first section gives a general overview of the leaching process, on the history and principles of acid mine drainage treatment, on the technical solution and applicability of active and passive treatment systems such as oxic and anoxic ponds, channels, wetlands, and several other engineered reactor-based technologies and permeable reactive barriers. (ii) The second subchapter focuses on sulfidic ore bioleaching. The biochemistry of the process is presented and the activities of iron- and sulfur-oxidizing microorganisms are clarified. The biotechno-logical solutions in the monitoring, prevention, and treatment of the acidic leachate is in focus.

(iii) The third subchapter is also closely related to the topic of natural leaching and its consequences. A case study is introduced where sulfide-ore mining produced AMD and ARD. Ongoing mine closure, and complex site rehabilitation is aimed to overcome environmental damage and prevent any future damage. The study presents the engineering solution applied in the closure of the Gyöngyösoroszi metal mine in Hungary. It is currently under restoration and provides an opportunity to study and manage all the impacts of leaching on water, sediment, and soil. Leaching in the underground mine, leaching from various mine wastes and waste rocks resulted in hundreds of relatively well-defined contaminated sites in addition to the 15 km long watershed-scale diffuse pollution.