Environmental impact of the oil shale industry includes the consideration of issues such as land use, waste management, and water and air pollution caused by the extraction and processing of oil shale. Surface mining of oil shale deposits causes the usual environmental impacts of open-pit mining. In addition, the combustion and thermal processing generate waste material, which must be disposed of, and harmful atmospheric emissions, including carbon dioxide, a major greenhouse gas. Experimental in-situ conversion processes and carbon capture and storage technologies may reduce some of these concerns in future, but may raise others, such as the pollution of groundwater. Surface mining and retorting Land use and waste management Surface mining and in-situ processing requires extensive land use. Mining, processing, and waste disposal require land to be withdrawn from traditional uses, and therefore should avoid high density population areas. Oil shale mining reduces the original ecosystem diversity with habitats supporting a variety of plants and animals. After mining the land has to be reclaimed, process takes time and cannot necessarily re-establish the original biodiversity. The impact of sub-surface mining on the surroundings will be less than for open pit mines. However, sub-surface mining may also cause subsidence of the surface due to the collapse of mined-out area and abandoned stone drifts. Disposal of mining wastes, spent oil shale (including semi-coke) and combustion ashes needs additional land use. According to the study of the European Academies Science Advisory Council, after processing, the waste material occupies a greater volume than the material extracted, and therefore cannot be wholly disposed underground. According to this, production of a barrel of shale oil can generate up to 1.5 tonnes of semi-coke, which may occupy up to 25% greater volume than the original shale. This is not confirmed by the results of Estonia's oil shale industry. The mining and processing of about one billion tonnes of oil shale in Estonia has created about 360-370 million tonnes of solid waste, of which 90 million tonnes is a mining waste, 70–80 million tonnes is a semi-coke, and 200 million tonnes are combustion ashes. The waste material may consist of several pollutants including sulfates, heavy metals, and polycylic aromatic hydrocarbons (PAHs), some of which are toxic and carcinogenic. To avoid contamination of the groundwater, the solid waste from the thermal treatment process is disposed in an open dump (landfill or "heaps"), not underground where it could potentially reach clean