SGEM 2019. 19th International Multidisciplinary Scientific GeoConference. Volume 19. Ecology, Economics, Education and Legislation : conference proceedings. Albena, Bulgaria, 30 June - 6 July 2019. Issue 5.1 Ecology and Environmental Protection. 1. vyd. Sofia : STEF 92 Technology, 2019. 832 s. ISBN 978-619-7408-84-3. ISSN 1314-2704.