ALLUVIAL SINKHOLES OVER GYPSUM IN THE EBRO BASIN (SPAIN) - GENESIS AND ENVIRONMENTAL-IMPACT

The Ebro Basin is located in the northeast of Spain. Its central part is filled with gypsum and carbonate deposits of Neogene age. Quaternar y sediments overlie the Tertiary layers. Alluvial sinkholes have devel oped in this region, causing high economic losses. From the study of a erial photographs, from the compilation of historical cases of collaps e and from data on the lithological characteristics of the Quaternary and Tertiary deposits (their structure, morphology, hydrological chara cteristics, etc.), natural and human factors in sinkhole development h ave been evaluated. Among the natural variables that may be considered are the lithology of the Tertiary deposits, the thickness and geomech anical behaviour of the Quaternary cover, the morphology of the contac t between the Tertiary and Quaternary deposits, fracturing, content of sulphates in groundwater and the depth and seasonal variations of the groundwater level. The irrigation techniques of farmers and the prese nce of irrigation canals increase the level of the groundwater. Pumpin g activities may also be important locally.