New insights into the underground hydrology of the eastern Po Plain (northern Italy)

A series of studies concerning the surficial aquifers of the Emilia alluvial plain showed the existence of hydrochemical anomalies, suggesting a widespread vertical circulation of the various fluid phases hosted in the thick Pleisto-Olocene succession. The occurrence of liquid and gaseous fluid vents is easily detectable from their brackish character (with an electric conductivity, Ec, up to 18,000/22,000 �S/cm, at 25�C) and is related to various fracture systems that are difficult to study in detail. According to detailed stratigraphical and structural data (AGIP�s seismic sections and industrial wells) the main source of the vertical salt uprisings is the Middle Pleistocene marine �Clinoforms Lithosome� (�Subsyntheme Marine Quaternary 3�), showing a wide distribution in the Po Plain-Adriatic basin. The occurrence of very low Ec freshwaters (250 �S/cm) sometimes accompanied by thermal anomalies (up to 7�C lower than the average annual temperature) is an interesting problem, hard to solve. The hydrochemical and thermal anomalies recorded in the underground waters were often very fast and penecontemporaneous over 30-40 km distances. These anomalies are genetically related to the earthquakes of the inner Po Plain zone and adjacent North-Apennine Chain. The active tectonic control played by the frontal zone of the North-Apennine Chain is fundamental for the explanation of the recorded anomalies. Ground collapse and disarrangements recorded and monitored over time are related to the loss in volume, underneath the surface due to the sediment compaction processes and other causes; also reminiscent effects of the deep gas-hydrates liquefaction, occurring in the distant past, cannot be excluded. Paroxysmal (explosive) fluid (methane) episodes and embankment underseepage (�suffosion�) are not consistent with the recorded data.