PRESSURE AND MASS TRANSFER PROCESSES IN A SANDY AQUIFER LATERALLY RECHARGED BY THE PO RIVER (LOW PADANA PLAIN, NORTHERN ITALY):OBSERVATIONS BASED UPON SURFACE AND GROUND WATER CONTINUOUS MONITORING

Alluvial low lands (distal alluvial plains, deltaic regions) generally do not play an important role as regional aquifers due to such features as the wide occurrence of fine-grained sediments or the groundwater salinization for the proximity of the sea; an important exception are the sandy aquifer-systems connected to Pleistocenic/ Holocenic deposition and lateral recharge of big rivers. Such a system is occurring in the low Padana plain (northern Italy) along the Po river, with huge fine to medium sandy lithosomes, either shaped as shallow depth narrow and sinuous Holocenic paleo-river beds or as an areally continuous layer of medium sands (from about 10 to 30 m b.g.l.) deposited in the Upper Pleistocene by fluvio-glacial braided channels; in some areas the two types of lithosomes are amalgamating between each other. This ¸SPo sandy aquifer systemˇT is very interesting from an hydrogeologic stand-point having a regional occurrence, being recharged by the Po river during the high stage periods and showing an interesting potential of natural attenuation of surface waters percolating through the sandy porous medium. Continuous monitoring both of groundwater parameters (hydraulic head, specific electrical conductivity, groundwater temperature) and of Po river stage and rainfall gives the opportunity to put in evidence the lateral recharge processes, to evaluate pressure and mass transfer waves travel times, in relation to flood events of the river, and, as a consequence, to parametrize the aquifer and, through modelling, to evaluate the potential for implementing public groudwater supply abstraction schemes. The presented case study is relative to the sector of this Po-aquifer complex located in Emilia-Romagna region in the north-western corner of the province of Ferrara. Since July 2002 a monitoring programme, still going on, has been implemented, with the recording of the following parameters: hydraulic head in three piezometers, drilled, for about 30 m, down to the main sandy Upper Pleistocenic aquifer, generally confined or semi-confined and only locally unconfined when amalgamated with the upper sandy Holocenic paleo-river beds; river stage in the Po; rainfall. Water levels are recorded each 30 minutes, rainfall on an hourly basis. Piezometers are located along a transect perpendicular to the PO river, respectively at a distance of 2400,3600 and 5200 m. At a distance of 2400 m from the Po river the signal of pressure transfert, defined by the delay-time between the peaks of the river stage and of the groundwater head, is between 36 and 3 hours, with the lowest delay times associated to the presence of a different peak immediately preceding the considered one; the amplitude of head increase is low, between 0.1 and 0.2 m. Lower sensibility down to the complete fading away of the signal occurs between 2400 and 5200 m distance. Particularly interesting was the response of the system to the flood event of the Po between November 15th and December 29th 2002; the peak of the flood (actually the first one of two neighbor peaks) was transferred to 2400, 3600,5400 m away from the river with a delay time, respectively of 10, 14 and 14 days and with a head increase of, respectively, 1.7, 0.94 and 0.7 m; in the following period low order river peaks gave a delay time of pressure transfert of few hours. ItŠs distinctive the behaviour of the electrical conductivity signal at 3600 m distance: after the arriving of a fresh wate signal at 21 days delay (0.4 mS/cm) due to the mass transfer via the kinematic porosity, we had, after 4 days more, the arrival of a mineralized water signal (0.7 mS/cm), due to the squeezing by the recharge wave of the pore water immobilized in the microporosity.