Changes on groundwater flow and hydrochemistry of the Gran Sasso carbonate aquifer after 2009 L'Aquila earthquake

The earthquake that struck L'Aquila on April 6 2009 (Mw 6.3) directly affected the Gran Sasso aquifer. Co-seismic and post-seismic changes in groundwater discharge and in hydrochemistry, possibly induced by the earthquake, were observed. Spot and monitoring measurements of the spring discharge, of water table level and of the main physico-chemical parameters of spring waters (T, pH, electrical conductivity, major ions and Rn-222) were thus carried out to determine the effects of the L'Aquila earthquake on groundwater at regional and local scale, to be compared with available data collected since the 1990s. Short- and mid-term effects have been observed in the groundwater flow at recharge and discharge areas. The following short-term effects have been observed: i) the sudden disappearance of some springs located along the surface trace of the Paganica Fault; ii) an immediate discharge increase of the Gran Sasso highway tunnel drainages (+20%) and of other springs (+10%); iii) a progressive increase of the water table elevation (+1m) at the boundary of the Gran Sasso aquifer during the following month; iv) a sudden lowering of the water table in the recharge area. Similar post-seismic effects have been recorded in the following 20 months, when spring discharge and water table remain higher than the pre-seismic ones in discharge zones. A conceptual model of the earthquake consequences on the Gran Sasso aquifer is proposed herein. The short-term hydrologic effects registered immediately after the mainshock have been caused by a pore pressure increase related to elasto-static aquifer deformation. Apart from the contribution of seasonal recharge observed in 2009-10, mid-term effects observed in the 20 months following the mainshock suggest that there was a change in groundwater hydrodynamics. Supplementary groundwater that flows toward aquifer boundaries and springs in discharge areas reflects a possible increase in hydraulic conductivity in the recharge area. This increase is probably related to fracture cleaning and/or dilatancy. Additional monitoring including hydrochemical data allows a refinement of the proposed model. The outcomes of the hydrochemical spot sampling of the pre-seismic (2001-2007), post-seismic (April 2009) and after-seismic (July and September 2009, may 2010) periods, give the following insights: i) post-seismic groundwater of Tempera spring group was more mineralised and richer in Rn-222 than the pre-seismic one; ii) transient changes in pH and calcite saturation index involve the whole aquifer, moving from Tempera springs and spreading from the recharge to discharge areas, causing changes in groundwater hydrochemistry; iii) post-seismic gradual return to previous hydrochemical equilibrium. Both hydrodynamic and hydrochemical observation converge towards a non-permanent increase of the bulk hydraulic conductivity in the aquifer portion close to the Paganica Fault (recharge area and local discharge zone). This fact has caused a lowering of the water table and of the calcite saturation index in recharge areas and simultaneously an increase of water table and flow rate in discharge zones. Complete interpretation of both quantitative and hydrochemical data allows to determine the long-term consequences of this earthquake on the groundwater flow of the Gran Sasso carbonate massif.