Human-induced hydrogeological changes and sinkholes in the coastal gypsum karst of Lesina Marina area (Foggia Province, Italy)

In 1930, a canal was excavated to connect the Lesina Lagoon with the Adriatic Sea, modifying the track of the previous existing Acquarotta canal. The 1100 m long stretch of the canal situated next to the coast exposed unknown highly cavernous gypsum bedrock underlying a loose sandy cover. During the last two decades, a large number of cover collapse and cover suffosion sinkholes have formed along two bands situated next to the canal, impinging the adjacent Lesina Marina residential area. The area affected by subsidence has increased exponentially from 1999 to 2009. The tight spatial correlation between the sinkholes and the canal reveals that the subsidence phenomenon has been induced by the local hydrogeological changes caused by the canal in the coastal evaporite aquifer: (1) Local lowering of the average water table. (2) Deflection of the groundwater flow lines towards the canal and increase in flow velocity. (3) Amplification of the groundwater level oscillations, largely controlled by the tidal regime. (4) Local reversals in the groundwater flow, changing the canal temporarily from effluent to influent. These changes in the hydrogeological functioning of the system have favoured both internal erosion and karstification processes. Hydrochemical evidence reveals that gypsum dissolution is a currently active process favoured by fresh water and sea water mixing and cation-exchange processes. Most likely, the partial replacement of a concrete lining in the canal by pervious gabions in 1993 provided more adequate conditions for the evacuation of the sediments filling the karst conduits, accelerating internal erosion and sinkhole development.

Research Highlights
► Palaeo-karst reactivation ensuing change of hydrological conditions.
► Faster groundwater flow after a canal digging for tidal effects and reduced path.
► Increase of submerged gypsum mass solution and suffosion fillings erosion.
► Localization of sinkholes generation near the new canal cutting the gypsum mass.
► Enhancement of gypsum solubility in coastal aquifer mixing zone.