Human-induced hydrogeological changes and sinkholes in the coastal gypsum karst of Lesina Marina area (Foggia Province, Italy)
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.
Introduction
The formation of subsidence sinkholes results from the coeval or concatenated activity of two types of processes; subsurface dissolution and downward movement of the overlying material due to a lack of basal support (internal erosion and gravitational deformation). The main subsidence mechanisms include (Gutiérrez et al., 2008a, Gutiérrez et al., 2008b, Gutiérrez, 2010): collapse, which involves the upward propagation of cavities by brittle deformation and breakdown of rocks and/or soils; suffosion, by which incoherent cover material settles progressively as a result of its downward migration through voids in the underlying bedrock; sagging, which is the gradual ductile bending of rocks and/or mantling soils overlying a karstification zone. In carbonate karst areas, the effects of dissolution processes acting over human and engineering timescales are generally negligible (Beck, 2005) due to the relatively low solubility of limestones and dolomites. In contrast, karstification of evaporites over short periods of time may have a significant detrimental effect on ground stability; the equilibrium solubilities of gypsum and halite at 25 °C are 2.4 and 360 g/l, respectively. Sinkhole-forming subsidence processes may progress at high rates regardless of the nature of the bedrock, and they may be related to either active or pre-existing karstic cavities.
Cover collapse and cover suffosion sinkholes account for the vast majority of the sinkhole damage, since these are the sinkhole types with higher probabilities of occurrence. Moreover, a significant proportion of the damaging sinkholes is induced or accelerated by human activity (Beck, 2005, Waltham et al., 2005, Gutiérrez, 2010). In most cases, the human-induced increase in the sinkhole hazard is related to deliberated or unintentional anthropogenic changes in the hydrology. Surface and groundwater flows are the main driving agents in sinkhole formation. Some of the hydrological alterations that may trigger or accelerate sinkhole formation include (Gutiérrez, 2010, Cooper and Gutiérrez, in press): (1) lowering the water table. This hydrogeological change may be caused by water abstraction, de-watering karst aquifers for mining operations, declining the water-level in lakes or intercepting phreatic conduits by underground excavations. In Ukraine, dewatering of gypsum karst for sulphur mining has increased substantially the rate of gypsum dissolution and has favoured the occurrence of sinkholes within the area affected by the cones of depression (Sprynskyy et al., 2009). The rapid decline of the water level in the Dead Sea, has propitiated the circulation of aggressive water in contact with salt deposits, resulting in the continuous generation of sinkholes in the Israeli and Jordanian coasts of this lake (Frumkin and Raz, 2001, Yechieli et al., 2006). In 2006 the Nanjing Gypsum mine in China broke into a phreatic cavity, causing the flooding of the mine, a sharp drop in the local piezometric level and ground subsidence that severely damaged roads and buildings (Wang et al., 2008). (2) Rising the water table. This type of hydrological change is particularly dramatic when a reservoir is impounded for the first time. The unnaturally high hydraulic gradients induced by water reservoirs may cause the flushing out of the sediments filling karst conduits and produce the rapid dissolutional enlargement of discontinuities. Cooper and Gutiérrez (in press) report on numerous dams built on gypsiferous formations that have suffered from leakage and stability problems. (3) Increasing the water input. This hydrological alteration may be related to a wide variety of human activities, including irrigation, leakages from utilities (pipes, canals, and tanks), runoff concentration or diversion, injection of fluids. Gutiérrez et al. (2007) report a probability of occurrence of cover collapse sinkholes, largely induced by sheet-flooding irrigation, of 45–50 sinkholes/km2/year in a sector of the Ebro Valley evaporite karst. According to Jassim et al. (1997), accelerated sinkhole activity in Mosul city, Iraq, is partly due to drainage from septic tanks infiltrating into the gypsum karst. The most dramatic examples of catastrophic collapse sinkholes are related to the solution mining of salt formation, a practice that involves injecting fresh water and pumping brine with the consequent generation of large cavities (Johnson, 1997). In the city of Tuzla, Bosnia and Herzegovina, solution subsidence has caused subsidence at rates as high as 40 cm/year. Here, 2000 buildings have been demolished and 1500 people relocated (Mancini et al., 2008). (4) Permafrost thawing. This process leads to the incorporation of liquid and aggressive water to the system and a significant reduction in the mechanical strength of the soils, previously “cemented” by ice. The construction of the Bratsk Dam in a gypsum karst area in Siberia has caused the progressive thawing of the permafrost and the formation of numerous sinkholes (Eraso et al., 1995). (5) Disrupting the groundwater flow. The underground water paths may be altered by local changes in the base level through the excavation of underground mines and drainage trenches. In the Cardona salt diapir, NE Spain, the interception of a phreatic conduit by a salt mine gallery caused the flooding of the mine and karst conduit system by fresh water from a nearby river, resulting in massive dissolution, widespread subsidence and the abandonment of the mine (Lucha et al., 2008). In this paper we analyze the impact of the hydrological changes caused by the excavation of the new Acquarotta canal in a mantled gypsum karst on the development of the sinkholes that affect the Lesina Marina residential area and its vicinity (Fig. 1). The main peculiarities of this cases study include: (1) The gypsum karst has been developed in a coastal area. Consequently its formation has been influenced by eustatic sea level changes and fresh water and sea water mixing processes. (2) The Acquarotta Canal connects the Lesina Lagoon with the Adriatic Sea (Fig. 1). The water level in the canal has a complex oscillating behaviour and the lag time and magnitude of the response of the aquifer to these changes depends on the distance to the oscillating base level. (3) The canal was excavated through unappreciated well-developed gypsum karst covered by a very loose and mobile sandy cover.
Section snippets
Geological and geomorphological setting
From the geotectonic point of view, the study area forms part of the Apulia region, which is the emerged sector of the Adria Plate. The latter, in turn, constitutes the foreland of both the Apennine and Dinaric orogens. Previous to the excavation of the northern reach of the Acquarotta Canal in 1930, the only bedrock outcrop in the area was located in the Pietre Nere Point (Fig. 2), a rocky headland situated at the mouth of this artificial canal. This exposure, consisting of basic igneous rocks
Hydrology of the Lesina Lagoon and anthropogenic changes
The hydrology of the Lesina Lagoon is controlled by multiple natural and anthropogenic factors. At the present-time, the lagoon is mostly fed by (a) 22 small streams with a drainage area of around 530 km2, (b) 6 main karstic springs situated in the NW sector of the carbonate Gargano Promontory, (c) direct precipitation, and (d) sea water inflow through the Acquarotta and Schiapparo canals. The maximum seasonal discharge of the springs has been estimated at 1130 l/s (Cotecchia and Magri, 1966).
Methodology
The investigation carried out was aimed at identifying and assessing the factors that control the development of the sinkholes impinging Lesina Marina residential area. The original working hypothesis was that the active subsidence phenomenon could be linked to hydrogeological changes caused by the excavation of the artificial Acquarotta Canal between the sea and the lagoon. This hypothesis was supported by the clear spatial association between the sinkholes and the canal, which was excavated
Hydrogeology
The sinkholes that started to occur in Lesina Marina area in 1990 result from the reactivation of a previously unknown mantled gypsum karst. The excavation of the Acquarotta Canal in the evaporitic bedrock, connecting the sea and the lagoon, has caused decisive alterations in the hydrological behaviour of the karst aquifer. Changes in the groundwater flow rate and direction and the continuous renewal of unsaturated water in contact with gypsum have activated internal erosion processes in
Hydrochemistry
In September 1999, 33 groundwater samples were collected at different depths from 18 monitoring wells. Moreover, in order to have saline end-members, one water sample was taken from the open sea and other two from the Acquarotta Canal, at its southern end and at the seaside mouth (Fig. 16, Table 1). Groundwater TDS (Total Dissolved Solids) ranges from 0.3 g/l to 36.6 g/l, increasing generally along the flow lines towards the canal. The lowest value was obtained in borehole S25 located in the
Sinkholes
Subsidence activity has affected the Acquarotta Canal soon after its construction in 1930. To our knowledge, the first account of a sinkhole occurrence in the area adjacent to the canal corresponds to an official report dating back to 1990. At the present-time, the great majority of the sinkholes are located within the canal and in two relatively narrow bands situated on its flanks (Fig. 4, Fig. 20, Fig. 21). The sinkholes tend to form clusters and alignments with a prevalent NW–SE orientation (
Discussion and final considerations
The Acquarotta canal, built in 1930 to connect the Lesina Lagoon with the Adriatic Sea, was excavated in intensively karstified gypsum bedrock mantled by loose sandy deposits. According to borehole data, karstic cavities represent around 30% of the evaporite bedrock and reach up to 13 m in height. The conduit network extends well below the sea level and is partially filled by sandy and clayey unconsolidated deposits. Most of the sinkholes in Lesina Marina area have formed in the last two
Acknowledgements
The work carried out by Francisco Gutiérrez has been supported by the project CGL2010-16775 (Spanish Ministry of Science and Education and FEDER). The authors thank Dr. F. Javier Gracia (University of Cádiz) for his review of the sections related to the geology and geomorphology of the study area.
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