Detection of preferential infiltration pathways in sinkholes using joint inversion of self-potential and EM-34 conductivity data

The percolation of water in the ground is responsible for measurable electric potentials called self‐potentials. These potentials are influenced by the distribution of the electrical conductivity of the ground. Because sinkholes are associated both with self‐potential and electrical conductivity anomalies, a joint inversion of EM‐34 conductivity and self‐potential data is proposed as a way of delineating the location of these features. Self‐potential and EM conductivity data were obtained at a test site in Normandy (France) where sinkholes and crypto‐sinkholes are present over a karstic area in a chalk substratum overlain by clay‐with‐flint and loess covers. The presence of sinkholes and crypto‐sinkholes is associated with negative self‐potential anomalies with respect to a reference electrode located outside the area where the sinkholes are clustered. The sinkholes also have a conductivity signature identified by the EM‐34 conductivity data. We used the simulated‐annealing method, which is a global optimization technique, to invert jointly EM‐34 conductivity and self‐potential data. Self‐potential and electrical conductivity provide clear complementary information to determine the interface between the loess and clay‐with‐flint formations. The sinkholes and crypto‐sinkholes are marked by depressions in this interface, focusing the groundwater flow towards the aquifer contained in the chalk substratum.     

The origin,typology, spatial distribution and detrimental effects of the sinkholes developed in the alluvial evaporite karst of the Ebro River valley downstream of Zaragoza city (NE Spain)

Three types of sinkhole have been mapped in a 50 km² stretch of the Ebro River valley downstream of Zaragoza: large collapse sinkholes, large shallow subsidence depressions and small cover-collapse sinkholes. The sinkholes relate to the karstification of evaporitic bedrock that wedges out abruptly downstream, giving way to a shale substratum. Twenty-three collapse sinkholes, up to 50 m in diameter by 6 m deep, and commonly hosting saline ponds, have been identified in the floodplain. They have been attributed to the upward stoping of dissolutional cavities formed within the evaporitic bedrock by rising groundwater flows. Twenty-four large shallow subsidence depressions were mapped in the floodplain. These may reach 850 m in length and were formed by structurally controlled interstratal karstification of soluble beds (halite or glauberite? and gypsum) by rising groundwater flow and the progressive settlement of the overlying bedrock and overburden sediments. A total of 447 small cover-collapse, or dropout, sinkholes have been recognized in a perched alluvial level along the southern margin of the valley. These sinkholes result from the upward propagation of voids through the alluvial mantle caused by the downward migration of detrital sediments into dissolutional voids. The majority of these sinkholes, commonly 1·5–2 m in diameter, are induced by human activities. Over the karstic bedrock, there is a significant increase in sinkhole density downstream. This is interpreted as being a result of the evaporitic bedrock wedging out and the convergence of the groundwater flow lines in the karstic aquifer. The collapse sinkholes in this area, locally with a probability of occurrence higher than 140 sinkholes/km²/year, cause substantial damage to the linear infrastructures, buildings and agriculture, and they might eventually cause the loss of human lives. Copyright © 2006 John Wiley & Sons, Ltd.     

Probabilistic sinkhole modelling for hazard assessment

A method for quantitatively assessing sinkhole susceptibility (spatial probability) and hazard (spatio‐temporal probability) has been developed and independently tested in a 50 km² sector of the Ebro Valley evaporite karst. Three genetic types of sinkholes have been mapped in the floodplain and a terrace surface: 947 small cover‐collapse sinkholes (type 1, terrace), large collapse sinkholes (type 2, floodplain) and large subsidence depressions (type 3, floodplain). The type 1 sinkhole inventory includes two temporal populations: 447 sinkholes formed before 24 November 2005, and 500 between that date and 2 November 2006. Sinkhole susceptibility models have been elaborated analysing the statistical relationships between the sinkholes of the 2005 inventory and a set of potential conditioning factors. The independent evaluation (validation) of the susceptibility models by means of several strategies (random, sequentially excluded, and temporal) has allowed us to select the most significant variables for each sinkhole type and assess quantitatively the quality of models; which are reasonable for the three sinkhole types. Validation has also provided information on the contribution of specific variables and the effect of changing their accuracy to the prediction capability of models. Susceptibility models for type 3 sinkholes have been validated satisfactorily with the 2006 sinkhole inventory (temporal validation). The best susceptibility model has been transformed into a hazard map considering the frequency of sinkholes that occurred in each susceptibility class between 2005 and 2006, as well as their average size. The susceptibility and hazard models obtained could be used as an objective basis for the application of mitigation measures, either of preventive or corrective nature. Copyright © 2008 John Wiley & Sons, Ltd.     

Anthropogenic sinkholes in the territory of the city of Naples (Southern Italy)

About 190 anthropogenic sinkholes occurred within the territory of the city of Naples (Southern Italy) between 1915 and 2010.In the study area, the genesis of sinkholes can be ascribed to two major factors, often strongly interacting with each other: the existence of a complex network of underground man-made cavities, and the inadequacy of the sewage disposal system.Rainfall has been identified as the main triggering factor combined with anthropogenic activity. Based on such predisposing factors, in addition to the geological setting, a susceptibility map of the territory has been realised.     

DInSAR measurements of ground deformation by sinkholes,mining subsidence,and landslides,Ebro River,Spain

Differential Interferometric Synthetic Aperture Radar (DInSAR) has been applied to detect and measure ground deformation in a stretch of the Ebro River valley (Spain) excavated in salt‐bearing evaporites. The capability of the Small Baseline Subset (SBAS) DInSAR technique to detect ground displacement is analyzed comparing the DInSAR results with the available geomorphological information. The deformation map derived from 27 European Remote Sensing (ERS) satellite images covering more than five years provides sub‐centimeter displacement measurements in zones coinciding with known active sinkholes and landslides. Moreover the map provides the first account of mining subsidence in the area. The measured deformation rates reach 1·68 cm/y for the sinkholes, 0·80 cm/y for the landslides and 1·45 cm/y for the area affected by mining subsidence. The SBAS DInSAR technique provided deformation measurements in a small proportion (5–10%) of the known active sinkholes and landslides. This limitation is mainly due to the lack of coherence in agricultural areas, the spatial resolution of the deformation map (pixel size of 90 m), and the parallelism between the ERS satellite line of sight and the linear escarpment on which most of the landslides occur. Despite this, the interferometric technique provides valuable data that complement traditional geomorphological studies including the quantification of the deformation phenomena, the identification of mining subsidence otherwise only recognizable by geodetic methods, and the detection of creep deformation which might correspond to premonitory indicators of catastrophic sinkholes and landslides capable of causing the loss of lives. Detailed DInSAR studies combined with field data would be required to improve the analysis of each deformation area. Copyright © 2009 John Wiley & Sons, Ltd.     

Detection of sinkholes using 2D electrical resistivity imaging

Sinkholes in dolomitic areas are notoriously difficult geophysical targets, and selecting an appropriate geophysical solution is not straightforward. Electrical resistivity imaging, or tomography (RESTOM) is well suited to mapping sinkholes because of the ability of the technique for detecting resistive features and discriminating subtle resistivity variations. RESTOM surveys were conducted at two sinkhole sites near Pretoria, South Africa. The survey areas are located in the dolomites of the Lyttelton Formation, which forms part of the Malamani Subgroup and Chuniespoort Group of the Transvaal Supergroup. The survey results suggest that RESTOM is an ideal geophysical tool to aid in the detection and monitoring of sinkholes and other subsurface cavities.     

Quantifying potential recharge in mantled sinkholes using ERT

Potential recharge through thick soils in mantled sinkholes was quantified using differential electrical resistivity tomography (ERT). Conversion of time series two-dimensional (2D) ERT profiles into 2D volumetric water content profiles using a numerically optimized form of Archie's law allowed us to monitor temporal changes in water content in soil profiles up to 9 m in depth. Combining Penman-Monteith daily potential evapotranspiration (PET) and daily precipitation data with potential recharge calculations for three sinkhole transects indicates that potential recharge occurred only during brief intervals over the study period and ranged from 19% to 31% of cumulative precipitation. Spatial analysis of ERT-derived water content showed that infiltration occurred both on sinkhole flanks and in sinkhole bottoms. Results also demonstrate that mantled sinkholes can act as regions of both rapid and slow recharge. Rapid recharge is likely the result of flow through macropores (such as root casts and thin gravel layers), while slow recharge is the result of unsaturated flow through fine-grained sediments. In addition to developing a new method for quantifying potential recharge at the field scale in unsaturated conditions, we show that mantled sinkholes are an important component of storage in a karst system.     

Using δ18O and δ2H to Detect Hydraulic Connection Between a Sinkhole Lake and a First-Magnitude Spring

Oxygen and hydrogen isotopes were used in this study to detect a hydraulic connection between a sinkhole lake and a karst spring. In karst areas, surface water that flows to a lake can drain through sinkholes in the lakebed to the underlying aquifer, and then flows in karst conduits and through aquifer matrix. At the study site located in northwest Florida, USA, Lake Miccosukee immediately drains into two sinkholes. Results from a dye tracing experiment indicate that lake water discharges at Natural Bridge Spring, a first-magnitude spring 32 km downgradient from the lake. By collecting weekly water samples from the lake, the spring, and a groundwater well 10 m away from the lake during the dry period between October 2019 and January 2020, it was found that, when rainfall effects on isotopic signature in spring water are removed, increased isotope ratios of spring water can be explained by mixing of heavy-isotope-enriched lake water into groundwater, indicating hydraulic connection between the lake and the spring. Such a detection of hydraulic connection at the scale of tens of kilometers and for a first-magnitude spring has not been previously reported in the literature. Based on the isotope ratio data, it was estimated that, during the study period, about 8.5% the spring discharge was the lake water that drained into the lake sinkholes.     

TEM study of the geoelectrical structure and groundwater salinity of the Nahal Hever sinkhole site, Dead Sea shore, Israel

Since the 1990s a large number of sinkholes have appeared in the Dead Sea (DS) coastal area. Sinkhole development was triggered by the lowering of the DS level. In the literature the relationship between the sinkholes and the DS level is explained by intrusion of relatively fresh water into the aquifer thereby dramatically accelerating the salt dissolution with creation of subsurface caverns, which in turn cause sinkholes. The main goal of our project was detection and localization of relatively fresh groundwater. During our study we used the transient electromagnetic method (TEM) to measure the electrical resistivity of the subsurface. As a test site we selected Nahal Hever South which is typical for the DS coast. Our results show that resistivity of the shallow subsurface reflects its vertical and lateral structure, e.g., its main hydrogeological elements explain the inter-relations between geology, hydrogeology, and sinkholes. The TEM method has allowed detailed differentiation of layers (clay, salt, etc.) in the subsurface based on their bulk resistivity. The 10 m-thick salt layer composed of idiomorphic crystals of halite deposited during the earlier Holocene period was extrapolated from borehole HS-2 through the study area. It was found that in Nahal Hever the typical value of the bulk resistivity of clay saturated with the DS brine varies between 0.2 and 0.3 Ωm, whereas saturated gravel and sandy sediments are characterized by resistivity between 0.4 and 0.6 Ωm. The high water salinity of the aquifer (enveloping the salt layer) expressed in terms of resistivity is also an important characterization of the sinkhole development mechanism. The electrical resistivity of the aquifer in the vicinity of the salt unit and its western border did not exceed 1 Ωm (in most cases aquifer resistivity was 0.2-0.6 Ωm) proving that, in accordance with existing criteria, the pores of the alluvial sediments are filled with highly mineralized DS brine. However, we suggest that the criterion of the aquifer resistivity responsible for the salt dissolution should be decreased from 1 Ωm to 0.6 Ωm corresponding to the chloride concentration of approximately 100 g/l (the chloride saturation condition reaches 224 g/l in the northern DS basin and 280 g/l in the southern one).Based on TEM results we can reliably conclude that in 2005, when most of sinkholes had appeared at the surface, salt was located within a very low resistivity environment inside sediments saturated with DS brine. Intrusion of relatively fresh groundwater into the aquifer through the 600 × 600 m²area affected by sinkholes has not been observed.     

Evaporite dissolution and development of karst features on the rolling plains of the texas panhandle

More than 400 dolines, consisting of collapse sinkholes and closed depressions, occur in Hall and Briscoe Counties, Texas. Of these, at least 36 sinkholes and two depressions formed between 1940 and 1972. Collapse sinks are typically circular and range up to 100 m in diameter and 15 m in depth. Closed depressions are irregularly-shaped, internally-drained depressions up to 2.4 km in length. Surface fractures up to 1.5 km in length are associated with certain closed depressions. Karst features are forming as a result of intrastratal dissolution of Upper Permian evaporites, primarily halite and secondarily gypsum. Salt dissolution and overburden collapse are evident from geophysical log cross-sections and occur at depths ranging from 195 to 275 m. Dissolution is progressing down dip and mean dissolution rates for salt beds in the Hall County area range from 0.29 to 7.73 cm/yr. The valley of the Prairie Dog Town Fork of the Red River is an area of ground-water discharge for flow systems moving eastward from topographically high areas of the Rolling Plains and from the eastern rim of the High Plains. Halite beds in contact with these waters undergo dissolution. Salt springs and brine seeps, some associated with sinkholes and depressions, occur in the discharge area and indicate that dissolution is an ongoing process.     

基于修正SWAT模型的岩溶地区非点源污染模拟初探——以横港河流域为例

岩溶流域含水系统的主要特征之一是连通地表的落水洞等垂直管道将近水平的地下暗河联系起来,降水及其形成的地表径流可以通过这些管道迅速地灌入地下河系,从而改变了水及其所携带的非点源污染物质在垂直与水平方向的传输速度与数量,使岩溶流域内地表-地下之间的物质交换与传输过程变得比较复杂;应用广泛的SWAT模型在模拟岩溶地区的水文、水质时会存在一些不足与局限.为此,本文针对岩溶水系统特征,引入落水洞、伏流、暗河的水文过程以及主要营养盐的输移过程,修正SWAT模型原有的水文循环过程及相关算法,改变其只适用于松散均匀介质流域非点源污染模拟的单一特征,并研究建立适合于岩溶流域的非点源污染模型和相应的模拟方法.选取横港河流域岩溶地区作为非点源污染的对象,应用修正后的模型通过控制性的模拟方法和敏感性性分析,定量评估落水洞、伏流、暗河等岩溶特征对氮、磷等主要非点源污染物质输移的影响及其带来的时空效应,并进一步探讨落水洞、伏流、暗河等对地表-地下水文与营养盐的交互作用及转换机理.结果表明,岩溶特征对流域的氮、磷负荷有增加作用,其中总磷的增加明显大于总氮的增加,总磷和总氮的增量分别为0.86%和2.12%;植被岩溶指数的增加会导致流域可溶性磷、有机磷的产出量增加,有机氮、地表产流中硝酸氮和沉积磷的产出量则居其次,落水洞改变了降雨的产流方式,增加了落水洞所在流域的有机磷和有机氮的产出,其增量变化在0~0.7和0~0.3 kg/hm²之间.     

Catchment‐scale conceptual modelling of water and solute transport in the dual flow system of the karst critical zone

Hydrological and hydrochemical processes in the critical zone of karst environments are controlled by the fracture‐conduit network. Modelling hydrological and hydrochemical dynamics in such heterogeneous hydrogeological settings remains a research challenge. In this study, water and solute transport in the dual flow system of the karst critical zone were investigated in a 73.5‐km² catchment in southwest China. We developed a dual reservoir conceptual run‐off model combined with an autoregressive and moving average model with algorithms to assess dissolution rates in the “fast flow” and “slow flow” systems. This model was applied to 3 catchments with typical karst critical zone architectures, to show how flow exchange between fracture and conduit networks changes in relation to catchment storage dynamics. The flux of bidirectional water and solute exchange between the fissure and conduit system increases from the headwaters to the outfall due to the large area of the developed conduits and low hydraulic gradient in the lower catchment. Rainfall amounts have a significant influence on partitioning the relative proportions of flow and solutes derived from different sources reaching the underground outlet. The effect of rainfall on catchment function is modulated by the structure of the karst critical zone (e.g., epikarst and sinkholes). Thin epikarst and well‐developed sinkholes in the headwaters divert more surface water (younger water) into the underground channel network, leading to a higher fraction of rainfall recharge into the fast flow system and total outflow. Also, the contribution of carbonate weathering to mass export is also higher in the headwaters due to the infiltration of younger water with low solute concentrations through sinkholes.     

Geoelectric structure of the Ein Gedi sinkhole occurrence site at the Dead Sea shore in Israel

Hazardous sinkholes started to appear in alluvial fans and unconsolidated sediments along the western Dead Sea coast in 1990. Since then hundreds of sinkholes have appeared from north to south along the shoreline. The Electrical Resistivity Tomography (ERT) method was used to achieve a better understanding of the subsurface geoelectric structure at the sinkhole development sites, taking into account that electric parameters (such as resistivity or conductivity) are very sensitive to formation properties and their variations in time. Fifteen image lines were surveyed at the Ein Gedi area during a period of active sinkhole development (in 2001–2002) over an area of 300 × 550 m². Resistivity cross-sections and maps were constructed from 2-D linear surveys. The process of sinkhole formation in the surveyed area is located in a strip 50–70 m wide and 300–500 m long, extending approximately in a north–south direction. The sinkholes are arranged along a tortuous line within this strip. On resistivity maps and sections this U-shaped zone appears as an alternation of high resistivity anomalies of 350–1000 Ωm (at sinkhole group locations) with narrow background resistivity zones of 50–100 Ωm. The large size of resistivity anomalies (250 × 300 m²), which are considerably greater than those of the sinkholes, form one of the features of the sinkhole sites in the Ein Gedi area. The anomalies continue down to the water table or even deeper (maximum of 25–35 m depth). A low resistivity layer of 1–8 Ωm underlies them. The combined analysis of the image results and other geophysical data shows that high resistivity anomalies are associated with the decompaction of the soil mass at the sinkhole development sites and surrounding areas. Recent studies have shown that sinkholes in the Ein Gedi area are developing along the salt western edge located at a depth of 50 m. The subsurface high resistivity anomaly conforms to the sinkhole line (and salt boundary). They are presumably located above the great dissolution caverns at the salt edge. The heterogeneity of the resistivity structure within the high resistivity anomaly (seen in both lateral and vertical planes) confirms that a disintegration of internal formation structure takes place. Away from the sinkhole sites the subsurface resistivity distribution is homogeneous.     

Evaluation of trenching,ground penetrating radar (GPR) and electrical resistivity tomography (ERT) for sinkhole characterization

This paper explores the suitability and advantages of combining the trenching technique with geophysical surveys [ground penetrating radar (GPR) and electrical resistivity tomography (ERT)] for sinkhole characterization in a mantled karst area. The approach is applied to two active sinkholes concealed by anthropogenic deposits and formed by contrasting subsidence mechanisms; collapse and sagging. The ERT section acquired across the collapse sinkhole images the clayey fill of the depression as an obvious low resistivity area, showing the approximate location of the sinkhole edges. Spatially dense GPR surveys provide information on the position of the boundaries of the concealed subsidence structures and their three‐dimensional (3D) internal geometry, revealing the dominant subsidence mechanism. We illustrate the impact of several factors on the quality of the GPR data such as sinkhole size, nominal frequency of the antennas, antenna shielding, and the presence of backfilled excavations and above‐surface objects. Trenches provided detailed information on the subsurface structure of the sinkhole, subsidence magnitude, partitioning of the strain, and the position of the sinkhole edges, especially when they are deep enough and excavated across the central sector and perpendicular to the boundaries. The stratigraphic and structural relationships observed in the trench were then used to infer the spatial evolution of the sinkholes (e.g. enlargement), their kinematic behavior (episodic versus progressive), and to differentiate discrete subsidence events and their associated magnitude. Numerical dates were used to estimate average subsidence rates and the recurrence of subsidence events. Such integrated data sets may be used as an objective basis to forecast the future behavior of potentially damaging sinkholes and to assess the associated hazard and risk. Copyright © 2013 John Wiley & Sons, Ltd.     

Sinkholes Due to Groundwater Withdrawal in Tazerbo Wellfield,SE Libya

The desert of eastern Libya forms one of the most arid regions of the Sahara. The Great Man‐Made River Project (GMRP) was established. It transports millions of cubic meters of water a day from desert wellfields to the coastal cities, where over 80% of the population lives. The Tazerbo Wellfield is one of the wellfields designed within the GMRP, delivering water to the eastern coast of Libya through an underground pipe network. Tazerbo Wellfield consists of 108 production wells; each well was designed to pump 100 L/s. The planned total groundwater withdrawal from all wells is 1 million m³/d. The deep sandstone aquifer (Nubian sandstone) is covered by a thick mudstone‐siltstone aquitard and is being heavily pumped. The aquifer and fine‐grained sediments of the aquitard may be compacted resulting in land subsidence as a result of high exploitation. Local sinkholes have developed in the area of Tazerbo since the start of the pumping from the wellfield in 2004. These sinkholes have been caused mainly by lowering of the piezometric heads due to the withdrawal of groundwater. In this study, a hydrogeological investigation is presented about the effect of large groundwater pumping from the Nubian sandstone aquifer in Tazerbo Wellfield, SE Libya, based on physical parameters for 108 production wells and 23 observation wells.     

Characterizing the variability of transit time distributions and young water fractions in karst catchments using flux tracking

Hydrological and biogeochemical processes in karst environments are strongly controlled by heterogeneous fracture-conduit networks. Quantifying the spatio-temporal variability of water transit time and young water fractions in such heterogeneous hydrogeological systems is fundamental to linking discharge and water quality dynamics in the karst critical zone. We used a tracer-aided conceptual hydrological model to track the fate of each hour of rain input individually. Using this approach, the variability of transit time distributions and young water fraction were estimated in the main landscape units in a karst catchment of Chenqi in Guizhou Province, Southwest China. The model predicted that the mean young water (i.e., <~2 months old) fraction of ground conduit flow is 0.31. Marked seasonal variabilities in water storage and hydrological connectivity between the conduit network and fractured matrix, as well as between hillslopes and topographic depression, drive the dynamics of young water fraction and travel time distributions in each landscape unit. Especially, the strong hydrological connectivity between the land surface and underground conduits caused by the direct infiltration through large fractures and sinkholes, leads the drastic increasement in young water fraction of runoff after heavy rain. Even though the contribution of young water to runoff is greater, the strong mixing and drainage of small fractures accelerate the old water release during high flows during the wet season. It is notable that the young water may sometimes be the most contaminated component contributing to the underground conduit network in karst catchments, because of the direct transfer of contaminants from the ground surface with rain water via large fractures and sinkholes.     

Ground fissures within the Main Ethiopian Rift: Tectonic,lithological and piping controls

Ground fissures, especially if they open due to a sudden collapse of the surface, is a serious risk for populated areas. Their common occurrence in unconsolidated sediments of the Main Ethiopian Rift was found to be mostly a result of piping. The fissures start by piping in linear sub-horizontal underground voids, which often propagate upwards resulting in ceiling collapse and formation of deep and long ground fissures with vertical walls. In the southern and central Main Ethiopian Rift the fissures pose a serious risk to infrastructure and settlements. The ground fissures are often linear (up to several kilometres long and often tens of metres deep) and accompanied by sinkholes (along the length). A detailed field mapping of the geological (rock composition, orientation and character of lithological boundaries, primary fabrics and brittle structures) and geomorphological features (especially a length, width and depth of fissures, sinkholes and gullies) followed by in situ seismic anisotropy measurements and a laboratory determination of the geomechanical properties of volcanoclastic deposits was carried out to investigate the ground fissures' origin. The conditions and factors leading to the formation of the ground fissures have been linked to: (a) the presence of regional normal faults and the associated extensional joints and (b) the alternation of lithological units with contrasting hydraulic permeability. The latter corresponds to a sequence of less permeable hard rocks (e.g., rhyolitic ignimbrites) overlain by heterogeneous, soft and permeable, unconsolidated volcaniclastic deposits with a low amount of clay (less than 10%). The ground fissures' occurrence has shown affiliation to areas which have a significantly high seismic anisotropy (more than 20% at the study sites), which can be used as a proxy to map out high risk areas prone to piping and ground fissure formation.     

Comparison of chemical hydrogeology of the carbonate peninsulas of Florida and Yucatan

Aquifers of the peninsulas of Florida and northern Yucatan are Tertiary marine carbonate formations showing many lithologic and faunal similarities. In addition, the tropical to subtropical climates of the two areas are similar, each having annual rainfall of about 1000 to 1500 mm.

Despite similarities in these fundamental controls, contrasts in the hydrologic and geochemical systems are numerous and striking. For example, Florida has many rivers; Yucatan has none. Maximum thickness of fresh ground water in Florida is about 700 meters; in the Yucatan it is less than 70 meters. In Florida the gradient of the potentiometric surface averages about 1 meter per kilometer; in the Yucatan it is exceedingly low, averaging about 0.02 meter per kilometer. In Florida the chemical character of water changes systematically downgradient, owing to solution of minerals of the aquifer and corresponding increases in total dissolved solids, sulfate, calcium, and Mg-Ca ratio; in the Yucatan no downgradient change exists, and dominant processes controlling the chemical character of the water are solution of minerals and simple mixing of the fresh water and the body of salt water that underlies the peninsula at shallow depth.

Hydrologic and chemical differences are caused in part by the lower altitude of the Yucatan plain. More important, however, these differences are due to the lack of an upper confining bed in Yucatan that is hydrologically equivalent to the Hawthorn Formation of Florida. The Hawthorn cover prevents recharge and confines the artesian water except where it is punctured by sinkholes, but sands and other unconsolidated sediments fill sinkholes and cavities and impede circulation. In the Yucatan the permeability of the entire section is so enormous that rainfall immediately infiltrates to the water table and then moves laterally to discharge areas along the coasts.     

Salt karst and tectonics: sinkholes development along tension cracks between parallel strike‐slip faults,Dead Sea,Jordan

This work deals with the tectonic interpretation of an alignment of more than 300 sinkholes stretching along the Jordanian coast of the Dead Sea, Ghor Al Haditha area. Its dimensions are 6 km long with a width of 600 m. Sinkholes appeared during the last decades as a consequence of the very rapid lowering of the lake level. The linear shape was inferred from ground collapse inventories carried out between 1991 and 2008. The lineament is replaced and analyzed in its structural setting at regional and local scales. Its direction (N 24° E) is sub‐parallel to the ones displayed by many focal mechanisms, especially the one associated with the earthquake of the 23 April, 1979 (mb = 5·1; N 20° E ± 5°), which is representative of all focal mechanisms calculated on a fault plane compatible with the general direction of the Jordan‐Dead Sea Transform fault system for the east coast of the Dead Sea area. The alignment of sinkholes is constituted by 13 minor linear segments separated by as many empty spaces. Four minor linear units present an en‐echelon arrangement from which one can deduce the presence of a local extensional stress field. In this context, the sinkhole locations provide information of subsurface discontinuities interpreted as hidden fractures. In a close future, such results could support the work of decision‐makers and engineers in the projected development of the area. Copyright © 2009 John Wiley & Sons, Ltd.     

Deep seated fault-related volcanogenic H2S as the key agent of high sinkhole concentration areas

Karst collapse sinkholes (KCS) are a peculiar karst morphology resulting from the collapse of the topographic surface caused by subsurface karstification. In the Southern Apennines these phenomena are not randomly distributed but concentrate in several zones, named High Sinkhole Concentration Areas (HSCA), showing peculiar geological, structural and hydrogeological conditions. Gas vents and mineral springs occur widely in these HSCA, and are often stricty related to the KCS. Starting from four representative areas, the aim of this study is to ascertain if there is a genetic link between peculiar mineralogical vs geochemical features of densely fractured/altered rocky masses in KCS, gas vents/springs occurrences and diffuse hypogenic karstification. By means of a multidisciplinary approach (geological/geomorphological, mineralogical and geochemical) we selected and analysed four different kinds of samples related to KCS, gas vent and spring occurrences: (i) altered to deeply altered limestones sampled in the sinkholes; (ii) unaltered limestones sampled in close proximity to sinkhole areas; (iii) gypsum-rich crusts/patinae, precipitated near to gas vents; (iv) gypsum- and calcite-rich precipitates occurring at springs or inside caves. Among neoformed non-carbonate minerals, gypsum is virtually ubiquitous, halides (fluorite, halite, sylvite) also occur in small to trace amounts. The mineralogical assemblages of the different samples show similarities and may also be compatible with hypogenic speleogenesis and with a process of alteration of the carbonate bedrock by means of uprising mineralizing fluids along structural discontinuities. Stable isotopic compositions (S, O) display strong variability in δ³⁴S and δ¹⁸O for sulfate in the different areas, but a deep-seated sulfur source can be hypothesized for many of the studied KCS-related samples. This study has important implications for the relationships between areas of high concentration of sinkholes, regional fault systems, mechanical characteristics of rocks and the high seismicity typical of these areas of the Southern Apennines. © 2018 John Wiley & Sons, Ltd.