Quantitative sinkhole hazard assessment. A case study from the Ebro Valley evaporite alluvial karst (NE Spain)

Quantitative sinkhole hazard assessments in karst areas allow calculation of the potential sinkhole risk and the performance of cost-benefit analyses. These estimations are of practical interest for planning, engineering, and insurance purposes. The sinkhole hazard assessments should include two components: the probability of occurrence of sinkholes (sinkholes/km² year) and the severity of the sinkholes, which mainly refers to the subsidence mechanisms (progressive passive bending or catastrophic collapse) and the size of the sinkholes at the time of formation; a critical engineering design parameter. This requires the compilation of an exhaustive database on recent sinkholes, including information on the: (1) location, (2) chronology (precise date or age range), (3) size, and (4) subsidence mechanisms and rate. This work presents a hazard assessment from an alluvial evaporite karst area (0.81 km²) located in the periphery of the city of Zaragoza (Ebro River valley, NE Spain). Five sinkholes and four locations with features attributable to karstic subsidence where identified in an initial investigation phase providing a preliminary probability of occurrence of 0.14 sinkholes/km² year (11.34% in annual probability). A trenching program conducted in a subsequent investigation phase allowed us to rule out the four probable sinkholes, reducing the probability of occurrence to 0.079 sinkholes/km² year (6.4% in annual probability). The information on the severity indicates that collapse sinkholes 10–15 m in diameter may occur in the area. A detailed study of the deposits and deformational structures exposed by trenching in one of the sinkholes allowed us to infer a modern collapse sinkhole approximately 12 m in diameter and with a vertical throw of 8 m. This collapse structure is superimposed on a subsidence sinkhole around 80 m across that records at least 1.7 m of synsedimentary subsidence. Trenching, in combination with dating techniques, is proposed as a useful methodology to elucidate the origin of depressions with uncertain diagnosis and to gather practical information with predictive utility about particular sinkholes in alluvial karst settings: precise location, subsidence mechanisms and magnitude, and timing and rate of the subsidence episodes.     

Formation mechanism of large sinkhole collapses in Laibin,Guangxi, China

On June 3, 2010, a series of karst sinkholes occurred at Jili village surrounded by Gui-Bei highway, Wu-Ping highway and Nan-Liu High-Speed Railway in Laibin, Guangxi, China. The straight-line distances from an large sinkhole pit, 85 m in diameter and 38 m in depth, to the above mainlines are 200, 600 and 500 m, respectively. Several investigation methods including geophysical technology, borehole and well drilling, groundwater elevation survey and hydrochemistry analysis of groundwater were used to study the formation mechanisms of these sinkholes. Based on the results, the spatial distribution of the Jili underground river was confirmed with a strike of SN along the middle Carboniferous limestone bedrock and the Quaternary deposits controlled the sinkhole formation. In addition, both historical sinkhole events and analysis of the groundwater–air pressure monitoring data installed in the underlying karst conduit system indicate that sinkholes in this area are more likely induced by extreme weather conditions within typical karst geological settings. Extreme weather conditions in the study area before the sinkhole collapses consisted of a year-long drought followed by continuous precipitation with a daily maximum precipitation of 442 mm between May 31 and June 1, 2010. Typical geological conditions include the Jili underground river overlain by the Quaternary overburden with thick clayey rubble. Especially in the recharge zone of the underground river, a stabilized shallow water table was formed in response to the extreme rainstorm because of the presence of the thick clayey rubble. When the underground conduit was flooded through the cave entrance on the surface, air blasting may have caused the cave roof collapse followed by formation of soil cavities and surface collapses. Borehole monitoring results of the groundwater–air pressure monitoring show that the potential karst sinkhole can pose threats to Shanbei village, the High-Speed Railway and the Wu-Ping highway. Local government needs to be aware of any early indicators of this geohazard so that devastating sinkholes can be prevented in the future. The results also suggest that groundwater–air pressure monitoring data collected both the Quaternary deposits and the bedrock karst system provide useful indicators for potential sinkhole collapses in similar karst areas where sinkholes usually occur during rainy season or karst groundwater level is always under the rockhead.     

The influence of urbanization of sinkhole development in central Pennsylvania

The karsted limestone valleys of central Pennsylvania contain two populations of sinkholes. Solution sinkholes occur in the Champlainian limestone units along the margins of the valleys. Solution sinkholes are permanent parts of the landscape and, although a nuisance to construction, do not present other problems. The second population is the suffosional or soil-piping sinkholes These occur on all carbonate rock units including the Beekmantown and Gatesburg dolomites that comprise the two principal carbonate aquifers in the valley. Suffosional sinkholes are the principal land-use hazard. Suffosional sinkholes are transient phonomena. They occur naturally but are exacerbated by runoff modifications that accompany urbanization Suffosional sinkholes are typically 1.5–2.5 m in diameter depending on soil thickness and soil type. The vertical transport of soil to form the void space and soil arch that are the precursors to sinkhole collapse is through solutionally widened fractures and cross-joints and less often through large vertical openings in the bedrock. The limited solution development on the dolomite bedrock combined with soil thickness, seldom greater than 2 m, limits the size of the sinkholes. All aspects of suffosional sinkhole development are shallow processes: transport, piping, void and arch formation, and subsequent collapse take place usually less than 10 m below the land surface Factors exacerbating sinkhole development include pavement, street, and roof runoff which accelerates soil transport Such seemingly minor activities as replacing high grass and brush with mowed grass is observed to accelerate sinkhole development. Dewatering of the aquifer is not a major factor in this region     

Detection of sinkholes using 2D electrical resistivity imaging in the Cheria Basin (north–east of Algeria)

Sinkhole collapse is one of the main limitations on the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. 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. Two-dimensional electrical resistivity tomography surveys were conducted at two sinkhole sites near Cheria city where limestone is covered by about 10 m of clayey soils. A Wenner transect was conducted between the two sinkholes. The electrode spacing was 2 m. The length of transect is about 80 m. The survey results suggest that RESTOM is an ideal geophysical tool to aid in the detection and monitoring of sinkholes and other subsurface cavities.     

广西来宾市良江镇吉利村岩溶塌陷成因机制分析

2010年6月,一场极端暴雨事件过后,广西来宾市良江镇吉利村发生大规模岩溶塌陷,一个月内陆续产生了17处塌陷坑,其中3个塌陷坑逐渐扩大发展,最终形成一长约170 m,深约38 m的大塌陷坑,被称为“广西第一塌”。此次塌陷事件影响面积约1 500 m2,导致70户298人搬离家园。为了弄清吉利村岩溶塌陷的成因机制,在塌陷发生地区开展了地面调查、三维激光扫描、地球物理勘探、水文地质工程地质钻探、岩溶塌陷动力条件监测、降雨量监测等多项工作。分析结果表明:吉利岩溶塌陷为自然塌陷,是多种不良因素共同作用的结果,这些因素包括特殊的“水-土-岩”组合和极端暴雨,其中极端暴雨引起的地下水气压力的急剧变化是其主要动力因素;特殊的地质条件,决定了该区岩溶塌陷发生的特殊性,分析其形成过程,认为主要受气爆效应、水击效应和渗流潜蚀效应的综合作用。      

The application of GPR and electrical resistivity tomography as useful tools in detection of sinkholes in the Cheria Basin (northeast of Algeria)

Sinkhole collapse is one of the main limitations in the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. Ground-penetrating radar (GPR) and electrical resistivity imaging or tomography (RESTOM) are well suited to mapping sinkholes because of the ability of these two techniques for detecting voids and discriminating subtle resistivity variations. Nine GPR profiles and two-dimensional electrical resistivity tomography have been applied, with relative success, to locate paleo-collapses and cavities, and to detect and characterize karst at two sinkhole sites near Cheria City where limestone is covered by about 10 m of clayey soils. The survey results suggest that GPR and RESTOM are ideal geophysical tools to aid in the detection and monitoring of sinkholes and other subsurface cavities.     

Mechanism of sinkhole formation during groundwater-level recovery in karst mining area,Dachengqiao, Hunan province,China

The Meitanba Coal Mine area in Hunan province, China, had been impacted by severe cover collapse sinkholes since 1982 due to mine dewatering. After the coal mine was closed in February 2015, the groundwater level has increased significantly. A series of sinkholes were recorded in the study area during groundwater-level recovery. Analysis of monitoring results and in-situ investigation indicated that 13 sinkhole collapses were more likely induced by abrupt change of groundwater–air pressure in response to heavy rainfall from March 2015 to July 2016 when the groundwater level increased by as much as 76 m. When the karst conduit was flooded, a relatively sealed environment was formed between saturated sediments and flooded karst conduit. Implosion of entrapped air might have caused the cave roof to collapse followed by surface collapses in a short time. On the other hand, four sinkholes occurred in November 2016 when the groundwater levels were near the soil–bedrock interface at elevations between 52.5 and 58.9 m amsl and the groundwater-level increase was at slower paces. Field measurements indicate that the groundwater-level fluctuation at the soil–bedrock interface could enlarge the soil cavity and accelerate the subsoil erosion process.     

A new approach for forecasting the appearance of sinkholes near the Jinshazhou tunnel

The paper deals with a frequent and known problem of suffusion type of sinkhole formation in soil, due to water level oscillations above karstified bedrock. The authors measured groundwater pressure oscillations caused by pumping groundwater from the excavated tunnel to determine the reliable indicators for prediction of sudden soil collapse based on risk assessment. The statistical analysis was performed to determine suspicious values, which reliably predict the soil collapse. The results of analysis of monitored values indicate very good time correlation coefficients with the failures. The work comprises the following main parts: (1) survey of the geologic environment; (2) monitoring of hydrodynamic conditions; and (3) acquisition of a forecasting criterion based on the statistical analysis of anomalous monitoring data. From this research, it might be concluded that the appearance time of sinkholes and the abnormal values were strongly linearly dependent with a correlation coefficient of over 99 %. This conclusion will be a contribution to estimate the risk of karst collapse in a similar environment.     

Application of matrix analysis in delineating sinkhole risk areas along highway (I-70 near Frederick,Maryland)

Sinkhole collapse in the area of Maryland Interstate 70 (I-70) and nearby roadways south of Frederick, Maryland, has been posing a threat to the safety of the highway operation as well as other structures. The occurrence of sinkholes is associated with intensive land development. However, the geological conditions that have been developing over the past 200 million years in the Frederick Valley control the locations of the sinkholes. Within an area of approximately 8 km², 138 sinkholes are recorded and their spatial distribution is irregular, but clustered. The clustering indicates the existence of an interaction between the sinkholes. The point pattern of sinkholes is considered to be a sample of a Gibbsian point process from which the hard-core Strauss Model is developed. The radius of influence is calculated for the recorded sinkholes which are most likely to occur within 30 m of an existing sinkhole. The stochastic analysis of the existing sinkholes is biased toward the areas with intensive land use. This bias is adjusted by considering (1) topography, (2) proximity to topographic depressions, (3) interpreted rock formation, (4) soil type, (5) geophysical anomalies, (6) proximity to geologic structures, and (7) thickness of overburden. Based on the properties of each factor, a scoring system is developed and the average relative risk score for individual 30-m segments of the study area is calculated. The areas designated by higher risk levels would have greater risk of a sinkhole collapse than the areas designated by lower risk levels. This risk assessment approach can be updated as more information becomes available.     

The Lar Dam; an example of infrastructural development in a geologically active karstic region

The problems associated with the construction of the Lar Dam, Iran, provide a classic example of problems that can result from infrastructural development in an active geological terrain which includes the regional carbonate units. The Lar Dam lies in a heavily faulted and fractured region close to the active Damavand volcano. The fracturing has enhanced the production of karst and sinkholes in the limestone below the dam, and acidic ground-water run-off from the Damavand volcano increases the dissolution of the carbonate beds along the fractures to produce sinkholes below the dam. The initial problem of building the dam in such a region is compounded by the relationship between water loss due to enhanced sinkhole development and the remedial measures being taken to lessen this leakage. Drainage of the sub-reservoir caverns through leakage along fractures can lead to loss of hydrostatic support for the developing sub-reservoir caverns, and their consequent collapse. Furthermore, rapid changes in subterranean water levels would lead to rock-shattering hammer effects particularly during rapid rises in water level.     

Sinkhole characterization in the Dead Sea area using airborne laser scanning

Since the early 1980s, the Dead Sea coast has undergone a near catastrophic land deterioration as a result of a rapid lake-level drop. One conspicuous expression of this deterioration is the formation of sinkholes fields that puncture the coastal plains. The evolution of sinkholes along nearly 70-km strip has brought to a halt the regional development in this well-known and toured area and destroyed existing infrastructures. Great efforts are being invested in understanding the phenomena and in development of monitoring techniques. We report in this paper the application of airborne laser scanning for characterization of sinkholes. We demonstrate first the appropriateness of laser scanning for this task and its ability to provide detailed 3D information on this phenomenon. We describe then an autonomous means for their extraction over large regions and with high level of accuracy. Extraction is followed by their detailed geometric characterization. Using this high-resolution data, we show how sinkholes of 0.5 m radius and 25 cm depth can be detected from airborne platforms as well as the geomorphic features surrounding them. These sinkhole measures account for their embryonic stage, allowing tracking them at an early phase of their creation.     

Formation of regolith-collapse sinkholes in southern Illinois: Interpretation and identification of associated buried cavities

Three regolith-collapse sinkholes formed near the Dongola Unit School and the Pentecostal Church in the southern Illinois village of Dongola (Union County) during the spring of 1993. The sinkholes appeared over a three-month period that coincided with development of a new municipal well. The new well was drilled through clay-rich, valley-fill sediment into karstified limestone bedrock. The piezometric surface of the limestone aquifer is above land surface, indicating the presence of an upward hydraulic gradient in the valley and that the valley fill is acting as a confining unit. Pumping during development of the well lowered the piezometric surface of the limestone aquifer to an elevation below the base of the valley fill. It is hypothesized that drainage of water from the sediments, the resulting loss of hydrostatic pressure and buoyant force in overlying sediments, increased intergranular pressure, and the initiation of groundwater flow toward the well resulted in rapid sediment transport, subsurface erosion, and collapse of the valley-fill sediment. The sinkholes follow an approximately east-west alignment, which is consistent with one of the two dominant alignments of passages of nearby joint-controlled caves. A constant electrode-separation resistivity survey of the school playground was conducted to locate areas that might contain incipient sinkholes. The survey revealed a positive resistivity anomaly trending N75E in the southern part of the study area. The anomaly is linear, between 5 and 10 m wide, and its trend either intersects or is immediately adjacent to the three sinkholes. The anomaly is interpreted to be a series of pumping-induced cavities in the valley-fill sediments that formed over a preexisting crevice in the karstified bedrock limestone.     

Use of sinkhole and specific capacity distributions to assess vertical gradients in a karst aquifer

The carbonate-rock aquifer in the Great Valley, West Virginia, USA, was evaluated using a database of 687 sinkholes and 350 specific capacity tests to assess structural, lithologic, and topographic influences on the groundwater flow system. The enhanced permeability of the aquifer is characterized in part by the many sinkholes, springs, and solutionally enlarged fractures throughout the valley. Yet, vertical components of subsurface flow in this highly heterogeneous aquifer are currently not well understood. To address this problem, this study examines the apparent relation between geologic features of the aquifer and two spatial indices of enhanced permeability attributed to aquifer karstification: (1) the distribution of sinkholes and (2) the occurrence of wells with relatively high specific capacity. Statistical results indicate that sinkholes (funnel and collapse) occur primarily along cleavage and bedding planes parallel to subparallel to strike where lateral or downward vertical gradients are highest. Conversely, high specific capacity values are common along prominent joints perpendicular or oblique to strike. The similarity of the latter distribution to that of springs suggests these fractures are areas of upward-convergent flow. These differences between sinkhole and high specific capacity distributions suggest vertical flow components are primarily controlled by the orientation of geologic structure and associated subsurface fracturing.     

A review of natural sinkhole phenomena in Italian plain areas

Italian sinkholes, which are mainly related to karst phenomena (i.e., solution sinkholes, collapse sinkholes, etc.), are widespread along the Apennine ridge and in pedemontane areas where there are carbonatic bedrock outcrops. However, other collapses, which seem unrelated to karst dissolution, have been identified in plain areas with a thick sedimentary cover over buried bedrock. The main goal of this work is to study the geological, geomorphological, and structural setting of these areas to identify the possible mechanism of the generation and evolution of these collapses. About 750 cases were identified by research based on historical archives, specific geological literature, and information from local administrations. Geological, geomorphological, and hydro-geochemical surveys were conducted in 300 cases, supported by literature, borehole, and seismic data. A few examples were discarded because they could be ascribed to karst dissolution, volcanic origin (i.e., maar), or anthropogenic causes. Field studies regarding the other 450 cases are in progress. These cases occur along the Tyrrhenian margin (Latium, Abruzzo, Campania, Tuscany) in tectonic, coastal, and alluvial plains close to carbonate ridges. These plains are characterized by the presence of pressurized aquifers in the buried bedrock, overlaid by unconsolidated sediments (i.e., clay, sands, pyroclastic deposits, etc.). The majority of these collapses are aligned along regional master and seismogenetic faults. About 50% of the studied cases host small lakes or ponds, often characterized by highly mineralized springs enriched with CO₂ and H₂S. The Periadriatic margin does not seem to be affected by these phenomena, and only a few cases have been found in Sicily, Sardinia, and Liguria. The obtained scenarios suggests that this type of collapse could be related to upward erosion through vertical conduits (i.e., deep faults) caused by deep piping processes whose erosive strength is increased by the presence of acidic fluids. In order to distinguish these collapses from typical karst dissolution phenomena, they are defined as deep piping sinkholes (DPS).     

A genetic classification of sinkholes illustrated from evaporite paleokarst exposures in Spain

This contribution analyses the processes involved in the generation of sinkholes from the study of paleokarst features exposed in four Spanish Tertiary basins. Bedrock strata are subhorizontal evaporites, and in three of the basins they include halite and glauberite in the subsurface. Our studies suggest that formation of dolines in these areas results from a wider range of subsidence processes than those included in the most recently published sinkhole classifications; a new genetic classification of sinkholes applicable to both carbonate and evaporite karst areas is thus proposed. With the exception of solution dolines, it defines the main sinkhole types by use of two terms that refer to the material affected by downward gravitational movements (cover, bedrock or caprock) and the main type of process involved (collapse, suffosion or sagging). Sinkholes that result from the combination of several subsidence processes and affect more than one type of material are described by combinations of the different terms with the dominant material or process followed by the secondary one (e.g. bedrock sagging and collapse sinkhole). The mechanism of collapse includes any brittle gravitational deformation of cover and bedrock material, such as upward stoping of cavities by roof failure, development of well-defined failure planes and rock brecciation. Suffosion is the downward migration of cover deposits through dissolutional conduits accompanied with ductile settling. Sagging is the ductile flexure of sediments caused by differential corrosional lowering of the rockhead or interstratal karstification of the soluble bedrock. The paleokarsts we analysed suggest that the sagging mechanism (not included in previous genetic classifications) plays an important role in the generation of sinkholes in evaporites. Moreover, collapse processes are more significant in extent and rate in areas underlain by evaporites than in carbonate karst, primarily due to the greater solubility of the evaporites and the lower mechanical strength and ductile rheology of gypsum and salt rocks.     

Evaluation of susceptibility for terrain collapse and subsidence in karst areas,municipality of Iraquara,Chapada Diamantina (BA), Brazil

The morphological evolution of the karstic systems is associated with a set of physical and chemical processes, triggered by the dissolution of the rocks, related to percolation of groundwater and surface water, which consequently open underground voids and carve out peculiar forms of relief. Due to environmental and geotechnical aspects, this system is naturally more fragile and vulnerable than other natural systems and, therefore, has increasingly received the attention of the scientific community over the past decades. The objective of the study was to delimit zones with varying degrees of susceptibility for collapses and subsidence of sinkholes in the municipality of Iraquara, Chapada Diamantina (BA), Brazil, and to understand their geological and morphological determinant factors. Geological data, karst phenomenon map, and visual analysis in the field were used to categorize zones with different types of susceptibilities to the nucleation of new sinkholes based on a Hazard Index. This index was defined from the sum of geological hazard factors, lineament density, and sinkhole density. The areas that presented the highest susceptibility for terrain collapse and subsidence corresponded to regions where carbonate rocks outcrop, with high density of photolineaments and 2.62 sinkholes/km². Processes associated with terrain collapse and subsidence in karst areas consisted of a combination of various factors, hindering precise predictions. However, zones of different types of susceptibilities to terrain collapse and subsidence can be delimited when the relationships between these processes and their factors are understood. The Hazard Index proposed does not provide quantitative values for the probability of hazard susceptibility, but rather indicates areas that are more susceptible to terrain subsidence and collapse.     

Development and validation of sinkhole susceptibility models in mantled karst settings. A case study from the Ebro valley evaporite karst (NE Spain)

A preliminary sinkhole susceptibility analysis has been carried out in a stretch 50 km² in area of the Ebro valley alluvial evaporite karst (NE Spain). A spatial database consisting of a sinkhole layer and 27 thematic layers related to causal factors was constructed and implemented in a GIS. Three types of sinkholes were differentiated on the basis of their markedly different morphometry and geomorphic distribution: large subsidence depressions (24), large collapse sinkholes (23), and small cover-collapse sinkholes (447). The susceptibility models were produced analysing the statistical relationships between the mapped sinkholes and a set of conditioning factors using the Favourability Functions approach. The statistical analyses indicate that the best models are obtained with 6 conditioning factors out of the 27 available ones and that different factors and processes are involved in the generation of each type of sinkhole. The validation of two models by means of a random-split strategy shows that reasonably good predictions on the spatial distribution of future dolines may be produced with this approach; around 75% of the sinkholes of the validation sample occur on the 10% of the pixels with the highest susceptibility and about 45% of the area can be considered as safe.     

Analysis of a database for anthropogenic sinkhole triggering and zonation in the Naples hinterland (Southern Italy)

Anthropogenic sinkholes are generally due to the collapse of man-made underground caves and represent a major threat, especially in urban contexts, where they could cause damage to people, buildings and lifelines. The hinterland of Naples (Campania, Southern Italy) is one of the most urbanized areas of Italy and is historically affected by frequent sinkhole phenomena. This study analyzes a database of both man-made caves and sinkholes collected by the authors over several years. The aim is to comprehend the predisposing and triggering factors of sinkholes in order to attempt a zonation of occurrence. The predisposing factor resulted to be the presence of a preexisting network of caves within the Campanian Ignimbrite tuff and their geometric features. Generally, the shallower the depth of the chamber roof and the lower the thickness of the vault, the higher is the frequency of sinkholes. Furthermore, an intrinsic fragility is represented by the access shafts of vertical wells, usually filled in and abandoned. Meanwhile, the main triggering mechanism identified consists in saturation of the subsoil, due to water leaks coming from buried sewage and water pipelines. The macrozonation of sinkhole occurrence shows that the highest class is achieved where the tuff is shallow and both man-made caves and historical sinkholes are present.

     

Spatial distribution, morphometry and activity of La Puebla de Alfindén sinkhole field in the Ebro river valley (NE Spain): applied aspects for hazard zonation

A highly active collapse sinkhole field in the evaporitic mantled karst of the Ebro river valley is studied (NE Spain). The subsidence is controlled by a NW-SE trending joint system and accelerated by the discharge of waste water from a nearby industrial state. The morphometry, spatial distribution and temporal evolution of the sinkholes have been analysed. The volume of the sinkholes yields a minimum estimate of average lowering of the surface by collapse subsidence of 46 cm. The clustering of the sinkholes and the tendency to form elongated uvalas and linear belts, in a NW–SE direction have a predictive utility and allow the establishment of criteria for a hazard zonation. With the precipitation record supplied by a pluviograph and periodic cartographic and photographic surveys the influence of heavy rainfall events on the triggering of collapses has been studied.     

Groundwater fluxes into a submerged sinkhole area,Central Italy,using radon and water chemistry

The groundwater contribution into Green Lake and Black Lake (Vescovo Lakes Group), two cover collapse sinkholes in Pontina Plain (Central Italy), was estimated using water chemistry and a ²²²Rn budget. These data can constrain the interactions between sinkholes and deep seated fluid circulation, with a special focus on the possibility of the bedrock karst aquifer feeding the lake. The Rn budget accounted for all quantifiable surface and subsurface input and output fluxes including the flux across the sediment–water interface. The total value of groundwater discharge into Green Lake and Black Lake (∼540 ± 160 L s⁻¹) obtained from the Rn budget is lower than, but comparable with historical data on the springs group discharge estimated in the same period of the year (800 ± 90 L s⁻¹). Besides being an indirect test for the reliability of the Rn-budget “tool”, it confirms that both Green and Black Lake are effectively springs and not simply “water filled” sinkholes. New data on the water chemistry and the groundwater fluxes into the sinkhole area of Vescovo Lakes allows the assessment of the mechanism responsible for sinkhole formation in Pontina Plain and suggests the necessity of monitoring the changes of physical and chemical parameters of groundwater below the plain in order to mitigate the associated risk.