Vadose CO2 gas drives dissolution at water tables in eogenetic karst aquifers more than mixing dissolution

Most models of cave formation in limestone that remains near its depositional environment and has not been deeply buried (i.e. eogenetic limestone) invoke dissolution from mixing of waters that have different ionic strengths or have equilibrated with calcite at different _pCO₂ values. In eogenetic karst aquifers lacking saline water, mixing of vadose and phreatic waters is thought to form caves. We show here calcite dissolution in a cave in eogenetic limestone occurred due to increases in vadose CO₂ gas concentrations and subsequent dissolution of CO₂ into groundwater, not by mixing dissolution. We collected high‐resolution time series measurements (1 year) of specific conductivity (SpC), temperature, meteorological data, and synoptic water chemical composition from a water table cave in central Florida (Briar Cave). We found SpC, _pCO₂ and calcite undersaturation increased through late summer, when Briar Cave experienced little ventilation by outside air, and decreased through winter, when increased ventilation lowered cave CO_2(g) concentrations. We hypothesize dissolution occurred when water flowed from aquifer regions with low _pCO₂ into the cave, which had elevated _pCO₂. Elevated _pCO₂ would be promoted by fractures connecting the soil to the water table. Simple geochemical models demonstrate that changes in _pCO₂ of less than 1% along flow paths are an order of magnitude more efficient at dissolving limestone than mixing of vadose and phreatic water. We conclude that spatially or temporally variable vadose CO_2(g) concentrations are responsible for cave formation because mixing is too slow to generate observed cave sizes in the time available for formation. While this study emphasized dissolution, gas exchange between the atmosphere and karst aquifer vadose zones that is facilitated by conduits likely exerts important controls on other geochemical processes in limestone critical zones by transporting oxygen deep into vadose zones, creating redox boundaries that would not exist in the absence of caves. Copyright © 2014 John Wiley & Sons, Ltd.     

Dissolution in a variably confined carbonate platform: effects of allogenic runoff,hydraulic damming of groundwater inputs,and surface–groundwater exchange at the basin scale

In variably confined carbonate platforms, impermeable confining units collect rainfall over large areas and deliver runoff to rivers or conduits in unconfined portions of platforms. Runoff can increase river stage or conduit heads in unconfined portions of platforms faster than local infiltration of rainfall can increase groundwater heads, causing hydraulic gradients between rivers, conduits and the aquifer to reverse. Gradient reversals cause flood waters to flow from rivers and conduits into the aquifer where they can dissolve limestone. Previous work on impacts of gradient reversals on dissolution has primarily emphasized individual caves and little research has been conducted at basin scales. To address this gap in knowledge, we used legacy data to assess how a gradient of aquifer confinement across the Suwannee River Basin, north‐central Florida affected locations, magnitudes and processes of dissolution during 2005–2007, a period with extreme ranges of discharge. During intense rain events, runoff from the confining unit increased river stage above groundwater heads in unconfined portions of the platform, hydraulically damming inputs of groundwater along a 200 km reach of river. Hydraulic damming allowed allogenic runoff with SI_CAL < ?4 to fill the entire river channel and flow into the aquifer via reversing springs. Storage of runoff in the aquifer decreased peak river discharges downstream and contributed to dissolution within the aquifer. Temporary storage of allogenic runoff in karst aquifers represents hyporheic exchange at a scale that is larger than found in streams flowing over non‐karst aquifers because conduits in karst aquifers extend the area available for exchange beyond river beds deep into aquifers. Post‐depositional porosity in variably confined carbonate platforms should thus be enhanced along rivers that originate on confining units. This distribution should be considered in models of porosity distribution used to manage water and hydrocarbon resources in carbonate rocks. Copyright © 2013 John Wiley & Sons, Ltd.     

Heterogeneous distributions of CO2 may be more important for dissolution and karstification in coastal eogenetic limestone than mixing dissolution

Mixing dissolution, a process whereby mixtures of two waters with different chemical compositions drive undersaturation with respect to carbonate minerals, is commonly considered to form cavernous macroporosity (e.g. flank margin caves and banana holes) in eogenetic karst aquifers. On small islands, macroporosity commonly originates when focused dissolution forms globular chambers lacking entrances to the surface, suggesting that dissolution processes are decoupled from surface hydrology. Mixing dissolution has been thought to be the primary dissolution process because meteoric water would equilibrate rapidly with calcium carbonate as it infiltrates through matrix porosity and because pCO₂ was assumed to be homogeneously distributed within the phreatic zone. Here, we report data from two abandoned well fields in an eogenetic karst aquifer on San Salvador Island, Bahamas, that demonstrate pCO₂ in the phreatic zone is distributed heterogeneously. The pCO₂ varied from less than log ?2.0 to more than log ?1.0 atm over distances of less than 30 m, generating dissolution in the subsurface where water flows from regions of low to high pCO₂ and cementation where water flows from regions of high to low pCO_2. Using simple geochemical models, we show dissolution caused by heterogeneously distributed pCO₂ can dissolve 2.5 to 10 times more calcite than the maximum amount possible by mixing of freshwater and seawater. Dissolution resulting from spatial variability in pCO₂ forms isolated, globular chambers lacking initial entrances to the surface, a morphology that is characteristic of flank margin caves and banana holes, both of which have entrances that form by erosion or collapse after cave formation. Our results indicate that heterogeneous pCO₂, rather than mixing dissolution, may be the dominant mechanism for observed spatial distribution of dissolution, cementation and macroporosity generation in eogenetic karst aquifers and for landscape development in these settings. Copyright © 2015 John Wiley & Sons, Ltd.     

Tidal pumping and biogeochemical processes: Dissolution within the tidal capillary fringe of eogenetic coastal carbonates

Growing evidence suggests microbial respiration of dissolved organic carbon (DOC) may be a principal driver of subsurface dissolution and cave formation in eogenetic carbonate rock. Analyses of samples of vadose zone gasses, and geochemical and hydrological data collected from shallow, uncased wells on San Salvador Island, Bahamas, suggest tidally varying water tables may help fuel microbial respiration and dissolution through oxygenation. Respiration of soil organic carbon transported to water tables generates dysaerobic to anaerobic groundwater, limiting aerobic microbial processes. Positive correlations of carbon dioxide (CO₂), radon-222 (²²²Rn) and water table elevation indicate, however, that tidal pumping of water tables pulls atmospheric air that is rich in oxygen, and low in CO₂ and ²²²Rn, into contact with the tidal capillary fringe during falling tides. Ratios of CO₂ and O₂ in vadose gas relative to the atmosphere indicate this atmospheric oxygen fuels respiration within newly-exposed, wetted bedrock. Deficits of expected CO₂ relative to O₂ concentrations indicate some respired CO₂ is likely removed by carbonate mineral dissolution. Tidal pumping also appears capable of transferring oxygen to the freshwater lens, where it could also contribute to respiration and dissolution; dissolved oxygen concentrations at the water table are at least 5% saturated and decline to anaerobic conditions 1–2 m below. Our results demonstrate how tidal pumping of air to vadose zones can drive mineral dissolution reactions that are focused near water tables and may contribute to the formation of laterally continuous vuggy horizons and potentially caves. © 2020 John Wiley & Sons, Ltd.     

Water and air dynamics within a deep vadose zone of a karst massif: Observations from the Lukina jama–Trojama cave system (−1,431 m) in Dinaric karst (Croatia)

Extreme heterogeneity of karst systems makes them very challenging to study. Various processes within the system affect its global response, usually measured at karst springs. Research conducted in caves provides a unique opportunity for in situ analysis of separate processes in karst underground. The aim of the present study was to research the water and air dynamics within a deep karst system. Air and water basic physical parameters across the Lukina jama–Trojama cave system (?1,431 m) were continuously monitored during a 1‐year period. Recorded hydrograph of the siphon lake at the bottom of the cave was used to interpret the characteristics of an unexplored phreatic/epiphreatic conduit network. Water origin in the siphon was determined based on temperature and electrical conductivity. Air temperature and humidity monitoring revealed a strong inflow of air of sub‐zero temperature into the upper portion of the cave during winter. Cave passage morphology was interpreted as the main determinant of air dynamics, which caused ice to accumulate extensively in the upper portions of the cave and caused the temperature on the top of the homothermic zone to be significantly below the mean outside temperature. Air dynamics also lowered the temperature of water flowing through the cave vadose zone and feeding the phreatic zone of the massif. The pronounced temperature difference between the phreatic zone and the top of the homothermic zone probably contributed to the thermal gradient observed in the cave, which is steeper than in ice‐free caves in the area. Our results enabled the development of a conceptual model that describes coupling between air and water dynamics in the cave system and its surroundings.     

The role of antecedent groundwater heads in controlling transient aquifer storage and flood peak attenuation in karst watersheds

Transient storage of floodwaters in aquifers is known to attenuate peak flows in rivers and drive subsurface dissolution. Transient aquifer storage could be enhanced in watersheds overlying karst aquifers where caves facilitate surface and groundwater exchange. Few studies, however, have examined controls on, or magnitudes of, transient aquifer storage or flood peak attenuation in karstic watersheds. Here we evaluate flood peak attenuation with multiple linear regression analyses of 10 years of river and groundwater data from the Suwannee River, which flows over the karstic upper Floridan aquifer in north-central Florida and experiences frequent flooding. Regressions show antecedent river stage exerts the dominant control on magnitudes of transient aquifer storage, with recharge and time to peak having secondary controls. Specifically, low antecedent stages result in larger magnitudes of transient aquifer storage and thus greater flood attenuation than conditions of elevated antecedent stage. These findings suggest subsurface weathering, including cave formation and enlargement, caused by transient aquifer storage could occur on a more frequent basis in aquifers where groundwater table elevation is lowered due to anthropogenic or climatic influences. Our work also shows that measures of groundwater table elevation prior to an event could be used to improve predictive flood models. © 2018 John Wiley & Sons, Ltd.     

Interpretation of groundwater level monitoring results in karst aquifers: examples from the Dinaric karst

The paper presents an attempt to determine the characteristics of karst aquifers using information on groundwater level (GWL) in natural holes and boreholes with different data quantity and time resolution of GWL measurements. In this paper the particulars of karst aquifers were analysed for four examples from the Dinaric karst. In all four study areas, aquifers are formed in bare, deep and well‐developed Dinaric karst consisting of Cretaceous limestones. The first example represents a wide area of Imotsko polje in the karst. The aquifer was analysed on the basis of infrequent water level monitoring in natural karst water features (jamas, lakes, wells) and discharges of springs and rivers. The karst aquifer in this example is complex, non‐homogenous and variable in space and time, which is frequent in the Dinaric karst. Regardless of the aforementioned it was possible to determine its elementary characteristics. The second example represents 10 wells used for the water supply for the city of Pula. The GWL and salinity were measured once a week in the period between 1981 and 1996. Even though these measurements were relatively infrequent in space and time, they served as bases for assessment of average and maximum aquifer conditions as well as boundaries of saltwater intrusion. In the third example only a portion of aquifer of the karst spring Blaz, which is in the contact with the Adriatic Seas, has been analyzed. It is a spring with an intrusion of salt water. For purposes of study of saltwater intrusion, 26 piezometers were drilled in its vicinity in which GWL, salinity and temperature were measured once a day during 168 days, a period comprising one complete cycle of seawater intrusion and retreat. These measurements proved the existence of dispersed discharge from the aquifer into the sea and its non‐homogeneity in space. In the fourth example GWL was measured continuously in 10 deep (up to 300 m) piezometers in the hinterland of the Ombla Spring catchment. The measurement period lasted 2 years (January 1988 to December 1989). The analyses are made with hourly data. The results made it possible to determine numerous characteristics of the karst aquifer and a significant non‐homogeneity of groundwater distribution in karst aquifers, depending more on the underground karst phenomena than the surface karst forms. Copyright © 2000 John Wiley & Sons, Ltd.     

Organic carbon inputs,common ions and degassing: rethinking mixing dissolution in coastal eogenetic carbonate aquifers

Caves deliver freshwater from coastal carbonate landscapes to estuaries but how these caves form and grow remains poorly understood. Models suggest fresh and salt water mixing drives dissolution in eogenetic limestone, but have rarely been validated through sampling of mixing waters. Here we assess controls on carbonate mineral saturation states using new and legacy geochemical data that were collected in vertical profiles through three cenotes and one borehole in the Yucatan Peninsula. Results suggest saturation states are primarily controlled by carbon fluxes rather than mixing. Undersaturation predicted by mixing models that rely on idealized end members is diminished or eliminated when end members are collected from above and below actual mixing zones. Undersaturation due to mixing is limited by CO₂ degassing from fresh water in karst windows, which results in calcite supersaturation. With respect to saline groundwater, controls on capacity for mixing dissolution were more varied. Oxidation of organic carbon increased pCO₂ of saline groundwater in caves (pCO₂ = 10^–2.06 to 10^–0.96 atm) relative to matrix porosity (10^–2.39 atm) and local seawater (10^–3.12 atm). The impact of increased pCO₂ on saturation state, however, depended on the geochemical composition of the saline water and the magnitude of organic carbon oxidation. Carbonate undersaturation due to mixing was limited where gypsum dissolution (Cenote Angelita) or sulfate reduction (Cenote Calica) increased concentrations of common ions (Ca²⁺ or HCO₃^?, respectively). Maximum undersaturation was found to occur in mixtures including saline water that had ion concentrations and ratios similar to seawater, but with moderately elevated pCO₂ (Cenote Eden). Undersaturation, however, was dominated by the initial undersaturation of the saline end member, mixing was irrelevant. Our results add to a growing body of literature that suggests oxidation of organic carbon, and not mixing dissolution, is the dominant control on cave formation and enlargement in coastal eogenetic karst aquifers. Copyright © 2016 John Wiley & Sons, Ltd.     

Tectonic uplift,sea level changes and Plio‐Pleistocene evolution of a coastal karst system: the Mount Saint Paul (Palawan,Philippines)

The St. Paul karst (Palawan, Philippines) is a tropical coastal karst, consisting of towers, cones, huge depressions and large caves. This area hosts the Puerto Princesa Subterranean River (PPSR, 24 km long), whose main entrance is a large spring along the coast and which is one of the largest cave complexes in eastern Asia. A geomorphological study performed by several field surveys and a morphometric analysis of the digital terrain model (DTM) and 3D cave models, allowed formulation of a first evolutionary framework of the karst system. The DTM was extracted from maps and aerial photographs in order to find different generations of ‘relict’ landforms, through the morphometric analysis of topographic surface and karst landforms. Several features suggest a long and multi‐stage evolution of the karst, whose age ranges from Pliocene to present. The southern and northern sectors of the area differ in their altimetric distribution of caves. In the southern sector, some large caves lie between 300 and 400 m asl and were part of an ancient system that developed at the base level of a past river network. In the northern sector, some mainly vadose caves occur, with a phreatic level at 120–130 m asl. An important phase of base‐level cave development is well documented in the inactive passages of PPSR at 50–80 m asl. Morphological features, such as horizontal solution passages and terraced deposits, suggest a phase of stillstand of the base level, which is recorded in the topography as low‐relief surfaces at 40–50 m asl. The age of this phase is probably Early Pleistocene, on the basis of assumed uplift rates. The more recent caves are still active, being located at the current sea level, but they show more than one cycle of flooding and dewatering (with calcite deposition). In the PPSR, several morphologic features, such as two main water level notches at +12·4 and +7·7 m asl and terraced alluvial deposits, suggest that the lower and active level passed through more than two high‐stands of sea level and so it could have formed throughout most of the Middle‐Late Pleistocene. Copyright © 2010 John Wiley & Sons, Ltd.     

Using periodic hydrologic and geochemical sampling with limited continuous monitoring to characterize remote karst aquifers in the Kaweah River Basin,California, USA

Hydrogeologic field work in remote settings is often challenging: assessing spring behaviour and aquifer characteristics can be expensive in both time commitment and resources needed to assess these systems. In this study, we document the hydrology and geochemistry of 47 perennial karst springs in the Kaweah River, a mountain river basin in the Sierra Nevada, California. After preliminary hydrogeochemical characterization and grouping, selected springs were continuously monitored to further assess aquifer characteristics in each group. Later, in areas without previous dye‐tracing work, traces were conducted to establish connections between large sinking streams and springs. The springs have a wide range of inter‐spring and intra‐spring variability in discharge and geochemistry. We assessed this variability by performing statistical comparisons with spring chemistry and principal components analysis of all measured variables. Results show that springs can be divided into two distinct groups: high elevation springs of the Mineral King Valley and lower elevation springs throughout the rest of the basin. Continuous discharge, temperature and specific conductivity data from four springs (two from each group) were then used to characterize the hydrograph recession behaviour of springs in each group. Both groups showed statistically similar baseflow recession slopes, suggesting that both groups contain baseflow storage compartments with similar hydrogeologic properties. The biggest difference between each group is the variability in amount of water remaining in the aquifer during baseflow conditions. High elevation springs have lower baseflow discharges, relative to peak flow, than lower elevation springs, despite the fact that more precipitation falls at higher elevation. This is likely caused by differences in the amount of soil and epikarst storage, which are related to recent geomorphic events: high elevation aquifers were glaciated as recent as 41 thousand years ago (kya), while there is no evidence that low elevation aquifers were glaciated. As a result, lower elevations have developed thicker soils, weathered bedrock and epikarst. Copyright © 2016 John Wiley & Sons, Ltd.     

Springflow hydrographs: eogenetic vs. telogenetic karst

Matrix permeability in the range of 10(-11) to 10(-14) m(2) characterizes eogenetic karst, where limestones have not been deeply buried. In contrast, limestones of postburial, telogenetic karst have matrix permeabilities on the order of 10(-15) to 10(-20) m(2). Is this difference in matrix permeability paralleled by a difference in the behavior of springs draining eogenetic and telogenetic karst? Log Q/Q(min) flow duration curves from 11 eogenetic-karst springs in Florida and 12 telogenetic-karst springs in Missouri, Kentucky, and Switzerland, plot in different fields because of the disparate slopes of the curves. The substantially lower flow variability in eogenetic-karst springs, which results in the steeper slopes of their flow duration curves, also makes for a strong contrast in patterns (e.g., "flashiness") between the eogenetic-karst and telogenetic-karst spring hydrographs. With respect to both spring hydrographs and the flow duration curves derived from them, the eogenetic-karst springs of Florida are more like basalt springs of Idaho than the telogenetic-karst springs of the study. From time-series analyses on discharge records for 31 springs and published time-series results for 28 additional sites spanning 11 countries, we conclude that (1) the ratio of maximum to mean (Q(max)/Q(mean)) discharge is less in springs of eogenetic karst than springs of telogenetic karst; (2) aquifer inertia (system memory) is larger in eogenetic karst; (3) eogenetic-karst aquifers take longer to respond to input signals; and (4) high-frequency events affect discharge less in eogenetic karst. All four of these results are consistent with the hypothesis that accessible storage is larger in eogenetic-karst aquifers than in telogenetic-karst aquifers.     

Eogenetic karst hydrology: insights from the 2004 hurricanes, peninsular Florida

Eogenetic karst lies geographically and temporally close to the depositional environment of limestone in warm marine water at low latitude, in areas marked by midafternoon thunderstorms during a summer rainy season. Spring hydrographs from such an environment in north-central Florida are characterized by smooth, months-long, seasonal maxima. The passage of Hurricanes Frances and Jeanne in September 2004 over three field locations shows how the eogenetic karst of the Upper Floridan Aquifer responds to unequivocal recharge events. Hydrographs at wells in the High Springs area, Rainbow Springs, and at Morris, Briar, and Bat Caves all responded promptly with a similar drawn-out rise to a maximum that extended long into the winter dry season. The timing indicates that the typical hydrograph of eogenetic karst is not the short-term fluctuations of springs in epigenic, telogenetic karst, or the smoothed response to all the summer thunderstorms, but rather the protracted response of the system to rainfall that exceeds a threshold. The similarity of cave and noncave hydrographs indicates distributed autogenic recharge and a free communication between secondary porosity and permeable matrix-both of which differ from the hydrology of epigenic, telogenetic karst. At Briar Cave, drip rates lagged behind the water table rise, suggesting that recharge was delivered by fractures, which control the cave's morphology. At High Springs, hydrographs at the Santa Fe River and a submerged conduit apparently connected to it show sharp maxima after the storms, unlike the other cave hydrographs. Our interpretation is that the caves, in general, are discontinuous.     

Quantifying the role of karstic groundwater in a snowmelt-dominated hydrologic system

River discharge in mountainous regions of the world is often dominated by snowmelt, but base flows are sustained primarily by groundwater storage and discharge. Although numerous recent studies have focused on base-flow discharge in mountain systems, almost no work has explicitly investigated the role of karst groundwater in these systems across a full range of flow conditions. We directly measured groundwater discharge from 48 karst springs in the Kaweah River and its five forks in the Sierra Nevada mountains, California, United States. Relationships between spring and river discharge showed that karst aquifers and springs provide significant storage and delayed discharge to the river. Regression models showed that, of all potential seasonal groundwater storage compartments in the river basin, the area of karst (0.1–4.4%) present provides the best explanation of base-flow recession in each fork of the Kaweah River (directly measured contributions from karst springs ranged from 3.5 to 16% during high-flow to 20 to 65% during base-flow conditions). These results show that, even in settings where karst represents a small portion of basin area, it may play an over-sized role in seasonal storage and water resources in mountain systems. Karst aquifers are the single most important non-snow storage component in the Kaweah River basin, and likely provide similar water storage capacities and higher base flows in other mountain river systems with karst when compared with systems without karst.     

The role of gypsum and/or dolomite dissolution in tufa precipitation: lessons from the hydrochemistry of a carbonate–sulphate karst system

The precipitation of freshwater carbonates (tufa) along karstic rivers is enhanced by degassing of carbon dioxide (CO₂) downstream of karstic springs. However, in most karstic springs CO₂ degassing is not enough to force the precipitation of tufa sediments. Little is known about the role of dissolution of gypsum or dolomite in the hydrochemistry of these systems and how this affects the formation of tufa deposits. Here we present a monitoring study conducted over a year in Trabaque River (Spain). The river has typical karst hydrological dynamics with water sinking upstream and re‐emerging downstream of the canyon. Mixing of calcium–magnesium bicarbonate and calcium sulphate waters downstream of the sink enhances the dissolution of carbonates and potentially plays a positive role in the formation of tufa sediments. However, due to the common‐ion effect, dissolution of dolomite and/or gypsum causes precipitation of underground calcite cements as part of the incongruent dissolution of dolomite/dedolomitization process, which limits the precipitation of tufa sediments. Current precipitation of tufa is scant compared to previous Holocene tufa deposits, which likely precipitated from solutions with higher saturation indexes of calcite (SIcc values) than nowadays. Limited incongruent dissolution of dolomite/dedolomitization favours higher SIcc values. This circumstance occurs when waters with relatively high supersaturation of dolomite and low SO₄^2? composition sink in the upper sector of the canyon. In such a scenario, the process of mixing waters enhances the exclusive dissolution of limestones, preventing the precipitation of calcite within the aquifer and favouring the increase of SIcc values downstream of the springs. Such conditions were recorded during periods of high water level of the aquifers and during floods. This research shows that the common‐ion effect caused by the dissolution of gypsum and/or dolomite rocks can limit [or favour] the precipitation of tufa sediments depending on the occurrence [or not] of incongruent dissolution of dolomite/dedolomitization. Copyright © 2016 John Wiley & Sons, Ltd.     

Seasonal δ13CDIC sourcing and geochemical flux in telogenetic epikarst of south-central Kentucky

Telogenetic epikarst carbon sourcing and transport processes and their associated hydrogeochemical responses are complex and dynamic. Carbon dioxide (CO₂) transport rates in the epikarst zone are often driven by hydrogeochemical responses, which influence carbonate dissolution and conduit formation. This study examines the influence of land use on carbon sourcing and carbonate dissolution kinetics through a comparative analysis of separate, but similar, epikarst systems in south-central Kentucky. The use of high-resolution hydrogeochemical data from multiple data loggers and isotope analysis from collected water samples reflects the processes within these epikarst aquifers, which are estimated to contribute significantly to bedrock dissolution. Results indicate that, in an agricultural setting, long-term variability and dissolution is governed by seasonal production of CO₂ . In a more urbanized, shallower epikarst system, land cover may affect CO₂ transport between the soil and underlying bedrock. This concentration of CO₂ potentially contributes to ongoing dissolution and conduit development, irrespective of seasonality. The observed responses in telogenetic epikarst systems seem to be more similar to eogenetic settings, which is suggested to be driven by CO₂ transport occurring independent of high matrix porosity. The results of this study indicate site-specific responses with respect to both geochemical and δ¹³C_DIC changes on a seasonal scale, despite regional geologic similarities. The results indicate that further comparative analyses between rural and urban landscapes in other karst settings is needed to delineate the impact of land use and seasonality on dissolution and carbon sourcing during karst formation processes. © 2019 John Wiley & Sons, Ltd.     

Simulation of spring flows from a karst aquifer with an artificial neural network

In China, 9·5% of the landmass is karst terrain and of that 47,000 km² is located in semiarid regions. In these regions the karst aquifers feed many large karst springs within basins of thousands of square kilometres. Spring discharges reflect the fluctuation of ground water level and variability of ground water storage in the basins. However, karst aquifers are highly heterogeneous and monitoring data are sparse in these regions. Therefore, for sustainable utilization and conservation of karst ground water it is necessary to simulate the spring flows to acquire better understanding of karst hydrological processes. The purpose of this study is to develop a parsimonious model that accurately simulates spring discharges using an artificial neural network (ANN) model. The karst spring aquifer was treated as a non‐linear input/output system to simulate the response of karst spring flow to precipitation and applied the model to the Niangziguan Springs, located in the east of Shanxi Province, China and a representative of karst springs in a semiarid area. Moreover, the ANN model was compared with a previous time‐lag linear model and it was found that the ANN model performed better. Copyright © 2007 John Wiley & Sons, Ltd.     

Speleogenesis of an exhumed hydrothermal sulphuric acid karst in Cambrian carbonates (Mount San Giovanni,Sardinia)

In the past few years the systematic study of caves intercepted by mine workings in southwest Sardinia has permitted us to observe morphologies due to rare speleogenetic and minerogenetic processes related to ancient hydrothermal activity. These relic morphologies are slowly being overprinted by recent speleogenetic processes that tend to obscure the hypogene origin of these caves. A combined geomorphological and mineralogical investigation has permitted a fairly detailed reconstruction of the various phases of evolution of these caves. Cave formation had already started in Cambrian times, but culminated in the Carboniferous, when most of the large voids still accessible today were formed. A key role in carbonate dissolution was played by sulphuric acid formed by the oxidation of the polymetallic ores present in the rocks since the Cambrian. During the Quaternary a variety of minerals formed inside the caves: calcite and aragonite, that yielded sequences of palaeo‐environmental interest, and also barite, phosgenite, hydrozincite, hemimorphite and many others. These minerals are in part due to a phreatic thermal hypogenic cave forming phase, and in part to later epigene overprinting in an oxidizing environment rich in polymetallic ores. Massive gypsum deposits, elsewhere typical of this kind of caves, are entirely absent due to dissolution during both the phreatic cave formation and the later epigenic stage. Copyright © 2013 John Wiley & Sons, Ltd.     

Reconstructing fluvial incision rates based on palaeo-water tables in chalk karst networks along the Seine valley (Normandy,France)

Quantifying rates of river incision and continental uplift over Quaternary timescales offer the potential for modelling landscape change due to tectonic and climatic forcing. In many areas, river terraces form datable archives that help constrain the timing and rate of valley incision. However, old river terraces, with high-level deposits, are prone to weathering and often lack datable material. Where valleys are incised through karst areas, caves and sediments can be used to reconstruct the landscape evolution because they can record the elevation of palaeo-water tables and contain preserved datable material. In Normandy (N. France), the Seine River is entrenched into an extensive karstic chalk plateau. Previous estimates of valley incision were hampered by the lack of preserved datable fluvial terraces. A stack of abandoned phreatic cave passages preserved in the sides of the Seine valley can be used to reconstruct the landscape evolution of the region. Combining geomorphological observations, palaeomagnetic and U/Th dating of speleothem and sediments in eight caves along the Lower Seine valley, we have constructed a new age model for cave development and valley incision. Six identified cave levels up to ∼100 m a.s.l. were formed during the last ~1 Ma, coeval with the incision of the Seine River. Passage morphologies indicate that the caves formed in a shallow phreatic/epiphreatic setting, modified by sediment influxes. The valley's maximum age is constrained by the occurrence of late Pliocene marine sand. Palaeomagnetic dating of cave infills indicates that the highest-level caves were being infilled prior to 1.1 Ma. The evidence from the studied caves, complemented by fluvial terrace sequences, indicates that rapid river incision occurred during marine isotope stage (MIS) 28 to 20 (0.8–1 Ma), with maximal rates of ~0.30 m ka⁻¹, dropping to ~0.08 m ka⁻¹ between MIS 20–11 (0.8–0.4 Ma) and 0.05 m ka⁻¹ from MIS 5 to the present time. © 2020 John Wiley & Sons, Ltd.     

Assessment of groundwater recharge processes through karst vadose zone by cave percolation monitoring

Recharge processes of karst aquifers are difficult to assess given their strong heterogeneity and the poorly known effect of vadose zone on infiltration. However, recharge assessment is crucial for the evaluation of groundwater resources. Moreover, the vulnerability of karst aquifers depends on vadose zone behaviour because it is the place where most contamination takes place. In this work, an in situ experimental approach was performed to identify and quantify flow and storage processes occurring in karst vadose zone. Cave percolation monitoring and dye tracing were used to investigate unsaturated zone hydrological processes. Two flow components (diffuse and quick) were identified and, respectively, account for 66% and 34% of the recharge. Quickflow was found to be the result of bypass phenomenon in vadose zone related to water saturation. We identify the role of epikarst as a shunting area, most of the storage in the vadose zone occurring via the diffuse flow component in low permeability zones. Relationship between rainfall intensity and transit velocity was demonstrated, with 5 times higher velocities for the quick recharge mode than the diffuse mode. Modelling approach with KarstMod software allowed to simulate the hybrid recharge through vadose zone and shows promising chances to properly assess the recharge processes in karst aquifer based on simple physical models.     

A high‐altitude temporary spring in a compartmentalized carbonate aquifer: the role of low‐permeability faults and karst conduits

The aim of this research was to refine the actual conceptual model related to the activation of high‐altitude temporary springs within the carbonate Apennines in southern Italy. The research was carried out through geophysical, hydrogeological, hydrochemical and isotopic investigations at the Acqua dei Faggi experimental site during five hydrologic years. The research demonstrated that, in carbonate aquifers where low‐permeability faults cause the aquifer system to be compartmentalized, high‐altitude temporary springs may be recharged by groundwater. In such settings, neither surface water infiltration in karst systems nor perched temporary aquifers play a role of utmost importance. The rare (once or a few time a year) activation of such springs is due to the fact that groundwater unusually reach the threshold head that allows the spring to flow. The activation of the studied high‐altitude temporary spring also depended on relationships between a low‐permeability fault core and a karst system that locally interrupts the low‐permeability barrier. In fact, when the hydraulic head did not reach the karst system, the concentrated head loss within the fault core did not allow the spring to flow, because the groundwater entirely flowed through the fault towards the downgradient compartment. Copyright © 2009 John Wiley & Sons, Ltd.