Shallow Karst Research Articles

Characteristics of shallow buried karst and its safety distance to tunnel in wuxi city, China

Subway tunneling construction in shallow buried karst region is a common challenge. Survey boreholes and cross-hole seismic CT data showed that 29 karst caves exist at elevation of −17 m to −37 m in Si–He section of the Wuxi Metro Line 4, which are consisted of 18 filled caves and 11 empty caves and constitute substantial dangers to the stability and quality of metro construction projects. Based on their size, position, and type of filling, we divided these shallow karst caves into four types. Aimed at exploring the instability of the tunnel after excavation, numerical analyses have been developed by using the Finite difference method. Both the effects of the distance between tunnel and karst caves and the effects of the filling of the cavities on the overall stability of the rock mass around the tunnel have been investigated. According to the design criteria, safety distance between tunnel and karst caves was determined. The monitoring data of tunnel during excavation proved the rationality and accuracy of the analysis results. The research results can provide technical reference for the design and construction of other proposed tunnels that are located in the regions of shallow buried karst in Jiangsu Province.

Mechanical characterization of subway tunnel construction in urban shallow distributed karst stratums

To address the challenges posed by rapid urban and population growth, China has undertaken extensive urban subway construction. In order to explore the mechanical characteristics of subway tunnel construction in urban shallow karst environment, based on the first phase project of Guiyang rail transit line 3, the concept of “distributed Karst” is proposed according to the occurrence characteristics of karst in urban environment. The dry ice and model soil were used to characterize the karst strata, and the similar model test study on the influence of different volume karst rates on subway tunnel construction was carried out. The influence of tunnel excavation on surrounding rock disturbance, internal force of arch structure and ground settlement under distributed karst stratum condition is studied comprehensively. The following key findings emerged:(1) Karst presence causes an abrupt change in the release curve of surrounding rock earth pressure. Higher degrees of karst development result in smaller changes in earth pressure on the arch top and shoulder after tunnel excavation, with final earth pressure changes of 91.1 kPa, 83.5 kPa, and 74.8 kPa for volumetric karst rates of 5%, 10%, and 20%, respectively. (2) Distributed karst in the tunnel arch has a limited impact on the lateral excavation displacement of the arch girdle but significantly affects the vertical settlement of the tunnel vault. Increasing volumetric karst rates by twofold (from 5% to 10% and 20%) reduces arch settlement by 48.9% and 55.6%, respectively. (3) At volumetric karst rates of 5%, 10%, and 20%, the axial force values at the tunnel top are 58.2 kN, 52.8 kN, and 50.3 kN, while the corresponding bending moment values are 49.6 kN-m, 37.1 kN-m, and 26.0 kN-m, respectively, indicating a more significant difference in bending moment values than axial force values. (4) Karst presence leads to increased surface settlement, with a 48.8% increase in peak ground settlement observed as the volumetric karst rate rises from 5% to 20%.

Porosity characterization using medical computed tomography scans of two deeply buried paleokarst aquifers in Silurian carbonates, southern Ontario, Canada

Paleokarst and paleokarst aquifers are not as well-documented as shallow karst, and studies of pores and pore networks are rare. Paleokarst lacks the open conduits typical of karst due to compaction, infilling, and diagenetic recrystallization by deep burial, so groundwater movement is through matrix porosity. In this case study, porosity networks in two saline aquifers in carbonate paleokarst of the Silurian Guelph Formation and Salina A-1 Carbonate Unit in southern Ontario (Canada) have been studied in drill cores at microscopic to macroscopic scales, utilizing medical computed tomography (CT) scans, optical petrography, macroscopic core examination, and scanning electron microscopy. The CT scans provided nondestructive three-dimensional visualization and quantification of pore distribution, size and volume, pore connections, and estimates of total porosity, similar to gas porosimetry. In the A-1 Carbonate, 50% of the pore volume comprises layers of macropores associated with algal laminations, with good horizontal connections. In the Guelph Formation, vuggy macropores contribute most of the pore volume. They are connected through subvertical fractures and rubble-filled karst conduits and by abundant small pores and micropores with poor horizontal connections. The Guelph paleokarst represents a longer period of subaerial exposure as evidenced by its greater thickness and geographic extent, former karst conduits, and near-total destruction of primary sedimentary fabrics. The findings provide real-world examples for construction of laboratory scale models of paleokarst aquifers, demonstrate the value of a multi-scale approach to porosity studies, and showcase the value of medical CT scans with its unique ability to visualize pore connections within drill cores.

Geographic and transport controls of temperature response in karst springs

Thermal responses of groundwater are useful signatures to reveal the recharge-discharge process and characterize aquifer structure in karst groundwater. Four karst springs with varying circulation depths ranging from ∼ 60 to 820 m in South China, were used to decipher the mechanism of heat exchange between recharge water and surrounding rock formations. An analytical solution of heat transfer in karst conduit was proposed to simulate the peak or trough temperatures of pulse flow. Hydraulic diameter, flow velocity, input temperature and circulation depth in karst aquifer, mainly depending on the recharge water volume after rainfall events, are important transport factors to control the thermal equilibrium depth and the efficiency of heat exchange that directly controls the behavior of warm peaks or cold troughs in groundwater temperature. In shallow karst aquifers wherein the circulation depth is less than its theoretical thermal equilibrium depth, the recharge water does not sufficiently exchange heat with the surrounding rocks, resulting in clear event-scale warm peaks in summer and cold troughs in winter. Otherwise, the recharge water will firstly reach a thermal equilibrium state and then mix with the warmer base flow at deeper location, which is verified in a karst spring with anomalous event-scale temperature drops after rainfall events all year around. The thermal response simulations obtained from this work provide new ways and insights to characterize geographic structure of karst aquifer and elucidate heat transfer mechanism in karst aquifer systems.

Late Pleistocene to Holocene Palaeohydrological History of the Thermal-Spring-Fed Lake Pețea (NW Romania) Revealed by Radiocarbon Dating and Complex Sedimentological Investigations

Understanding sedimentation processes in response to past hydrogeological and climatic changes and capturing millennial-scale variations is a key focus of lacustrine paleoenvironmental research. This study presents the first high-resolution chronology and sedimentary data for the small thermal-spring-fed Lake Pețea, NW Romania, and unravels the evolutionary history of the lake harboring a unique endemic fauna. Its small size and single source of water make it particularly sensitive to hydrological changes. In the recent past, over-exploitation of the thermal water has led to the complete drying up of the lake and the extinction of its fauna. Nevertheless, past spatio-temporal variation of environmental factors, in particular the fluctuation of lake levels and water temperature, must have had a significant impact on the survival and evolution of the endemic mollusk fauna. This fact makes this study particularly important. Based on our results, a three-stage sedimentary evolution occurred, mainly controlled by major climate-driven hydrological changes also seen in regional records, i.e., 17.5–14.5 ka shallow eutrophic lake, 14.5–5.5 ka oligotrophic carbonate-rich lake, and 5.5–0.5 ka shallow eutrophic lake. A major lowstand at 11.7–10.2 ka due to drier climate was followed by progressively rising water levels up to 5 ka followed by a drop. The main control on lake level fluctuations and sedimentary phases was the varying input of thermal water due to recurring increased/decreased recharge of the underground shallow karst water system. The driving factor of thermal water discharge was different during the Late Glacial than the Holocene. It was the warming of the climate at 14.5 ka cal BP and melting of regional ice sheets in addition to increased precipitation that created an oligotrophic lake by recharging the underground thermal water system. Conversely, during the Holocene, increasing/decreasing moisture availability driven by major climate forcings was in control of thermal water recharge, erosion, and fluctuating lake levels.

Caves demonstrate decrease in rainfall recharge of southwest Australian groundwater is unprecedented for the last 800 years

Billions of people worldwide rely on groundwater. As rainfall in many regions in the future is projected to decrease, it is critical to understand the impacts of climate change on groundwater recharge. The groundwater recharge response to a sustained decrease in rainfall across southwest Australia that began in the late 1960s was examined in seven modern speleothems and drip waters from four caves. These show a pronounced increase or uptick in regional drip water and speleothem oxygen isotopic composition (δ18O) that is not driven by a change in rainfall δ18O values, but is in response to the shallow karst aquifers becoming disconnected from rainfall recharge due to regional drying. Our findings imply that rainfall recharge to groundwater may no longer be reliably occurring in this region, which is highly dependent on groundwater resources. Examination of the longer speleothem record shows that this situation is unprecedented over the last 800 years.

Migration of dissolved carbon on bare karst slopes in soil in response to natural rainfall events

Soil carbon in karst critical zones is an essential part of the global carbon pool, and the migration of dissolved carbon is a vital component of the carbon cycle in such regions. The unique “dual hydrological structure” makes the path and process of migration of soil dissolved carbon driven by rainfall unclear in karst slopes, which ultimately hinders the accurate evaluation of the carbon sink effect in such regions. Therefore, based on field monitoring of natural rainfall, this study evaluated the process and characteristics of the spatial migration of soil dissolved carbon in bare karst slopes and its response to natural rainfall (rainfall duration and amount). The results showed that the concentration of dissolved carbon transported in surface runoff was significantly lower than that transported within the soil layer; however, carbon loss displayed the opposite trend. Rainfall factors (rainfall amount and duration) and runoff volume significantly affected soil dissolved carbon, contributing 46.91% to soil dissolved carbon. Rainfall factors significantly affect soil dissolved carbon migration by influencing runoff. In addition, the hydrological path is an important factor affecting soil dissolved carbon transport. Our study shows that the soil and bedrock interface (SBI) and shallow karst fissures (SKF) are the main routes of dissolved carbon migration in soils in bare karst slopes. Dissolved carbon migrating along SKF is a vital contributor to the “missing carbon sink” in karst regions, and dissolved carbon migrating along the SBI is potentially an important contributor to the “missing carbon sink.” This study provides primary data and is expected to serve as a reference for revealing the mechanism of karst carbon migration and transformation and accurately evaluating the carbon sink effect in karst areas.

Effects of vegetation restoration types on soil erosion reduction of a shallow karst fissure soil system in the degraded karst areas of Southwestern China

AbstractShallow karst fissures are the main hydrological paths, underground loss channels, and one of the most critical root habitats in fragile karst rocky desertification areas. Revegetation may alter the type of vegetation cover in fissures, resulting in changes to soil properties and erosion reduction. In this study, we evaluate the influence of revegetation on the soil erosion reduction of fissure‐soil–plant systems (FSPS) for three vegetation types (herbs, shrubs, and trees) in FSPS and use bare FSPS without vegetation as a control in a karst rocky desertification area. The results show that the soil physicochemical properties of FSFS degraded first and then improved after abandonment, resulting in the order of soil erosion reduction of FSPS with vegetation succession being: shrub < herb < bare < tree. Vegetation restoration can significantly improve the soil erosion resistance of the 10–20 cm soil layer in the fissure zone, making it stronger than that in the non‐fissure zone, thus reducing the risk of underground leakage on the slope. The results obtained in this study deepen our knowledge of the impact of vegetation restoration type on soil erosion reduction and has significant implications for guiding vegetation restoration and preventing underground leakage in karst areas.

Response of OC, TN, and TP deposition mediated by aquatic photosynthetic community structures in shallow karst surface waters under different land uses

Land use change alters the hydrochemical features, nutrient outputs, and community structure of aquatic photosynthetic organisms in watersheds and has an important impact on C, N, and P biogeochemical processes. In shallow water environments, sediments are the most important burial sites for C, N, and P; however, the factors underlying the control of their deposition by land use changes remain unclear. In this study, the relationship among hydrochemical features, aquatic photosynthetic organism community structure, and C, N, and P deposition in surface waters associated with different land uses was studied at the Shawan Karst Water-Carbon Cycle Test Site, Puding, SW China, by combining field monitoring and laboratory experiments performed over a complete hydrological year from September 2018 to August 2019. The results indicate that (1) OC and TN deposition showed small differences among ponds associated with five land uses, while TP was significantly higher in ponds associated with shrubland and grassland than in ponds of cultivated land, bare soil, and bare rock. (2) Cultivated land increased OC and TN deposition by increasing N and P output and planktonic algae biomass in surface waters, while grassland and shrubland ponds mainly by increasing DIC output and macrophyte biomass. (3) Compared with cultivated land, grassland and shrubland significantly enhanced TP deposition by promoting the deposition of calcium-bound P and biogenic P from macrophytes and their epiphytic algae in surface waters. In conclusion, the shift of cultivated land and bare soil to grassland and shrubland may be conducive to the formation of benign aquatic ecosystems and stabilization of C, N, and P sinks in karst shallow surface waters.

Relationships between biomass of phytoplankton and submerged macrophytes and physicochemical variables of water in Lake Caohai, China: Implication for mitigation of cyanobacteria blooms by CO2 fertilization

Reducing nutrient input in aquatic ecosystems has been widely recognized as a way to mitigate eutrophication. The paradigm of nutrient limitation by N and P is pervasively accepted, without considering the possibility of C limitation. However, our assumption that C might affect the aquatic biological composition, lacks fieldwork support. We tested the co-limitation of C/N/P on the growth of submerged macrophytes and phytoplankton in the Caohai Lake, a typical shallow karst lake in southwestern China. This study documented the seasonal variations in the concentration and comparative availability of nutrients C/N/P, in addition to physicochemical parameters, submerged macrophytes biomass, and phytoplankton composition in the lake. Our results show that dissolved inorganic carbon plays a substantial role in enhancing the primary production of phytoplankton and macrophytes, which suggests that primary production can be suppressed not only by P and/or N (as generally believed) but also by C, owing to the unique physicochemical characteristics that prevail in typical karst regions. Additionally, submerged macrophytes have been found to inhibit phytoplankton growth. Further analysis showed that the proportion of Cyanobacteria over Chlorophyta and Bacillariophyta throughout the year was reduced at dissolved carbon dioxide/total nitrogen (CO2/TN) ratios (>0.2) and dissolved carbon dioxide/total phosphorus (CO2/TP) ratios (>10) in this study. Isotopic evidence further indicated that the abundance of Cyanobacteria was reduced when CO2 >30 µmol/L. In summary, the combined results suggest that when controlling N and P, CO2 fertilization may significantly affect both the growth and composition of phytoplankton communities/submerged macrophytes and carbon sequestration in karst surface water ecosystems. This implies a new “paradigm” in the mitigation of cyanobacteria blooms by adding more CO2 to aquatic ecosystems with low pCO2, during which carbon sequestration may also be enhanced.

Detecting hypogenic karst features in the northeastern Delaware Basin, west Texas: Applications of Full Tensor Gradient (FTG) gravity data

Extensive karst development in carbonate and evaporate strata often occur in the Upper Permian formations of the Delaware Basin, west Texas. Subsurface karst feature detection is essential since they may cause severe drilling geohazards. In order to detect subsurface karst features, we present an interpretation approach that integrates high-resolution, high-precision airborne full tensor gradiometry (FTG) gravity data, structural interpretations, density and lithological variations derived from well logs, and published seismic refraction data, with regional cross sections to produce a geologically plausible 3D FTG model in the northeastern Delaware Basin. Both traditional gravity (Tz) and FTG (Tzz) density inversions were carried out on three shallow sedimentary layers of the Ochoan-lower Permian section, demonstrating the subtle density distribution in these strata. Detailed low-density karst boundaries were then interpretated by making use of the intrinsic properties of the gradient components, Txz and Tyz, which are: 1) edge detectors, because anomaly maxima and minima are located directly over source boundaries, and 2) indicators of the direction of density change across these boundaries. Source boundaries, or faults, mapped from these components were integrated and combined with regional geology and measured densities to verify existing faults, infer new faults, and predict probable locations and shapes of karst cavities in the southwestern and northeastern parts of the study area. Our results show that the central and southwestern parts of the study area may represent higher drilling risks due to an increase in shallow karst geohazards. This interpretation approach may be used to detect fault systems and geometries of low-density or high-density source geobodies worldwide.

Evidence of the climate control of strong seismicity in the Italian Apennines through groundwater recharge

This work explores the possibility that the destructive earthquakes occurring along the Apennine Chain in Italy are systematically triggered by groundwater recharge. The focus is on multi-year transitions toward phases of wet climate rather than on short-term heavy rainfall occurring in a few days or seasonally. The analysis takes into consideration the earthquakes with a moment magnitude of Mw ≥5.8 that have occurred since 1901. Their time distribution is compared with the fluctuations of the self-calibrated Palmer Drought Severity Index (scPDSI), an indicator of soil moisture, here assumed to be a proxy for groundwater recharge. It is found that the scPDSI evolved through six main oscillations lasting from 11 to 25 years, and that, with one exception, the strongest earthquake in each phase is placed within 2 years from the maximum of soil moisture. Based on a statistical test for pairs of point processes, such a coincidence indicates a significant synchrony between the two phenomena. In particular, the two strongest earthquakes of the study period (the 1915 Marsica earthquake and the 1980 Irpinia–Basilicata earthquake, with moment magnitudes of Mw 7.1 and 6.8, respectively) occurred exactly in the year of two of the largest peaks of scPDSI. The connection between wet climate conditions and the occurrence of strong earthquakes is further investigated by comparing the time distribution of Mw ≥6.1 historical earthquakes that have occurred since 1200 AD with the evolution of the Great Aletsch Glacier in Switzerland, representative of water accumulation at the continental level. Even in this case, the earthquakes clustered during time periods of increased precipitation and lower water evaporation, corresponding to the extreme phases of the Little Ice Age. The agreement of the results at different time scales and using different climate indexes leads to postulate a significant and systematic role of groundwater recharge in the triggering of large earthquakes along the Apennines. It is also suggested that the earthquakes might be triggered by pore-pressure propagation, where the necessary hydraulic continuity is made possible by the intersection of the shallow karst structures with the seismogenic faults.

Biomass Allocation, Root Spatial Distribution, and the Physiological Response of Dalbergia odorifera Seedlings in Simulated Shallow Karst Fissure-Soil Conditions

Karst rocky desertification (KRD) is a typical fragile ecological environment with its key and difficult management point being vegetation restoration. Therefore, it is crucial to determine the adaptation mechanisms of suitable plants for ecological restoration in KRD areas. D. odorifera is a tall leguminous, woody plant with high medicinal and wood value. This study aimed to explore the adaptation strategy of the D. odorifera root system to the shallow karst fissure-soil (SKF-S) habitats. The growth, biomass, spatial root distribution, morphological characteristics, and physiological responses of D. odorifera seedlings under different treatments were studied in pots simulating SKF-S habitats. Through the experiments conducted, the following conclusions were obtained: (I) D. odorifera enhanced its ability to acquire limited resources through an allocation adjustment strategy (adjusting the biomass allocation strategy, increasing the root shoot ratio, prioritizing organ leaves and 3-level roots), which effectively offset some of the adverse effects; (II) with an increase in the stress severity, D. odorifera improved its resource acquisition adaptive strategy by reducing the root diameter and increasing the contact area with soil; (III) the spatial development characteristics of its root system were mainly manifested in the ability to grow vertically, deeper, compared to a horizontal extension; (IV) D. odorifera did not passively endure rocky desertification stress but actively improved its metabolism through root metabolic activity and SOD enzyme activity.

Water Quality Monitoring of Five Karst Springs within a Pastureland in Southwest Polk County, Missouri

ABSTRACT The fractured nature of karst terrain makes these areas highly susceptible to contamination. Besides aquifer contamination, impaired karst springs can negatively impact the aquatic habitat of their receiving streams. Five springs of a shallow karst aquifer in southwest Polk County, Missouri, and their receiving stream were selected to construct a baseline of water quality data, using nitrate (NO3) as indicator. The main land use in the study area is cattle farming. Water samples were collected during a 12-month period and analyzed for pH, turbidity, Ca, Mg, HCO3, Cl, SO4, and NO3. Results show that each spring had its own range of values. NO3 and Cl values were below contamination levels and only in a few instances exceeded threshold levels reported for karst areas (NO3-N <3 mg/L; Cl <13 mg/L), implying that the number of cattle per acre is maintained at a sustainable level and leaking from the Springfield Plateau aquifer to the underlying Ozark aquifer poses a minor threat. Interestingly, NO3 and Cl did not correlate despite their common source, suggesting that they undergo different chemical and microbiological processes along their path. The yearly nitrate export from the Little Sac River was estimated as 286.8 Mg, and the maximum N load was observed in March. Monitoring of springs and streams is recommended for this environmentally fragile area.

Past fires and post-fire impacts reconstructed from a southwest Australian stalagmite

Speleothem records of past environmental change provide an important opportunity to explore fire frequency and intensity in the past, and the antecedent climatic conditions leading to fire events. Here, fire sensitive geochemical signals in a stalagmite from Yonderup Cave, a shallow cave in Western Australia, are compared to well-documented wildfire events that occurred in recent decades. The results are extended to identify wildfires during the growth interval of the speleothem (1760 CE–2005 CE). Principal component analysis of the stalagmite time-series revealed distinct peaks in a combination of phosphorus and metal (aluminium, zinc, copper and lead) concentrations in response to known fire events, which are interpreted to have come from ash. Varying responses in the geochemical signal in the pre- and post-European colonisation period are likely linked to changes in land management, fire frequency and fire intensities. Of note is what we infer to be a particularly intense fire event concluded to have occurred in 1897 ± 5 CE, which climate sensitive proxies (δ18O and bedrock-derived elements) indicate was preceded by a multi-decadal dry period that began in the late 1860s. The intensity of the deduced fire event is supported by a peak in P that is 6.3 times higher than elsewhere in the speleothem, accompanied by a peak in Zn. It was also inferred that changes to the surface-cave hydrology occurred as a result of this event, indicated by increased input of colloidal organic material onto the stalagmite that was interpreted to be caused by shallow karst bedrock fracturing from the heat-induced deformation that is observed after intense fires. These findings show the potential for speleothems to provide dated records of fire intensity and recurrence intervals. Further development could lead to a better understanding of the climate-fire relationship and the effects of land-management practices on wildfire frequency and intensity.

Seed and Fruiting Phenology Plasticity and Offspring Seed Germination Rate in Two Asteraceae Herbs Growing in Karst Soils with Varying Thickness and Water Availability

Shallow soil with low water availability is the key limiting factor for plant growth and reproduction in vulnerable karst regions. Annual herbs are pioneers adapted to these areas; however, little is known about the responses of their seeds and infructescence, and the germination of their offspring to these limited water and soil resources. In this study, we investigated how the seed and fruiting phenology plasticity and offspring seed germination rates of two annual Asteraceae herbs (Xanthium sibiricum and Bidens pilosa) respond to the harsh karst soil environment, by assessing the seed number, seed biomass and offspring seed germination rate. X. sibiricum and B. pilosa were grown under three soil thicknesses and three water availability levels in a full two-way randomized block design. The key results were as follows: (1) The number and biomass of progenies (infructescence and seeds) of X. sibiricum decreased with the decline of soil thickness and/or water availability (P <0.05). The infructescence and seed biomass of B. pilosa increased with the decline of water availability. (2) Seed quantity and seed biomass of X. sibiricum showed no correlation after their parents experienced resource reductions. A significant positive relationship between seed number and seed biomass was observed in B. pilosa (P <0.05). (3) The offspring seed germination rate of X. sibiricum did not change with the decrease of soil thickness under three levels of water treatment. However, the offspring seed germination rate of B. pilosa decreased significantly with the decrease of soil thickness under the control water level (P<0.05). The results show that X. sibiricum tends to improve its competitiveness by ensuring the quantity and quality of offspring in order to adapt to the shallow karst soils and dry karst habitats. In contrast, B. pilosa adapts to the unfavorable karst habitats by a risk-sharing strategy. B. pilosa produces more and bigger seeds to in an attempt to expand its survival range and escape from the unfavorable living environment, but this results in a lower seed number and germination rate of its progeny under the karst soil resource reduction.

Seasonal Differences in Water-Use Sources of Impatiens hainanensis (Balsaminaceae), a Limestone-Endemic Plant Based on “Fissure-Soil” Habitat Function

The southwestern mountains of Hainan Island are the southernmost region with tropical karst landform in China. The frequent alternation of dry and wet seasons leads to the loss of the mineral nutrients of limestone, creating karst fissure habitats. Plants living in karst fissure habitats for long periods of time have developed local adaptation mechanisms correspondingly. In the paper, hydrogen–oxygen stable isotope technology was applied to determine the water-use sources of Impatiens hainanensis in the dry and wet seasons, hoping to expound the adaptation mechanism of I. hainanensis in karst fissure habitats to the moisture dynamics in the wet and dry seasons. In the wet season (May to October, 2018), the air humidity is relatively high in the I. hainanensis habitat; in the dry season (November 2018 to April 2019), there is a degree of evaporation. In the wet season, fine-root biomass increases with soil depths, while coarse-root biomass decreases with soil depths; in the dry season, fine-root biomass is lower and coarse-root biomass is higher compared with the wet season. It was found that the average rainfall reached 1523 mm and the main water-use sources were shallow (0–5 cm) and middle (5–10 cm) soil water, epikarst water, and shallow karst fissure water during the wet season; the average rainfall reached 528 mm, and the deep (10–15 cm) soil water and shallow karst fissure water were the main water-use sources during the dry season. Fog water has a partial complementary effect in the dry season. The differences in the distribution of root biomass and each source of water in the wet and dry seasons of I. hainanensis also reflect the different water-use strategies of I. hainanensis in the wet and dry seasons. In both dry and wet seasons, I. hainanensis formed a water-use pattern dominated by soil water and shallow fissure water (0–15 cm) under the influence of the “fissure-soil-plant” system in the karst region.

Long-term time series of environmental tracers reveal recharge and discharge conditions in shallow karst aquifers in Hungary and Slovakia

Study regionThree karst regions in Hungary and Slovakia. Study focusTime series of environmental tracers both in the groundwater recharge and discharge provide important insights into how a karst water system works. The aim of the present work was to study the response of discharging karst waters to recharge using time series of environmental tracers, such as tritium, stable water isotopes, noble gases and SF6. New hydrological insights for the regionOur results show that sampling frequency plays a significant role in detecting short residence times (months): the seasonality of 18O isotope composition of a selected karst spring indicates a 10 % contribution of recent water with a residence time of half a year. The contribution of an older component can be proven by the decrease of the tritium content of the waters, which compares to the decreasing trend of the tritium time series of the precipitation. However, the tritium concentrations are just slightly lower than those of the precipitation and the recharge water, hence the residence times of these shallow springs are supposed to be short. 3H/3He and SF6 apparent ages confirm this to be between 0 and 10 years, with a median of 1.4 years. Our study demonstrates that long-term time series are preferable to provide better estimation to the age distribution than individual, short-term investigations.

A Hydrostratigraphic Framework for the Paleozoic Bedrock of Southern Ontario

Groundwater systems in the intermediate to deep subsurface of southern Ontario are poorly understood, despite their value for a number of societal uses. A regional hydrostratigraphic framework is a necessary precursor for improving our understanding of groundwater systems and enabling development of a 3-D hydrostratigraphic model to visualize these groundwater systems. This study is a compilation and integration of published and unpublished geological, hydrogeological, hydrochemical and isotopic data collected over the past 10 years to develop that framework.Bedrock is covered by a thin veneer of surficial sediments that comprise an aquifer/aquitard system of considerable local variability and complexity. Aquifers in the bedrock are thin and regionally extensive, separated by thick aquitards, within a well-defined lithostratigraphic framework and a well-developed hydrochemical depth zonation comprising a shallow fresh water regime, an intermediate brackish to saline sulphur water regime, and a deep brine regime of ancient, evaporated seawater. Occurrence and movement of groundwater in shallow bedrock is principally controlled by modern (Quaternary) karstic dissolution of subcropping carbonate and evaporite rocks, and in the intermediate to deep subsurface by paleokarst horizons developed during the Paleozoic. Flow directions in the surficial sediments of the shallow groundwater regime are down-gradient from topographic highs and down the regional dip of bedrock formations in the intermediate regime. Shallow karst is the entry point for groundwater penetration into the intermediate regime, with paleo-recharge by glacial meltwater and limited recent recharge by meteoric water at subcrop edges, and down-dip hydraulic gradients in confined aquifers. Hydraulic gradient is up-dip in the deep brine regime, at least for the Guelph Aquifer and the Cambrian Aquifer, with no isotopic or hydrochemical evidence of infiltration of meteoric water and no discharge to the surface.Fourteen bedrock hydrostratigraphic units are proposed, and one unit comprising all the surficial sediments. Assignment of lithostratigraphic units as hydrostratigraphic units is based principally on hydrogeological characteristics of Paleozoic bedrock formations in the intermediate to deep groundwater regimes, below the influence of modern meteoric water. Carbonate and evaporite rocks which form aquitards in the subsurface may form aquifers at or near the surface, due to karstic dissolution by acidic meteoric water, necessitating compromises in assignment of hydrostratigraphic units.

Microbial diversity and physicochemical characteristics of tropical karst soils in the northeastern Yucatan peninsula, Mexico

Shallow karst soils can support productive forests, but their microbiota has been poorly studied in tropical areas where they accomplish relevant biogeochemical functions. Therefore, we studied the bacterial, archaeal, and fungal communities of Rendzic Leptosols from a tropical sub-humid area in the Peninsula of Yucatan (Mexico), aiming to know these microorganisms and the physicochemical characteristics that shape their communities. The soils were moderately alkaline (pH ~8.4), with a high content of organic-C (17–29%) and a clay fraction >43%. The microbial component was estimated at 2.8 × 107 16S rDNA gene copies/g (Bacteria/Archaea) and 9.3 × 104 ITS copies/g (Fungi). The microbial communities were divergent among subsites, possibly due to soil microheterogeneity, and less than 1.5% of the phylotypes were observed in at least 65% of the samples. These shared microorganisms were affiliated to phyla often found in forest soils such as Acidobacteria, Actinobacteria, Rokubacteria, and Mortierellomycota, which are reported as effective organic matter transformers. N-fixing bacteria (Bradyrhizobium) and nitrifiers (Thaumarchaeota and Nitrospira) were also detected. pH was the main factor associated with the bacterial and archaeal community structure. The functional prediction suggests that soil microorganisms have a high potential for carbon metabolism and the biosynthesis of antibiotics, besides polymer and pollutant degradation, such as phenolic substances and PAHs. Consequently, these shallow black soils harbor a diverse biosphere that can readily transform litter material and fix-N to increase the C- and N-soil pools, supporting thus a productive low forest.