Hydrogeological Processes Research Articles

Not All Rivers Are Created Equal: The Importance of Spring-Fed Rivers under a Changing Climate

In the Western United States, volcanic spring-fed rivers are anticipated to become increasingly more important for salmonids and other native fishes, as these rivers will retain coldwater habitats as the climate warms. Despite this, little is known about the hydro-biogeochemical interactions within these ecosystems. A review of existing literature on spring-fed rivers, coupled with a decade of research on volcanic spring-fed rivers of northern California, finds that these systems are exceptionally productive and exhibit stable environmental conditions. These unique conditions stem from hydrogeologic processes typical of young volcanic terrains. Aquatic macrophytes, common to some nutrient-rich spring-fed systems, play a disproportionate role in hydrologic and geomorphic processes by facilitating ecological interactions and velocity conditions that improve juvenile salmonid growth. We find that volcanic spring-fed rivers are also resilient to climate change, due not only to their ability to dampen water temperature changes through deep groundwater flow but also because of their nutrient-driven high ecosystem productivity, which may enable coldwater species to metabolically compensate for marginal increases in water temperature. Understanding the fundamental geomorphic and ecological differences between these rare ecosystems and their numerically dominant runoff rivers is essential for developing long-term conservation strategies for coldwater species under a rapidly changing climate.

Concepts on groundwater resources

When dealing with groundwater and water resources in general, various conceptual terms are used by different individuals, experts and organizations. They are often applied without a clear–cut definition. This produces poor precision in the language used, misinterpretations and uncertain evaluations that make results difficult to compare. Proposing a terminology is not a simple task if it has to agree with both the hydrogeological processes and the related environmental issues involved. However, this is a task that has to be done. Lack of precise definition of terms can be found in the Water Plans carried out by the different Water Districts in Spain, as well as in the legal norms. This also happens in Ibero-America, although in some cases glossaries are included in the administration and legal texts. This paper presents a synthetic proposal, following the sequence of hydrological–hydrogeological processes, highlighting some of the commonly used terms. Some of them may coexist, but the others should be discarded. Some of the terms to be considered when referring to groundwater are: use, demand, consumption, recharge, resource, exploitation, intensive exploitation, water mining, and recovery time. The English terms are given, and their Spanish and Catalan trnaslation.

Predicting Electrokinetic Coupling and Electrical Conductivity in Fractured Media Using a Fractal Distribution of Tortuous Capillary Fractures

Electrokinetics methods have attracted increasing interest to characterize hydrogeological processes in geological media, especially in complex hydrosystems such as fractured formations. In this work, we conceptualize fractured media as a bunch of parallel capillary fractures following the fractal size distribution. This conceptualization permits to obtain analytical models for both the electrical conductivity and the electrokinetic coupling in water saturated fractured media. We explore two different approaches to express the electrokinetic coupling. First, we express the streaming potential coupling coefficient as a function of the zeta potential and then we obtain the effective charge density in terms of macroscopic hydraulic and electrokinetic parameters of porous media. We show that when the surface electrical conductivity is negligible, the proposed models reduces to the previously proposed one based on a bundle of cylindrical capillaries. This model opens up a wide range of applications to monitor the water flow in fractured media.

Effect of slippery boundary on solute transport in rough-walled rock fractures under different flow regimes

Fundamental understanding of solute transport behaviors in rock fractures is of great importance to many hydrogeological processes. The fate of fluid-borne solutes is inherently linked to fluid flow process in rock fractures, which is usually theoretically and numerically analyzed under the assumption of classic no-slip boundary condition. However, fluid slippage at rock surfaces is possibly present in some geological environments, such as hydrophilicity change in rock surfaces exposed to organic substances, or the immobile wetting film covering rock surfaces. These situations would induce a non-zero velocity at the boundary walls for flowing fluid, i.e., fluid slippage, which leads to a violation of the no-slip assumption. Nonetheless, the effect of slippery boundary on fluid-borne solute transport is poorly understood for rock fractures. This study systematically investigated the slippery boundary effect on solute transport in rough-walled rock fractures under different flow regimes. A series of pore-scale simulation results showed that the slippery boundary has a profound influence on both microscale and macroscale solute transport behaviors in rock fractures. Such an influence became more significant with increasing hydraulic gradient, accompanied by the flow regime transitioning from Darcy to non-Darcian. For Darcy flow regime, the slippery boundary exerted its influence on solute transport by altering velocity distribution pattern in rock fracture; while for non-Darcian flow regime, the enlarged eddy volume and promoted mass transfer process by the slippery boundary profoundly affected solute transport behaviors. Further discussion indicated that the predictive models with parameters estimated from the conventional no-slip boundary condition cannot directly apply to the slip one. Moreover, the effect of slippery boundary would possibly have an impact on larger-scale fractured rock aquifers which requires further verification. The results and findings enrich the understanding of solute transport in fractured rocks where the slippery boundary is prevalently present, and contribute to developing predictive models for complex geological environments.

The use of regional and alluvial groundwater by riparian trees in the wet‐dry tropics of northern Australia

AbstractRiparian trees play a critical role in the ecological function of rivers, yet are threatened by anthropogenic change to the hydrological cycle. Identifying the sources of water used by riparian trees can inform sustainable water policy. We used isotopic analysis complemented by measurements of plant water relations to assess water sources for riparian trees at two sites with contrasting hydrogeological processes; one with an alluvial aquifer overlaying an aquitard, and one where fault‐induced preferential pathways in the aquitard allowed the flow of deeper, older groundwater from a regional aquifer to the alluvium. At both sites, plant water potential, stomatal conductance, and plant water isotope composition in the xylem sap of riparian trees were collected from two landscape positions, the riverbank and floodplain. We used a Bayesian mixing model (MixSIAR) to assess differences in the proportion of water sources for sites and landscape positions. We found that xylem water isotope values differed between the two sites in line with their hydrogeological characteristics, with trees at the regional aquifer site using water sourced from the regional groundwater and trees at the site with only an alluvial aquifer present using a mixture of water sources, with no dominant source identified. Higher plant predawn water potential values at the regional site indicated greater water availability and support the inference that plants were using more groundwater at the regional site compared to the alluvial site. Trees closer to the river had higher isotope values, indicative of surficial water sources i.e. shallow soil water and river water. Our findings show that the water sources used by riparian trees reflect local hydrogeology and resource availability. Water managers should identify and protect plant water sources to ensure maintenance of riparian trees.

The use of Radon (Rn222) isotopes to detect groundwater discharge in streams draining Table Mountain Group (TMG) aquifers

Environmental isotopes have been used for decades as natural tracers in studies aimed at understanding complex hydrogeological processes such as groundwater and surface water interactions. Radon (Rn222) is a naturally occurring, radioactive isotope which is produced from radium (Ra226) during the radioactive decay series of uranium (U238). Since U238 is present in most geological substrates, Rn222 is produced in various lithological structures and subsequently transported with groundwater through fractures and pore spaces in an aquifer towards surface water discharge points in rivers and springs. This study aimed to determine (i) the concentration of Rn222 within both surface water and groundwater in Table Mountain Group (TMG) aquifer systems, and (ii) the feasibility of using Rn222 isotopes as a natural tracer in groundwater-surface water interaction studies. This study was conducted in a highly fractured TMG aquifer system near Rawsonville, South Africa. Surface water from two perennial rivers (i.e. Gevonden and Molenaars), together with groundwater from a nearby borehole, were sampled and their corresponding Rn222 concentrations measured. Our study found median Rn222 concentrations in the Gevonden River of 76.4 Bq∙L-1 and 67.2 Bq∙L-1 in the dry and wet seasons, respectively. Nearly 12% of surface water samples exceeded 100 Bq∙L-1. These abnormally high Rn222 concentrations can only be attributed to the influx of groundwater with extremely high Rn222 concentrations. Under ambient (no pumping) conditions, Rn222 concentrations in groundwater range between 130–270 Bq∙L-1. However, when the borehole was pumped, and inflowing water from the surrounding aquifer was sampled, even higher Rn222 concentrations (391–593 Bq∙L-1) were measured. These extremely high Rn222 concentrations in groundwater are believed to be attributed to the underlying granitic geology and the prevalence of faults. The use of Rn222 isotopes as an environmental tracer in groundwater–surface water interaction studies is therefore regarded as a feasible option in similar highly fractured aquifer systems.

Analysis of the Hydrogeological Processes formation Condition in the Baku City Zone and Evaluation of the Subsoil Waters Suitability for Verdures Irrigation for Using

Analysis of the Hydrogeological Processes formation Condition in the Baku City Zone and Evaluation of the Subsoil Waters Suitability for Verdures Irrigation for Using

Hydrochemical Zoning and Chemical Evolution of the Deep Upper Jurassic Thermal Groundwater Reservoir Using Water Chemical and Environmental Isotope Data

A comprehensive hydrogeological understanding of the deep Upper Jurassic carbonate aquifer, which represents an important geothermal reservoir in the South German Molasse Basin (SGMB), is crucial for improved and sustainable groundwater resource management. Water chemical data and environmental isotope analyses of δD, δ18O and 87Sr/86Sr were obtained from groundwater of 24 deep Upper Jurassic geothermal wells and coupled with a few analyses of noble gases (3He/4He, 40Ar/36Ar) and noble gas infiltration temperatures. Hierarchical cluster analysis revealed three major water types and allowed a hydrochemical zoning of the SGMB, while exploratory factor analyses identified the hydrogeological processes affecting the water chemical composition of the thermal water. Water types 1 and 2 are of Na-[Ca]-HCO3-Cl type, lowly mineralised and have been recharged under meteoric cold climate conditions. Both water types show 87Sr/86Sr signatures, stable water isotopes values and calculated apparent mean residence times, which suggest minor water-rock interaction within a hydraulically active flow system of the Northeastern and Southeastern Central Molasse Basin. This thermal groundwater have been most likely subglacially recharged in the south of the SGMB in close proximity to the Bavarian Alps with a delineated northwards flow direction. Highly mineralised groundwater of water type 3 (Na-Cl-HCO3 and Na-Cl) occurs in the Eastern Central Molasse Basin. In contrast to water types 1 and 2, this water type shows substantial water-rock interaction with terrestrial sediments and increasing 40Ar/36Ar ratios, which may also imply a hydraulic exchange with fossil formation waters of overlying Tertiary sediments.

Assessment of spring flows in Indian Himalayan micro-watersheds – A hydro-geological approach

Springs are an indispensable source of freshwater for mountain communities in the Indian Himalayan Region (IHR). Owing to the synergistic impact of anthropogenic and climatic factors, numerous perennial springs and streams in the region, are either becoming ephemeral or drying out, thus impacting the local people. Water scarcity, poverty, and limited scope of alternate livelihoods further reduce the communities’ resilience. Our focus is to assess the potential of reviving drying springs with the help of hydro-geological studies in water-scarce villages. The methodology involved high-resolution data monitoring of springs and first-order streams in two headwater micro-watersheds, namely, Shiv gadera and Haraita, in the rural Himalayas of Uttarakhand, India, with unalike topography and geology. To understand the hydro-geological processes and assess the flow regimes and aquifer storage dynamics, we applied water balance, correlation, flow duration, master recession curves analysis, and geological studies. The univariate and bivariate analysis shows that Shiv gadera has a better system memory, indicating a larger storage capacity than Haraita. The spring hydrograph responses also append that Shiv gadera has better storage and has a homogenous aquifer feed. The water balance, however, is in positive storage only during the rainy months in both the sites. The hydro-geological characterization from hydrograph analysis, recession analysis, and field surveys shows that Shiv gadera has intricate flow networks and slow flow velocities while Haraita is characteristic of transmissive fractured rocks. The spring flows in Shiv gadera are observed to be perennial and more groundwater contributes to spring discharges while Haraita exhibits intermittent to ephemeral nature. The recession curves also indicate uniform geology, a distinctive feed from recharge area, and slow emptying of the aquifer, while Haraita exhibits shallow storage and quick responses to storms. Spring flows in Shiv gadera show better stability than Haraita, as indicated by Q10/Q90 and Q50/Q90 measurements. These inferences qualify Shiv gadera as having a better chance of responding to management and treatments, thus a better potential for revival. The combination of hydrologic time series analysis and geological characterization used in this study could be a valuable approach for assessing spring revival in the IHR and has a potential for implementation across other parts of the Himalayas.

Effects of large-scale heterogeneity and temporally varying hydrologic processes on estimating immobile pore space: A mesoscale-laboratory experimental and numerical modeling investigation

The advection-dispersion equation (ADE) often fails to predict solute transport, in part due to incomplete mixing in the subsurface, which the development of non-local models has attempted to deal with. One such model is dual-domain mass transfer (DDMT); one parameter that exists within this model type is called immobile porosity. Here, we explore the complexity of estimating immobile porosity under varying flow rates and density dependencies in a large-scale heterogeneous system. Immobile porosity is estimated experimentally and using numerical models in 3-D flow systems, and is defined by domains of comparatively low advective velocity instead of truly immobile regions at the pore scale. Tracer experiments were conducted in a mesoscale 3-D tank system with embedded large impermeable zones and the generated data were analyzed using a numerical model. The impermeable zones were used to explore how large-scale structure and heterogeneity affect parameter estimation of immobile porosity, assuming a dual-porosity model, and resultant characterization of the aquifer system. Spatially and temporally co-located fluid electrical conductivity (σf) and bulk apparent electrical conductivity (σb)—using geophysical methods—were measured to estimate immobile porosity, and numerical modeling (i.e., SEAWAT and R3t) was conducted to explore controls of the immobile zones on the experimentally observed flow and transport. Results showed that density-dependent flow increased the hysteresis between measured fluid and bulk electrical conductivity, resulting in larger interpreted immobile pore-space estimates. Increasing the dispersivity in the model simulations decreased the estimated immobile porosity; flow rate had no impact. Overall, the results of this study highlight the difficulty faced in determining immobile porosity values in field settings, where hydrogeologic processes may vary temporally. Our results also highlight that immobile porosity is an effective parameter in an upscaled model whose physical meaning is not necessarily clear and that may not align with intuitive interpretations of a porosity.

Toward Using Seismic Interferometry to Quantify Landscape Mechanical Variations after Earthquakes

ABSTRACT In mountainous terrain, large earthquakes often cause widespread coseismic landsliding as well as hydrological and hydrogeological disturbances. A subsequent transient phase with high landslide rates has also been reported for several earthquakes. Separately, subsurface seismic velocities are frequently observed to drop coseismically and subsequently recover. Consistent with various laboratory work, we hypothesize that the seismic-velocity changes track coseismic damage and progressive recovery of landscape substrate, which modulate landslide hazard and hydrogeological processes, on timescales of months to years. To test this, we analyze the near-surface seismic-velocity variations, obtained with single-station high-frequency (0.5–4 Hz) passive image interferometry, in the epicentral zones of four shallow earthquakes, for which constraints on landslide susceptibility through time exist. In the case of the 1999 Chi-Chi earthquake, detailed landslide mapping allows us to accurately constrain an exponential recovery of landslide susceptibility with a relaxation timescale of about 1 yr, similar to the pattern of recovery of seismic velocities. The 2004 Niigata, 2008 Iwate, and 2015 Gorkha earthquakes have less-resolved constraints on landsliding, but, assuming an exponential recovery, we also find matching relaxation timescales, from ∼0.1 to ∼0.6 yr, for the landslide and seismic recoveries. These observations support our hypothesis and suggest that systematic monitoring of seismic velocities after large earthquakes may help constrain and manage the evolution of landslide hazard in epicentral areas. To achieve this goal, we end by discussing several ways to improve the link between seismic velocity and landscape mechanical properties, specifically by better constraining time-dependent near-surface strength and hydrogeological changes. Hillslopes displaying coseismic surface fissuring and displacement may be an important target for future geotechnical analysis and coupled to passive geophysical investigations.

Avaliação hidrogeológica de nascentes na Grande Bacia Artesiana centro-sul da Austrália

Artesian springs support important wetland ecosystems comprising highly specialised, endemic animal and plant species. Understanding the hydrogeological processes that sustain springs is critical to their management and characterisation of the connected groundwater system. This paper focuses on the springs of the Eulo and Bourke supergroups in the south-central part of the Great Artesian Basin in Queensland and New South Wales, Australia. These springs have been heavily impacted by aquifer drawdown, as a result of uncontrolled discharge of artesian water from pastoral bores. This study uses a stratigraphic model of the aquifer and aquitard sediments, a model of the potentiometric surface, existing information on the coverage of basement and surface structures, and a comprehensive field survey of all known springs to identify the association of springs with the edge of the geological basin, basement outcrop, basement highs, surface and deep basement faults, and a thinned aquitard. Many springs in the Eulo Supergroup are associated with basement highs and many springs are associated with deep faults, suggesting that reactivation of these faults may be an important cause of spring discharge. Springs in the Bourke Supergroup previously thought to be outcrop springs (gravity-fed), by virtue of their surface characteristics, are more likely to be artesian springs based on the interpretation of basement geology, surface faults and water chemistry, which align most closely to the Hooray Sandstone, the artesian aquifer underlying the springs.

Groundwater Level Forecast Via a Discrete Space-State Modelling Approach as a Surrogate to Complex Groundwater Simulation Modelling

Reliable and precise forecasts of future groundwater level fluctuations are crucial constituents of sustainable management of scarce water resources and design of remediation plans. Groundwater simulations and predictions are often performed by employing physically based models, which are not applicable in a majority of water scarce areas around the globe, particularly in the developing countries like Bangladesh due to data limitations. On the other hand, data-driven statistical forecast models have demonstrated their suitability to model nonlinear and complex hydrogeological processes to forecast short- and long-term groundwater level fluctuations. The purpose of this effort is to propose a non-physical based approach by utilizing a discrete Space-State model as a prediction tool to forecast future scenarios of groundwater level fluctuations. The present study utilizes the prediction focused approach of the system identification process in which the overall objective is to develop a pragmatic dynamic system model. The performance of the proposed approach is evaluated for groundwater level data at three observation wells of Tanore upazilla in Rajshahi district, Bangladesh. Historical weekly time series data of groundwater level fluctuations from the three observation wells for 39 (1980–2018) years is used to develop the time series model, which is used for future groundwater level predictions for a period of next 22 years (up to 2040). The findings demonstrate the conceivable applicability of the proposed discrete Space-State modelling approach in forecasting future scenarios of groundwater level fluctuations in the selected observation wells.

Prediction of hydrogeomechanical processes in undermining of water bodies

Rivers, arable land, engineering structures, urban buildings, and communications often fall into the zone of influence of mining operations. Insufficiently justified mining development often leads to negative environmental and social consequences of the violation or destruction of these objects. On the other hand, surface and underground waters often complicate the development of minerals, negatively affecting the safety of production. The paper deals with the interaction of hydrogeological and geomechanical processes near water bodies in complex mining and hydrogeological conditions. One field of research is based on the development of an engineering approach to the analysis of the interaction of hydrogeological and deformation processes. In this case, the results of instrumental field observations and experience of mining operations near water bodies are used. In this approach, it is assumed that six zones can be distinguished in the under-worked rock mass: collapsed rock; through cracks; partially through cracks; individual cracks that do not form a single system; the zone without discontinuities of continuity; the zone of high stresses and compression deformations. As an example of the application of this approach, the development of a mineral deposit under a river is considered and recommendations are made for the preservation of the earth’s surface from waterlogging. The second field of research is related to mathematical modeling and the use of crack theory. The results of the simulation showed that a spontaneous hydraulic fracturing crack may develop in the underworked rock mass, which can serve as a main channel for the penetration of underground or surface water into the mine workings. The nature of spontaneous hydraulic fracturing of rocks is determined by the fact that when a man-made impact on the rock mass, one of the main stresses of the rock mass can become less than the hydrostatic pressure of water. In this case, a hydraulic fracturing crack develops in the area where this condition is met. As an example, the breakthrough of water from a surface reservoir into an underground mine is considered. It is shown that at a low crack resistance of rocks, it is possible for a crack to grow into the mine, which may result in a sudden water breakthrough. The acquired knowledge expands the scientific basis for the development of appropriate supplements to the instructions for mining operations under water bodies.

A new rating system for hydrogeological risk management along railway infrastructures in Prealpine zone (northern Italy)

Railway infrastructures in mountain areas often develop along hillslopes affected by geomorphological and hydrogeological processes which might lead hazardous events. Therefore, specific tools for risk analysis and management are required. This paper develops a new rating system (Railway Hydrogeological Management System, RHMS), based on a heuristic method which considers the susceptibility to different types of slope instabilities, as well as the peculiar features affecting the railway vulnerability. The proposed method introduces an iterative approach for the risk assessment, based on the definition of acceptability thresholds for the residual risk. The application of this method to a test area pointed out its feasibility, as well as its operational capability to identify the critical sections of the infrastructure, in which protection or mitigation measures are needed in order to reduce the risk.

Tephrochronology and micromorphology of Theran tephra deposits at Palaikastro, Crete

This study uses tephrochronology and soil micromorphology to analyze the geochemical and physical properties of five tephra deposits found in Late Minoan IA contexts (excavated in 2003 and 2015) at the Bronze Age site of Palaikastro, Crete, Greece. Results confirm that the tephra originated from the Minoan Theran eruption and establish relative chronologies for LM IA site areas containing the Theran tephra. Micromorphological observations indicate that the tephra was initially deposited by aeolian processes and was redeposited or reworked by various hydrogeological processes into secondary depositional contexts. Findings are contextualized by comparison with data from Theran samples from Thera (Santorini) and from other East Cretan sites. Results signal that the settlement area at Palaikastro was partially or completely abandoned before tephra deposition, and the area and its surrounding landscape remained abandoned in the immediate aftermath of the Theran eruption.

Acquiring vulnerability indicators to geo-hydrological hazards: An example of mobile phone-based data collection

Geo-hydrological risk reduction is a key issue for local governments in Italy. In this context, a collaboration was undertaken between multiple actors in the La Spezia municipality aimed at: (i) monitoring building characteristics, using specific and valuable indicators, and (ii) increasing the knowledge of geo-hydrological hazards across residents and local land planners (iii) implementing local emergency civil protection plan. An extensive mobile data collection was carried out through apps specifically developed for Android and IOS mobile devices. The digital forms were differentiated on the basis of the potential hazard: one of 46 fields and one of 125 fields designed for buildings respectively located in flood prone areas and in medium to very high landslide susceptibility areas. The digital version of the forms was designed using the Open Data Kit (ODK) and GISCloud client-server approach. All the collected data, including geospatial locations and images, were automatically sent to a central server, stored and organized in a database. Geospatial data-analysis and maps resulted useful in evaluating possible impacts to exposed buildings to potential geo-hydrological processes. The proposed public participation method for data-gathering increased the knowledge across residents providing a better understanding of the urban systems, of the buildings condition and their relation respect to the geo-hydrological risk. The method can be considered as part of the decision support systems for civil protection purposes to better planning geo-hydrological mitigation measures. The application of mobile technology for data collection can be effectively used when local government resources are limited.

A New Approach in Determining the Decadal Common Trends in the Groundwater Table of the Watershed of Lake “Neusiedlersee”

Shallow groundwater is one of the primary sources of fresh water, providing river base-flow and root-zone soil water between precipitation events. However, with urbanization and the increase in demand for water for irrigation, shallow groundwater bodies are being endangered. In the present study, 101 hydrographs of shallow groundwater monitoring wells from the watershed of the westernmost brackish lake in Europe were examined for the years 1997–2012 using a combination of dynamic factor and cluster analyses. The aims were (i) the determination of the main driving factors of the water table, (ii) the determination of the spatial distribution and importance of these factors, and (iii) the estimation of shallow groundwater levels using the obtained model. Results indicate that the dynamic factor models were capable of accurately estimating the hydrographs (avg. mean squared error = 0.29 for standardized water levels), meaning that the two driving factors identified (evapotranspiration and precipitation) describe most of the variances of the fluctuations in water level. Both meteorological parameters correlated with an obtained dynamic factor (r = −0.41 for evapotranspiration & r = 0.76 for precipitation). The strength of these effects displayed a spatial pattern, as did the factor loadings. On this basis, the monitoring wells could be objectively distinguished into two groups using hierarchical cluster analysis and verified by linear discriminant analysis in 98% of the cases. This grouping in turn was determined to be primarily related to the elevation and the geology of the area. It can be concluded that the application of the data analysis toolset suggested herein permits a more efficient, objective, and reproducible delineation of the primary driving factors of the shallow groundwater table in the area. Additionally, it represents an effective toolset for the forecasting of water table variations, a quality which, in the view of the likelihood of further climate change to come, is a distinctive advantage. The knowledge of these factors is crucial to a better understanding of the hydrogeological processes that characterize the water table and, thus, to developing a proper water resource management strategy for the area.

Usage of antecedent soil moisture for improving the performance of rainfall thresholds for landslide early warning

Landslides triggered by heavy rains are increasing in number and creating severe losses in hilly regions across the world. Rainfall thresholds on regional and local-scales are being used for forecasting such events, for efficient early warning. Empirical and probabilistic approaches for defining rainfall thresholds are traditional tools which are being used as part of the forecasting system for rainfall induced landslides. Such methods are easy-to-use and are based on statistical analyses. They can be derived without looking into the complex hydro-geological processes involved in slope failures, but are often associated with the disadvantage of higher false alarms, limiting their applications in a regional landslide early warning system (LEWS). This study is an attempt to improve the performance of conventional meteorological thresholds by considering the effect of soil moisture, using a probabilistic approach. Idukki district in southern part of India is highly susceptible to landslides and has witnessed major socio-economical setbacks in the recent disasters happened in 2018 and 2019. This tourist hub is now in need of a landslide forecasting system, which can help in landslide risk reduction. This study attempts to understand the effect of averaged soil moisture estimates derived from passive microwave remote sensing data, for improving the performance of conventional empirical and probabilistic thresholds. For defining empirical thresholds, an algorithm-based approach such as Calculation of Thresholds for Rainfall-induced Landslides Tool (CTRL-T) has been used. Probabilistic thresholds were defined using a Bayesian approach, finding the posterior probability of occurrence using the marginal and conditional probabilities of the control parameters along with the prior probability of occurrence of landslide. The derived rainfall thresholds were quantitatively compared with the Bayesian probabilistic threshold derived using rainfall severity and soil wetness using an area under the curve (AUC) based on receiver operating characteristics (ROC) curve method. The results show that when the antecedent moisture content in soil is less, only severe rainfall events can trigger landslides in the study area; while less severe rainfall events can also trigger landslides when the soil is wet. The role of soil wetness in the initiation is used to improve the performance of the conventional methods, and a ROC approach was used for the statistical comparison of different models. Further, the results indicated that the probabilistic threshold using rainfall severity and soil wetness outperformed the conventional approaches with AUC of 0.96, being the most sensitive and specific among the models considered. This result opens new promising perspectives for the development of an operational LEWS in the Idukki district based on a combination of rainfall and soil moisture data. Moreover, this work contributes to strengthen the advancing trend of hydro-meteorological thresholds based on soil moisture, which is gaining a growing attention in landslide studies and that, to date, was lacking evidences in monsoon regions.

Spatial distribution, multivariate statistical analysis, and health risk assessment of some parameters controlling drinking water quality at selected primary schools located in the southwestern coastal region of Bangladesh

The southwestern coastal areas of Bangladesh have been severely suffering from the scarcity of safe drinking water due to the combined consequences of hydrogeological processes, groundwater depletion, saltwater intrusion, and adverse anthropogenic activities. This concern results in serious public health hazards, particularly for children at primary schools. This study was conducted to evaluate some physiochemical and bacteriological parameters of drinking water from tube wells (TWs) located at primary schools in Kalapara upazila, Patuakhali district, Bangladesh. Water samples were collected from 65 TWs. The levels of the examined parameters recorded the following ranges: pH 6.9–8.3, electrical conductivity 774.3–1987.0 μS/cm, Fe 0.12–15.78 mg/L, As 0.0–0.03 mg/L, and Cl− 9.0–354.0 mg/L. The results revealed that 11% of TWs exceeded the level of Fe recommended by the National Drinking Water Quality Standard in Bangladesh, while other physiochemical parameters fit with the national standards. The bacteriological analysis revealed that 60% of TWs delivered fecal coliform (FC)-free water, while 40% of TWs supplied total coliform (TC)-free water. However, about 26% of TWs had the potential to pose non-carcinogenic health problems for children from As, while only 5% of TWs were found to cause probable non-carcinogenic risks for adults. The calculated carcinogenic risks values for adult male and female were within the USEPA acceptable range (1 × 10−4–1 × 10−6), except for only one sampling point, while four sampling points (6.15% of TWs) exhibited carcinogenic risks for children. This result indicates that the ingestion of drinking water from those TWs may pose lifetime cancer risks to children.