Risk of groundwater contamination widely underestimated because of fast flow into aquifers.

Groundwater pollution threatens human and ecosystem health in many regions around the globe. Fast flow to the groundwater through focused recharge is known to transmit short-lived pollutants into carbonate aquifers, endangering the quality of groundwaters where one quarter of the world's population lives. However, the large-scale impact of such focused recharge on groundwater quality remains poorly understood. Here, we apply a continental-scale model to quantify the risk of groundwater contamination by degradable pollutants through focused recharge in the carbonate rock regions of Europe, North Africa, and the Middle East. We show that focused recharge is the primary reason for widespread rapid transport of contaminants to the groundwater. Where it occurs, the concentration of pollutants in groundwater recharge that have not yet degraded increases from <1% to around 20 to 50% of their concentrations during infiltration. Assuming realistic application rates, our simulations show that degradable pollutants like glyphosate can exceed their permissible concentrations by 3 to 19 times when reaching the groundwater. Our results are supported by independent estimates of young water fractions at 78 carbonate rock springs over Europe and a dataset of observed glyphosate concentrations in the groundwater. They imply that in times of continuing and increasing industrial and agricultural productivity, focused recharge may result in an underestimated and widespread risk to usable groundwater volumes.

Field Tracer Tests to Evaluate Transport Properties of Tryptophan and Humic Acid in Karst.

The monitoring of water quality, especially of karst springs, requires methods for rapidly estimating and quantifying parameters that indicate contamination. In the last few years, fluorescence-based measurements of tryptophan and humic acid have become a promising tool to assess water quality in near real-time. In this study, we conducted comparative tracer tests in a karst experimental site to investigate the transport properties and behavior of tryptophan and humic acid in a natural karst aquifer. These two tracers were compared with the conservative tracer uranine. Fluorescence measurements were conducted with an online field fluorometer and in the laboratory. The obtained breakthrough curves (BTCs) and the modeling results demonstrate that (1) the online field fluorometer is suitable for real-time fluorescence measurements of all three tracers; (2) the transport parameters obtained for uranine, tryptophan, and humic acid are comparable in the fast flow areas of the karst system; (3) the transport velocities of humic acid are slower and the resulting residence times are accordingly higher, compared to uranine and tryptophan, in the slower and longer flow paths; (4) the obtained BTCs reveal additional information about the investigated karst system. As a conclusion, the experiments show that the transport properties of tryptophan are similar to those of uranine while humic acid is partly transported slower and with retardation. These findings allow a better and quantitative interpretation of the results when these substances are used as natural fecal and contamination indicators.

Experimental field evidence for transport of microplastic tracers over large distances in an alluvial aquifer.

The transport of microplastic tracer particles in comparison to the solute conservative tracer uranine was experimentally investigated in a shallow alluvial aquifer over distances from 3.1 to 200 m by means of a natural-gradient tracer test. The microplastic particles (MPs) with diameters of 1, 2 and 5 µm were artificially injected into an observation well to simulate microplastic transport; water samples were taken at eleven observation wells further downgradient over a time span of 171 days. In total, 44 individual breakthrough curves of microplastics and uranine were obtained at all sampling sites, allowing a detailed analysis of the size-dependency of microplastics transport in porous media at field scale. Results clearly show that (i) microplastics of 1-5 µm can be transported in significant amounts in sand-and-gravel aquifers; (ii) peak concentrations of microplastics can exceed those of conservative solutes, in particular for longer flow distances; (iii) microplastic peak velocities are in a similar range or exceed those of conservative solutes; (iv) retardation and filtration processes did not efficiently attenuate microplastics in groundwater at the study site. To our best of knowledge, this is the first experimental field evidence for microplastics transport over large distances in an alluvial aquifer.

Author Correction: Global karst springs hydrograph dataset for research and management of the world's fastest-flowing groundwater.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Physicochemical and major ion data for springs in the Black Forest National Park, Germany.

The dataset in this article consists of the general physicochemical parameters (temperature, pH, specific electrical conductivity, dissolved oxygen, redox potential, alkalinity) and concentrations of major ions (Ca2+, Mg2+, K+, Na+, Cl-, SO4 2-, NO3 -) of water samples collected at 19 springs and the surface stream in the water catchment area of the upper Schönmünz river in the Black Forest National Park, Germany. Data on concentrations of dissolved organic carbon (DOC), total organic carbon (TOC), spectral absorbance at different wavelengths and fluorescence as well as microbiological indicators (E. coli, total coliforms, enterococci) are also reported. Sampling was conducted during five field campaigns between spring 2016 and spring 2017. Knowledge of the current physicochemical parameters and concentrations of dissolved organic and inorganic constituents provides a baseline to assess future changes and serves as a supplement to ongoing studies of the spring ecosystems. Understanding the specific processes influencing the water chemistry will aid in their effective protection. For more details and further discussion on this dataset, the reader is referred to the associated research article "Processes controlling spatial and temporal dynamics of spring water chemistry in the Black Forest National Park" [1].

Experimental and modeling evidence of kilometer-scale anomalous tracer transport in an alpine karst aquifer.

Karst aquifers are important drinking water resources, but highly vulnerable to contamination. Contaminants can be transported rapidly through a network of fractures and conduits, with only limited sorption or degradation, which usually leads to a fast and strong response at karst springs. During migration, contaminants can also enter less mobile zones, such as pools or water in intra-karstic sediments, or advance from conduits into the adjacent fractured rock matrix. As contaminant concentrations in the main flow path(s) decrease, contaminants may migrate back into the main flow path and reach the karst springs at low (but significant) concentrations over a long time span. This is the conventional interpretation for the oft-observed steep rising limb and the long-tailed falling limb of tracer breakthrough curves in karst systems. Here, field measurements are examined from an alpine karst system in Austria where a series of distinctive, long-tailed breakthrough curves (BTCs) of conservative tracers were observed over distances up to 7400 m. Recognizing that the conventional advection-dispersion equation (ADE) cannot usually quantify such behavior, two other modeling approaches are considered, namely the two-region non-equilibrium (2RNE) model, which explicitly includes mobile and immobile zones, and a continuous time random walk (CTRW) model, which is based on a physically-based, probabilistic approach that describes anomalous (or non-Fickian) transport behavior characteristic of heterogeneous systems such as karst. In most cases, the ADE and 2RNE models do not quantify the low concentrations at longer travel times. The CTRW, in contrast, accounts for the long-tailed breakthrough behavior found in this karst system.

Assessment of the impact of geogenic and climatic factors on global risk of urinary stone disease.

Urinary Stone Disease (USD) or urolithiasis has plagued humans for centuries, and its prevalence has increased over the past few decades. Although USD pathology could vary significantly among individuals, previous qualitative assessments using limited survey data demonstrated that the prevalence of USD might exhibit a distinctive geographical distribution (the so-called "stone belt"), without any knowledge about the characteristics and contribution factors of the belt. Here, we argue that the spatial distribution of USD can at least partly be explained by geogenic and climatic factors, as it correlates with the ambient geo-environmental conditions modulated by lithology/mineralogy, water quality and climate. Using a Bayesian risk model, we assessed the global risk of USD based on updated big data of four key geogenic factors: phosphorite mines (inventory >1600 points), carbonate rocks (at the scale of 1:40 million), Ca2+/Mg2+ molar ratio of river water (1.27 million samples distributed over 17,000 sampling locations), and mean air temperature (0.5o × 0.5° resolution) representing the climate. We quantitatively identified possible contributions of the factors to USD and delineated the regions with the high USD risk which stretched from southern North America, via the Mediterranean region, northeastern Africa, southern China to Australia, and roughly coincide with the world's major areas of carbonate outcropping. Under current climate conditions, the areas with the probabilities for the USD prevalence of ≥50% and ≥30% covered 3.7% and 20% of the Earth's land surface, respectively. By the end of the 21st century, such total areas could rise to 4.4% and 25% as a result of global warming. Since the USD data used in this study were quite heterogeneous, the prediction results needed further calibration with additional high-quality prevalence data in the future.

Processes controlling spatial and temporal dynamics of spring water chemistry in the Black Forest National Park.

Spring water chemistry is influenced by many factors, including geology, climate, vegetation and land use, which determine groundwater residence times and water-rock interaction. Changes in water chemistry can have a profound impact on their associated ecosystems. To protect these ecosystems and to evaluate possible changes, knowledge of the underlying processes and dynamics is important. We collected water samples at 20 locations during 5 campaigns within the water catchment area of the upper Schönmünz river in the Black Forest National Park, Southwest Germany and analyzed them hydro-chemically for their contents of inorganic constituents, organic carbon content, fluorescence properties as well as several physico chemical field parameters and spring discharge. Results show that water chemistry is strongly dependent on geology and that the response of dissolved organic carbon to changes in hydraulic conditions is highly dynamic. Due to increased flow through the upper soil layer during and after rain events, more organic carbon is extracted from the soil and transported with the water. Fluorescence EEM measurements indicate an allochthonous source of this organic carbon. This study can be used as baseline to assess future changes and serve as a supplement to ongoing studies of the spring ecosystems.

Global karst springs hydrograph dataset for research and management of the world's fastest-flowing groundwater.

Karst aquifers provide drinking water for 10% of the world's population, support agriculture, groundwater-dependent activities, and ecosystems. These aquifers are characterised by complex groundwater-flow systems, hence, they are extremely vulnerable and protecting them requires an in-depth understanding of the systems. Poor data accessibility has limited advances in karst research and realistic representation of karst processes in large-scale hydrological studies. In this study, we present World Karst Spring hydrograph (WoKaS) database, a community-wide effort to improve data accessibility. WoKaS is the first global karst springs discharge database with over 400 spring observations collected from articles, hydrological databases and researchers. The dataset's coverage compares to the global distribution of carbonate rocks with some bias towards the latitudes of more developed countries. WoKaS database will ensure easy access to a large-sample of good quality datasets suitable for a wide range of applications: comparative studies, trend analysis and model evaluation. This database will largely contribute to research advancement in karst hydrology, supports karst groundwater management, and promotes international and interdisciplinary collaborations.

RAYLEIGH WAVE AND WELL HEAD RESPONSE TO CALCULATE POROSITY IN THE EDWARDS AQUIFER OF SOUTH-CENTRAL TEXAS, USA.

We use the magnitude and centroid period of Rayleigh wave along with the amplitude of fluctuations of water level in a well to calculate effective porosity of a karst aquifer at the site scale. The radial and vertical displacements of Rayleigh wave are first related to the confining pressure of rock, which is then related to fluid pressure via the Gassmann equation. Three seismograms recorded at station 633A of the USARRAY and the induced responses of Well J-17 in the Edwards Aquifer (Texas) allow the calculation of an effective porosity between 17.0 and 24.4 percent, the average of which is close to the total porosity of core samples determined by geophysical well logs. This paper provides an innovative method to measure effective porosity in aquifers. Because of the long wavelengths of Rayleigh wave, the interdisciplinary approach is advantageous in that the resulting effective porosity is at the site scale which includes large conduits or voids.

Improved understanding of particle transport in karst groundwater using natural sediments as tracers.

Colloids and particles act as vectors for contaminant transport. In karst aquifers, particle transport is particularly efficient and plays critical roles in soil erosion and in the process of karstification. However, available techniques for particle tracing are either expensive or not representative for the transport of natural colloids and particles. We developed a new method for particle tracing, using natural sediments as artificial tracers, and first applied this method at a karst experimental site in the Alps. Suspended particles were injected into a swallow hole together with a conservative solute tracer for comparison. Breakthrough curves for 32 different particle size classes between 0.8 and 450 µm were recorded at a karst spring 230 m away using a mobile particle counter that allows quantitative detection at high temporal resolution. Results show that (i) sediments can be used as efficient particle tracers in karst groundwater; (ii) recoveries are similar for particles and solutes; (iii) mean velocity increases with increasing particle size; (iv) dispersion decreases with increasing particle size; (v) these observations point to exclusion processes. As a conclusion, this new experimental technique allows new insights into the transport and fate of colloids and particles in groundwater at affordable costs.

Fluorescence-based multi-parameter approach to characterize dynamics of organic carbon, faecal bacteria and particles at alpine karst springs.

Karst springs, especially in alpine regions, are important for drinking water supply but also vulnerable to contamination, especially after rainfall events. This high variability of water quality requires rapid quantification of contamination parameters. Here, we used a fluorescence-based multi-parameter approach to characterize the dynamics of organic carbon, faecal bacteria, and particles at three alpine karst springs. We used excitation emission matrices (EEMs) to identify fluorescent dissolved organic material (FDOM). At the first system, peak A fluorescence and total organic carbon (TOC) were strongly correlated (Spearman's rs of 0.949), indicating that a large part of the organic matter is related to humic-like substances. Protein-like fluorescence and cultivation-based determination of coliform bacteria also had a significant correlation with rs=0.734, indicating that protein-like fluorescence is directly related to faecal pollution. At the second system, which has two spring outlets, the absolute values of all measured water-quality parameters were lower; there was a significant correlation between TOC and humic-like fluorescence (rs=0.588-0.689) but coliform bacteria and protein-like fluorescence at these two springs were not correlated. Additionally, there was a strong correlation (rs=0.571-0.647) between small particle fractions (1.0 and 2.0µm), a secondary turbidity peak and bacteria. At one of these springs, discharge was constant despite the reaction of all other parameters to the rainfall event. Our results demonstrated that i) all three springs showed fast and marked responses of all investigated water-quality parameters after rain events; ii) a constant discharge does not necessarily mean constant water quality; iii) at high contamination levels, protein-like fluorescence is a good indicator of bacterial contamination, while at low contamination levels no correlation between protein-like fluorescence and bacterial values was detected; and iv) a combination of fluorescence measurements and particle-size analysis is a promising approach for a rapid assessment of organic contamination, especially relative to time-consuming conventional bacterial determination methods.

Evaluation of ß-d-glucuronidase and particle-size distribution for microbiological water quality monitoring in Northern Vietnam.

In many karst regions in developing countries, the populations often suffer from poor microbial water quality and are frequently exposed to bacterial pathogens. The high variability of water quality requires rapid assays, but the conventional cultivation-based analysis of fecal indicator bacteria, such as Escherichia coli (E. coli), is very time-consuming. In this respect, the measurement of the enzymatic activity of E. coli could prove to be a valuable tool for water quality monitoring. A mobile automated prototype device was used for the investigation of ß-d-glucuronidase (GLUC) activity at a remote karst spring, connected to a sinking surface stream, in Northern Vietnam. To assess the relationship between GLUC activity, discharge dynamics and contamination patterns, multiple hydrological, hydrochemical, physicochemical and microbiological parameters, including discharge, turbidity, particle-size distributions, and E. coli, were measured with high temporal resolution during ten days of on-site monitoring. A complex contamination pattern due to anthropogenic and agricultural activities led to high E. coli concentrations (270 to >24,200 MPN/100ml) and a GLUC activity between 3.1 and 102.2 mMFU/100ml. A strong daily fluctuation pattern of GLUC activity and particle concentrations within small size classes (<10µm) could be observed, as demonstrated by autocorrelations. A Spearman's rank correlation analysis resulted in correlation coefficients of rs=0.56 for E. coli and GLUC activity and rs=0.54 for GLUC activity and the concentration of 2-3µm particles. On an event scale, correlations were found to be higher (rs=0.69 and 0.87, respectively). GLUC activity and E. coli displayed a general contamination pattern, but with significant differences in detail, which may be explained by interferences of e. g. viable but non-culturable cells. Although further evaluations are recommended, GLUC activity is a promising, complementary parameter for on-site and near real-time water quality monitoring.

Uranium in groundwater--Fertilizers versus geogenic sources.

Due to its radiological and toxicological properties even at low concentration levels, uranium is increasingly recognized as relevant contaminant in drinking water from aquifers. Uranium originates from different sources, including natural or geogenic, mining and industrial activities, and fertilizers in agriculture. The goal of this study was to obtain insights into the origin of uranium in groundwater while differentiating between geogenic sources and fertilizers. A literature review concerning the sources and geochemical processes affecting the occurrence and distribution of uranium in the lithosphere, pedosphere and hydrosphere provided the background for the evaluation of data on uranium in groundwater at regional scale. The state of Baden-Württemberg, Germany, was selected for this study, because of its hydrogeological and land-use diversity, and for reasons of data availability. Uranium and other parameters from N=1935 groundwater monitoring sites were analyzed statistically and geospatially. Results show that (i) 1.6% of all water samples exceed the German legal limit for drinking water (10 µg/L); (ii) The range and spatial distribution of uranium and occasional peak values seem to be related to geogenic sources; (iii) There is a clear relation between agricultural land-use and low-level uranium concentrations, indicating that fertilizers generate a measurable but low background of uranium in groundwater.

Tracking changing X-ray contrast media application to an urban-influenced karst aquifer in the Wadi Shueib, Jordan.

Sewage input into a karst aquifer via leaking sewers and cesspits was investigated over five years in an urbanized catchment. Of 66 samples, analyzed for 25 pharmaceuticals, 91% indicated detectable concentrations. The former standard iodinated X-ray contrast medium (ICM) diatrizoic acid was detected most frequently. Remarkably, it was found more frequently in groundwater (79%, median: 54 ng/l) than in wastewater (21%, 120 ng/l), which is supposed to be the only source in this area. In contrast, iopamidol, a possible substitute, spread over the aquifer during the investigation period whereas concentrations were two orders of magnitude higher in wastewater than in groundwater. Knowledge about changing application of pharmaceuticals thus is essential to assess urban impacts on aquifers, especially when applying mass balances. Since correlated concentrations provide conclusive evidence that, for this catchment, nitrate in groundwater rather comes from urban than from rural sources, ICM are considered useful tracers.

Sources and processes affecting the spatio-temporal distribution of pharmaceuticals and X-ray contrast media in the water resources of the Lower Jordan Valley, Jordan.

The closed basin of the Lower Jordan Valley with the Dead Sea as final sink features high evapotranspiration rates and almost complete reuse of treated wastewater for irrigation farming. This study focuses on the water transfer schemes and the presence, spreading, and potential accumulation of pharmaceutical residues in the local water resources based on findings of a five-year monitoring program. Overall 16 pharmaceuticals and 9 iodinated X-ray contrast media were monitored in groundwater, surface water, and treated wastewater. A total of 95 samples were taken to cover all geographical settings and flow paths from origin (wastewater) to target (groundwater). Nine substances were detected in groundwater, with concentrations ranging between 11 ng/L and 33,000 ng/L. Sometimes, detection rates were higher than in comparable studies: Diatrizoic acid 75%, iopamidol 42%, iopromide 19%, iomeprol 11%, carbamazepine and iohexol 8%, ibuprofen 6%, and fenofibrate and iothalamic acid 3%. Concentrations in groundwater generally increase from north to south depending on the application of treated wastewater for irrigation. Almost all substances occurred most frequently and with highest concentrations in treated wastewater, followed by surface water and groundwater. As exception, diatrizoic acid was found more frequently in groundwater than in treated wastewater, with concentrations being similar. This indicates the persistence of diatrizoic acid with long residence times in local groundwater systems, but may also reflect changing prescription patterns, which would be in accordance with increasing iopamidol findings and surveys at local hospitals. Trend analyses confirm this finding and indicate a high probability of increasing iopamidol concentrations, while other substances did not reveal any trends. However, no proof of evaporative enrichment could be found. The high spatial and temporal variability of the concentrations measured calls for further systematic studies to assess the long-term evolution of organic trace substances in this reuse setting.

Transport and variability of fecal bacteria in carbonate conglomerate aquifers.

Clastic sedimentary rocks are generally considered non-karstifiable and thus less vulnerable to pathogen contamination than karst aquifers. However, dissolution phenomena have been observed in clastic carbonate conglomerates of the Subalpine Molasse zone of the northern Alps and other regions of Europe, indicating karstification and high vulnerability, which is currently not considered for source protection zoning. Therefore, a research program was established at the Hochgrat site (Austria/Germany), as a demonstration that karst-like characteristics, flow behavior, and high vulnerability to microbial contamination are possible in this type of aquifer. The study included geomorphologic mapping, comparative multi-tracer tests with fluorescent dyes and bacteria-sized fluorescent microspheres, and analyses of fecal indicator bacteria (FIB) in spring waters during different seasons. Results demonstrate that (1) flow velocities in carbonate conglomerates are similar as in typical karst aquifers, often exceeding 100 m/h; (2) microbial contaminants are rapidly transported toward springs; and (3) the magnitude and seasonal pattern of FIB variability depends on the land use in the spring catchment and its altitude. Different groundwater protection strategies that currently applied are consequently required in regions formed by karstified carbonatic clastic rocks, taking into account their high degree of heterogeneity and vulnerability.

Percolation and particle transport in the unsaturated zone of a karst aquifer.

Recharge and contamination of karst aquifers often occur via the unsaturated zone, but the functioning of this zone has not yet been fully understood. Therefore, irrigation and tracer experiments, along with monitoring of rainfall events, were used to examine water percolation and the transport of solutes, particles, and fecal bacteria between the land surface and a water outlet into a shallow cave. Monitored parameters included discharge, electrical conductivity, temperature, organic carbon, turbidity, particle-size distribution (PSD), fecal indicator bacteria, chloride, bromide, and uranine. Percolation following rainfall or irrigation can be subdivided into a lag phase (no response at the outlet), a piston-flow phase (release of epikarst storage water by pressure transfer), and a mixed-flow phase (increasing contribution of freshly infiltrated water), starting between 20 min and a few hours after the start of recharge event. Concerning particle and bacteria transport, results demonstrate that (1) a first turbidity signal occurs during increasing discharge due to remobilization of particles from fractures (pulse-through turbidity); (2) a second turbidity signal is caused by direct particle transfer from the soil (flow-through turbidity), often accompanied by high levels of fecal indicator bacteria, up to 17,000 Escherichia coli/100 mL; and (3) PSD allows differentiation between the two types of turbidity. A relative increase of fine particles (0.9 to 1.5 microm) coincides with microbial contamination. These findings help quantify water storage and percolation in the epikarst and better understand contaminant transport and attenuation. The use of PSD as "early-warning parameter" for microbial contamination in karst water is confirmed.

Origin and spatial-temporal distribution of faecal bacteria in a bay of Lake Geneva, Switzerland.

The origin and distribution of microbial contamination in Lake Geneva's most polluted bay were assessed using faecal indicator bacteria (FIB). The lake is used as drinking water, for recreation and fishing. During 1 year, water samples were taken at 23 points in the bay and three contamination sources: a wastewater treatment plant (WWTP), a river and a storm water outlet. Analyses included Escherichia coli, enterococci (ENT), total coliforms (TC), and heterotrophic plate counts (HPC). E. coli input flux rates from the WWTP can reach 2.5 x 10(10) CFU/s; those from the river are one to three orders of magnitude lower. Different pathogenic Salmonella serotypes were identified in water from these sources. FIB levels in the bay are highly variable. Results demonstrate that (1) the WWTP outlet at 30 m depth impacts near-surface water quality during holomixis in winter; (2) when the lake is stratified, the effluent water is generally trapped below the thermocline; (3) during major floods, upwelling across the thermocline may occur; (4) the river permanently contributes to contamination, mainly near the river mouth and during floods, when the storm water outlet contributes additionally; (5) the lowest FIB levels in the near-surface water occur during low-flow periods in the bathing season.