Groundwater Flow Paths Research Articles

Groundwater mean residence times of a subtropical barrier sand island

Abstract. Fresh groundwater on barrier islands is affected by changing sea levels and precipitation variability due to climate change and is also vulnerable to anthropogenic processes, such as contamination and groundwater over-abstraction. Constraining groundwater mean residence times (MRTs) and flow paths is essential for understanding and managing these resources. This study uses tritium (3H) and carbon-14 (14C) to determine the MRTs of groundwater along a transect across subtropical North Stradbroke Island, south-east Queensland, Australia. Hydraulic properties, major ion geochemistry and stable isotopes are used to validate residence times and to identify the processes responsible for their variability. 3H activities range from less than 0.01 to 1 TU (tritium units), which are values lower than those of local average rainfall (1.6–2.0 TU). 14C concentrations range from 62.5 to 111 pMC (percent modern carbon). Estimated MRTs determined using lumped parameter models and 3H activities range from 37 to more than 50 years. Recharge occurs over the entire island, and groundwater MRTs generally increase vertically and laterally towards the coastal discharge areas, although no systematic pattern is observed. MRTs estimated from 14C concentrations display similar spatial relationships but have a much greater range (from modern to approximately 5000 years). Water diversion and retention by lower-permeability units in the unsaturated parts of the dune systems are the most likely course for relatively long MRTs to date. The results indicate that the internal structures within the dune systems increase MRTs in the groundwater system and potentially divert flow paths. The structures produce perched aquifer systems that are wide-spread and have a significant influence on regional recharge. The geochemical composition of groundwater remains relatively consistent throughout the island, with the only irregularities attributed to old groundwater stored within coastal peat. The outcomes of this study enhance the understanding of groundwater flow, recharge diversion and inhibition for large coastal sand masses in general, especially for older sand masses that have developed structures from pedogenesis and dune movement. With respect to south-east Queensland, it allows the existing regional groundwater flow model to be refined by incorporating independent MRTs to test models' validity. The location of this large fresh groundwater reservoir, in dry and populous south-east Queensland, means that its potential to be used as a water source is always high. Background information on aquifer distribution and groundwater MRTs is crucial to better validate impact assessment for water abstraction.

Effects of large macropores on soil evaporation in salt marshes

The occurrence of macropores in salt marsh sediments is a natural and ubiquitous phenomenon. Although they are widely assumed to affect pore-water flow in salt marshes significantly, the mechanisms involved and their extent are not well understood. We conducted laboratory experiments and numerical simulations to examine the effect of macropores on soil evaporation. Soil columns packed with either sand or clay and with or without macropores were set up with watertables in the columns set at different levels. A high potential evaporation rate was induced by infrared light and a fan. The results showed that in the soil with a low saturated hydraulic conductivity (and thus a low water transport capacity), the macropore behaved as a preferential flow path for groundwater to recharge the surrounding soil during evaporation. The evaporated water originated largely from the macropore rather than the soil matrix, maintaining a high evaporation rate in comparison with a homogeneous soil. This effect was more pronounced for sediments with lower hydraulic conductivities and shallower watertables. These results improve our understanding of water flow and evaporation in salt marshes with continuous macropores between the soil surface and groundwater.

Characterization of recharge processes and groundwater flow paths using isotopes in the arid Santanghu basin, Northwest China

Isotopes and hydrochemistry were used to characterize the recharge and flow of groundwater in the arid Santanghu basin in Northwest China. The results of isotopic measurement and hydrochemical facies indicate that the predominant recharge mechanism is via rivers and streams. Modern recharge only occurs in the piedmont area of Moqinwula Mountains by ephemeral rivers, and the main recharge source is precipitation in the mountain areas at an elevation of about 2,200 m above sea level. Two recharge mechanisms were identified by stable isotope methods: the ephemeral stream recharge in the piedmont with the occurrence of evaporation during recharge, and subsurface inflow recharge from the mountain regions. Diffuse recharge derived from precipitation in the basin can be ignored because of the scarcity of precipitation and intense evaporation. Groundwater in the Paleogene and Neogene confined aquifers could represent recharge during cooler climatic conditions, rather than latitude or altitude effects, signified by tritium-free samples and significant depletion of heavy stable isotopes throughout the study area. Groundwater age data suggest that central faults are controlling the flow paths of the regional groundwater flow system. Groundwater moves from the piedmont plain to the basin lowland area through lateral flow, and changes flow direction to the spring zone of Hanshuiquan Oasis because of the barrier of the central faults. A conceptual model was defined, for better understanding of the groundwater recharge and flow systems. The major findings of this study have significant implications for groundwater protection and management in arid basins.

Rates of hydroxyapatite formation and dissolution in a sandstone aquifer: Implications for understanding dynamic phosphate behaviour within an agricultural catchment

Eutrophication, caused by excessive nutrient concentrations, is a major environmental issue and has significant impacts on both aquatic ecosystems and human health. Phosphorus (P) is a key element that contributes to this eutrophication response. As such, P concentrations are regulated under both the European Union Water Framework Directive (EU WFD) and Habitats Directive (EU HD). While P export to rivers from point sources is well-understood, diffuse sources, particularly that route through connected aquifer systems are less well-characterised. Based on data from a catchment in southern England where an Upper Greensand aquifer controls river baseflow, we hypothesise that dynamic precipitation and dissolution of phosphorus in the form of hydroxyapatite occurs along the groundwater flow path, affecting P storage and export. In addition, P may also occur as the less-reactive mineral fluorapatite or sorb onto, or substitute into, Fe-oxides, which also occur in the aquifer. To investigate hydroxyapatite kinetics in this context, batch experiments were conducted to precipitate the mineral onto sand grains, and then dissolved in a continuous-flow reactor under various pH and concentration conditions. The reactive transport of PO43- along a 1D flow path was modelled to simulate hydroxyapatite precipitation and dissolution. Fe-oxides were modelled as sorbing surfaces for phosphate under chemical conditions representative of the aquifer.Hydroxyapatite rapidly precipitated onto quartz sand grains from a solution that was supersaturated with respect to hydroxyapatite, thus demonstrating that an input of P to Ca-rich, alkaline porewaters can result in the precipitation of secondary hydroxyapatite. The dissolution rate of hydroxyapatite is strongly pH dependent, however within the aquifer porewaters, the solution is often close to equilibrium with respect to hydroxyapatite, which causes the dissolution rate to have a greater dependence on the PO43- concentration. Within the range of measured pH and PO43- concentrations, the dissolution rate ranges between 10-12.25 to 10-9.15 mol kg−1 s−1, contributing P to the catchment headwaters. Over a 1D flow path, there is dynamic precipitation and dissolution of hydroxyapatite, as well as PO43- sorption and desorption, over relatively short timescales. These results indicate that hydroxyapatite is likely to precipitate within the pore space of the aquifer, but can then dissolve and re-precipitate, adsorb and desorb, under expected spatial and temporal variations in pH and water chemistry within the aquifer. Hydroxyapatite may therefore represent a temporary pool of anthropogenic P, sourced from P-fertilisers, which represents a previously unrecognised pathway for anthropogenic P transfer between the soil surface and surface waters.

Determination of the major geochemical processes of groundwater along the Cretaceous-Tertiary boundary of Trichinopoly, Tamilnadu, India

The hydrogeochemical variations in groundwater are mainly influenced by lithology, residence time of water in the aquifer matrix, and anthropogenic activities. This study was focused on the geochemical variations of groundwater in different lithological units (Archaean, Cretaceous, Tertiary, and Quaternary) by understanding the major factors governing the geochemical variations in each lithology. The 71 groundwater samples were collected from these rock types, namely, Archaean (14), Cretaceous (37), Tertiary (11), Quaternary (9). The collected samples were measured for major ions and they were used for preparation of standard geochemical plots and ionic ratios. Factor analysis and factor score were used to identify the major factors controlling the hydrochemistry and their spatial distribution in the study area. In addition, geochemical model, WATEQ 4F was used to determine the saturation condition of carbonate and sulphate minerals in the groundwater. Na–Cl and mixed Ca–Na–HCO3 were the dominant hydrochemical facies irrespective of lithological units. The overall interpretation of geochemical data revealed that leaching of secondary salts, weathering and ion exchange reaction along the groundwater flow path through various lithological units, and anthropogenic influence from domestic sewages and agricultural activities constitute the major geochemical processes in the study area. Hence, this study brings out the multiple hydrogeochemical process in the complex geological terrain along the Cretaceous-Tertiary boundary.

Krypton-81 dating of the deep Continental Intercalaire aquifer with implications for chlorine-36 dating

Deep groundwater samples from the Continental Intercalaire (CI) aquifer in the Northern Tunisian Sahara have been analyzed for noble gases 3He, 4He, Ne and 81Kr, and for 14C to better constrain the groundwater residence time of this large transboundary aquifer. Its significant radiogenic 4He content and background-level 14C both indicate water older than a few tens of thousands of years. Distinct helium concentrations and Ne/He ratios suggest different groundwater flow paths through the Tozeur and Kebili regions. 81Kr is applied for the first time on the CI aquifer, providing more direct evidence for the presence of old groundwater with 81Kr/Kr ratios at 63% – 15% of the atmospheric value, corresponding to residence times of 150 – 630 ka. These newly obtained 81Kr ages enable a re-evaluation of the previously reported data from the Tozeur region in order to better constrain the input values of 36Cl/Cl and Cl. The initial 36Cl/Cl ratio is found to be 4 – 6 times larger than previously assumed, and the initial Cl concentration 6 – 15 times smaller. The results of 81Kr dating and the recalibrated 36Cl dating define an eastward age progression across >500 km of flow path from the Algerian to Tunisian sections. This study exemplifies the utility of 81Kr not only as an independent and conservative chronometer, but also as a supplemental measurement to better calibrate the parameters required for other age tracers, including cosmogenic 36Cl and radiogenic 4He.

Linking microbial community composition to hydrogeochemistry in the western Hetao Basin: Potential importance of ammonium as an electron donor during arsenic mobilization

Various functional groups of microorganisms and related biogeochemical processes are likely to control arsenic (As) mobilization in groundwater systems. However, spatially-dependent correlations between microbial community composition and geochemical zonation along groundwater flow paths are not fully understood, especially with respect to arsenic mobility. The western Hetao Basin was selected as the study area to address this limitation, where groundwater flows from a proximal fan (geochemical-group I: low As, oxidizing), through a transition area (geochemical-group II: moderate As, moderately-reducing) and then to a flat plain (geochemical-group III: high As, reducing). High-throughput Illumina 16S rRNA gene sequencing showed that the microbial community structure in the proximal fan included bacteria affiliated with organic carbon degradation and nitrate-reduction or even nitrate-dependant Fe(II)-oxidation, mainly resulting in As immobilization. In contrast, for the flat plain, high As groundwater contained Fe(III)- and As(V)-reducing bacteria, consistent with current models on As mobilization driven via reductive dissolution of Fe(III)/As(V) mineral assemblages. However, Spearman correlations between hydrogeochemical data and microbial community compositions indicated that ammonium as a possible electron donor induced reduction of Fe oxide minerals, suggesting a wider range of metabolic pathways (including ammonium oxidation coupled with Fe(III) reduction) driving As mobilization in high As groundwater systems.

Using cluster analysis for understanding spatial and temporal patterns and controlling factors of groundwater geochemistry in a regional aquifer

Understanding spatial and temporal patterns of groundwater geochemistry at the regional scale over a long time period is challenging, due to the lack of data and effective statistical approaches to characterize complex natural processes and anthropogenic activities. We applied a recently developed cluster analysis method to investigate spatial and temporal patterns and controlling factors of groundwater geochemistry in the confined aquifer of the Jianghan Plain, China. The cluster analysis is applied to a dataset of 13,024 groundwater geochemical measurements for 11 geochemical parameters of 1,184 groundwater samples collected over 23 years from 29 monitoring wells distributed over the Jianghan Plain. The cluster analysis yielded a classification of seven clusters, and the classification was confirmed by using principal component analysis and Stiff and Piper diagrams. Based on the spatial distribution of the seven clusters, the Jianghan Plain is separated into four geochemical zones (i.e., recharge zone, transition zone, flow-through zone, and discharge-mixing zone) along the regional groundwater flow path, which has not been attempted in the past. The temporal changes of groundwater geochemistry are controlled by short- and long-term factors of water-rock interactions and anthropogenic activities. A particular finding of this study is that the Three Gorges Reservoir has a long-term impact on groundwater geochemistry, because the reservoir increased river discharge to groundwater after 2009. This study demonstrates that using the cluster analysis method together with hydrogeochemical analysis can identify spatiotemporal patterns and controlling factors of groundwater geochemistry at the regional scale over a long monitoring period.

Hydrochemical evolution of groundwater in gabbro of the Troodos Fractured Aquifer. A comprehensive approach

Ophiolites often form complex, fractured, multi-lithological and highly compartmentalized aquifers that are difficult to characterize. Groundwater chemistry reflects water-rock interactions and can provide insights to groundwater recharge mechanisms, flow paths and relative residence time. A comprehensive hydrogeochemical modelling approach with PHREEQC has been developed to identify water-rock interactions and simulate groundwater evolution and mixing with connate water in fractured aquifers. The methodology was applied in the gabbro aquifers of Troodos Massif (Cyprus). The geochemical processes generating the most frequently observed water types were reproduced through modelling. Based on the overall results, a generic evolution path has been identified: CaMgHCO3 → MgCaHCO3 → CaMgNaHCO3 → Ca/Mg/Na/HCO3/SO4 → NaMgHCO3SO4 → NaSO4/Cl group. CaMgHCO3 and MgCaHCO3 types were modelled to occur predominantly in early stages of recharge. Deep unsaturated zones and longer flow paths result in gradual removal of Ca+2 and Mg+2 from groundwater, through secondary mineral precipitation, and in an increase of Na+2 and SO4−2. The most frequently occurring water types, CaMgNaHCO3 and MgCaNaHCO3, were modelled to occur at later stages of hydro-chemical evolution, after secondary mineral precipitation, and in closed conditions. In regional flow, where closed conditions with secondary mineral precipitation prevail, water types progressively shift from the HCO3 to the NaSO4 end-member group. In 38% of the 536 samples from gabbro aquifers in Troodos, groundwater mixing with connate water at a ratio of ≥0.999:0.001 occurs.

Hydrochemical and Isotopic Evolution of Groundwater Flowing Downstream of the Daqing River (Liaodong Bay, China)

Guo, Q.; Zhang, J.; Hu, Z., and Zhou, Z., 2020. Hydrochemical and isotopic evolution of groundwater flowing downstream of the Daqing River (Liaodong Bay, China). Journal of Coastal Research, 36(3), 608–618. Coconut Creek (Florida), ISSN 0749-0208.Hydrochemistry and specific environmental isotope ratios (δD, δ17O, and δ18O) were employed to investigate the hydrochemical and isotopic characteristics of groundwater in the downstream coastal plain of the Daqing River, China. The relative abundance of the major cations and anions of groundwater along the Daqing River changed from Ca2+, Mg2+ to Na+, Ca2+ and from to Cl–, respectively. The samples, divided into different groups on the basis of correlation coefficients, showed that the chemical composition of the groundwater samples near the Daqing River is approximately consistent with that of the Daqing River. The saturation indices calculated by PHREEQC software indicated that the main hydrogeochemical processes are evaporation and concentration along groundwater flow paths from the upper zone to the middle zone of the Daqing River basin. However, in the down zone, calcite, anhydrite, and dolomite become supersaturated. Isotopic data reveal that the groundwater is affected by the strong evaporation along the Daqing River. In the down zone of the Daqing River, the groundwater is polluted by the seawater.

Stream and floodplain restoration impacts riparian zone hydrology of agricultural streams.

While the influence of stream restoration on vertical and lateral hyporheic exchange has been documented, impacts on broader riparian zone hydrology have not been thoroughly investigated. We quantified riparian water table dynamics, hydraulic gradient, and groundwater flow paths and fluxes across a range of hydrologic conditions following natural channel design restoration (riparian regrading, in-channel cross-vane structure installation). Water table measurements were collected at least once per season for 2.5 years to capture water levels during baseflow conditions from networks of wells and piezometers at sites with different stream morphology (created riffle-cross-vane-scour pool complexes versus natural pools and riffles), restoration status (agricultural restored, unrestored, forested reference), and riparian characteristics (slope, soils, topography) in North Carolina, USA. The regraded riparian zone had higher near-stream water tables (< 0.5 m below ground surface) than the unrestored site. Riffle-cross-vane complexes induced a zone of low hydraulic gradient that spanned 30-40% of the riparian area, similar to groundwater dynamics near beaver dams. This effect persisted regardless of hydrologic condition (wet or dry) or scour pool status (functioning or filled-in). Riffles also promoted a lower near-stream hydraulic gradient at the forested site. Conversely, the influence of stream features on riparian groundwater was minimal at the unrestored site, where groundwater fluxes were controlled by hillslope inputs and riparian geomorphology. Overall, restoration enhanced stream-riparian zone hydrologic interaction beyond the immediate hyporheic zone. Our work stresses that cross-vanes, even when partially buried by sediments post-restoration, impact whole floodplain hydrology in a more significant way than shown by prior stream restoration studies.

Hydrogeological characteristics of foot caves in a karst peak-forest plain in South China

In karst areas of South China, isolated peaks in peak-forest plains often have foot caves near the water table. Originally, foot caves are formed by surface water on the plain flowing through the peak foot into the underground during the rainy season. Later, surface water might not flow into some foot caves anymore, but foot caves still connect with groundwater. Understanding the role of foot caves is important in comprehending regional groundwater flow and in controlling the damage caused by groundwater flow in cave sites. This study employed a multi-electrode resistivity method to locate karst voids or caves at Zengpiyan cave site, Guilin City (Guangxi Province). Boreholes were used to detect karst features, tracer tests were adopted to assess the flow path and velocity of groundwater, and hydrological observations were conducted on site to reveal groundwater changes. Of the 26 boreholes drilled, 14 found underground caves, with heights ranging from 0.3 to 17.5 m. Groundwater was stored and flowed in four distinct media zones: concentrated flow zone (CFZ), highly fractured zone, cave sediment zone, and matrix-with-fractures zone, in which the foot caves constitute the primary groundwater flow zone. The CFZ was characterized and found to have a minimum hydraulic slope of 0.2%. Thus, the study succeeded in identifying hydrogeological characteristics of foot caves in a karst peak-forest plain and a hydrogeological model of the foot caves was developed. The methodology provides a basis for flood control and groundwater pollution prevention in the hydrogeological unit in which foot caves are located.

Groundwater exploration in a typical southwestern basement terrain

ABSTRACT Integrated geophysical techniques were applied for groundwater exploration in a typical southwestern Nigeria basement terrain. EM-3D was primarily used to map out the precise subsurface conductive zones before adopting the recommended sounding locations with aid of Vertical Electrical Sounding (VES) technique. Hydraulic parameters were also estimated and plotted. Twenty Electromagnetic (EM) horizontal profiling and Schlumberger methods were conducted in E-W and N-S direction across the site. The points where the inflection of the raw real intersects the positive peak of the filtered real are diagnostic of major conductive zones suspected to be faults/fractures. Possible groundwater flow paths that identified as conductive zones were used in picking locations for VES deep sounding. The VES conducted on the conductive zones, which are labelled V1, V2, V3 and V4 were characterised by good aquifer resistivity values with relatively thick overburden, which are the major factors in determining the groundwater potential of an area in a typical basement complex, while those carried out on the non-conductive zones labelled V5 and V6 were characterised by poor indication to groundwater potential of the area.

Assessment of the geochemical evolution of groundwater quality near the El Kharga Oasis, Egypt using NETPATH and water quality indices

The Nubian Sandstone Aquifer (NSA) has been the main source of water in the western desert of Egypt since 1985. This aquifer is subject to excessive groundwater withdrawal in the region, causing declines in potentiometric heads and a deterioration of groundwater quality. Consequently, investigations were undertaken to provide baseline water quality information for groundwater management. Water quality data from wells in the study area were manipulated using a Geographic Information System (GIS), statistical analyses (SPSS), graphical simulations (such as Piper and Gibbs diagrams), water quality index (WQI) and a simple geochemical model (NETPATH). This assessment and the use of hydrochemical indices indicated that chemical reactions between the aquifer matrix and groundwater are likely to be the main factors controlling the chemical composition of groundwater in the area. Also, the results of WQI showed that the majority of the collected water points (about 89%) were unsuitable for potable use due to elevated iron concentrations. The NETPATH modeling results suggest that dedolomitization, silicate weathering and dissolution of halite processes are the main geochemical processes that influence changes in the chemical composition along groundwater flow-paths.

Stable Isotopes of Water and Nitrate for the Identification of Groundwater Flowpaths: A Review

Nitrate contamination in stream water and groundwater is a serious environmental problem that arises in areas of high agricultural activities or high population density. It is therefore important to identify the source and flowpath of nitrate in water bodies. In recent decades, the dual isotope analysis (δ15N and δ18O) of nitrate has been widely applied to track contamination sources by taking advantage of the difference in nitrogen and oxygen isotope ratios for different sources. However, transformation processes of nitrogen compounds can change the isotopic composition of nitrate due to the various redox processes in the environment, which often makes it difficult to identify contaminant sources. To compensate for this, the stable water isotope of the H2O itself can be used to interpret the complex hydrological and hydrochemical processes for the movement of nitrate contaminants. Therefore, the present study aims at understanding the fundamental background of stable water and nitrate isotope analysis, including isotope fractionation, analytical methods such as nitrate concentration from samples, instrumentation, and the typical ranges of δ15N and δ18O from various nitrate sources. In addition, we discuss hydrograph separation using the oxygen and hydrogen isotopes of water in combination with the nitrogen and oxygen isotopes of nitrate to understand the relative contributions of precipitation and groundwater to stream water. This study will assist in understanding the groundwater flowpaths as well as tracking the sources of nitrate contamination using the stable isotope analysis in combination with nitrate and water.

Role of Natural and Anthropogenic Influence on the Salinization of Groundwater from Basement Aquifers in the Middle Part of Mono River Basin, Togo

This study aimed to identify the role of natural and anthropogenic influence on geochemical processes controlling groundwater salinization in the middle portion of the Mono River basin, Togo. Multivariate statistical analysis, geochemical masse balance modeling, and conventional graphical methods were applied to the chemical dataset obtained from 110 groundwater samples collected during two campaigns (March-May 2011 and April 2014). The results showed that groundwater is predominantly fresh, acidic to circumneutral and mixed cations-HCO-3 water types. The proportion of ions (Na+, K+, Cl- and SO2-4) occurring ubiquitously in a saline environment increases substantially with TDS. Two factors, defined as natural and anthropogenic, account for 76.5% of the hydrochemical dataset variance. Based on the hierarchical cluster analysis, groundwater samples are classified into four clusters that corroborate the factor analysis results and illustrate different hydrochemical evolution stages along a topographically-driven groundwater flow path. The primary natural process contributing to cations and bicarbonate ions is the weathering of silicate minerals and possibly secondary carbonates by infiltrating water enriched with soil CO2 gas. Groundwater salinization indicators including Cl-, SO2-4 and NO-3, originate from potential sources such as human and animal wastes, agrochemicals, and deforestation. Thermodynamic equilibrium conditions ranged from saturation to oversaturation with respect to quartz, chalcedony, amorphous silica, calcite, dolomite, sepiolite, and talc. In conjunction with the lowering of CO2 partial pressure and evaporation, these conditions restrict acidic hydrolysis of primary silicates but magnify the groundwater salinization process in the lower elevation areas where soil crusts’ formation is favorable. This study showed the effective use of multivariate statistical analysis and complementary techniques in identifying the significant factors and processes controlling groundwater chemistry.

Natural isotopes support groundwater origin as a driver of mire type and biodiversity in Slitere National Park, Latvia

Slitere National Park in Latvia is home to rich fens with many endangered and threatened plant species. This study aims to address how the hydrological systems affect vegetation biodiversity (cf. Wolejko et al. 2019) in the mire systems of the National Park: the base-rich inter-dune mires and extremely base-rich calcareous fens. Groundwater samples from these areas were collected for measurements of ion composition and natural isotopes of C, H and O. Also, we simulated groundwater flow paths from the highest local topographical point (a nearby sandy plateau) to the sea, and calculated the residence times of these groundwater flows. The results show that the inter-dune mires are supplied by a mixture of local and regional groundwater systems. The groundwater supply at one of the inter-dune mires was dominated by local groundwater flow from adjacent dunes, but we also detected a small input of calcareous water. This dominance of local groundwater may have resulted from the presence of drainage ditches and a small stream that drains into the Baltic Sea. In contrast, the extremely base-rich fens were found to be solely dependent on regional groundwater which is likely to discharge at the plateau foothills due to the presence of fault structures. Thus, the mires in Slitere National Park are not as undisturbed as was previously believed. Drainage may have affected the original hydrological flow paths. Further research on the extent of these changes is recommended to preserve the endangered species and high biodiversity of these fens. Also, in order to trace the origin of groundwater flows, further investigation into the larger landscape beyond the plateau might be required.

Understanding the inorganic carbon transport and carbon dioxide evasion in groundwater with multiple sulfate sources during different seasons using isotope records

The geochemical cycle of carbon under anthropogenic activities controls the transport of carbon between surface sediments, hydrosphere and atmosphere, especially for atmospheric and marine carbon dioxide. Currently, the anthropogenically-induced carbon cycle in groundwater is an important part of the cycle in inland waters and cannot be underestimated as a geochemical process affecting the global carbon cycle. Therefore, there is a need to conduct deep research using isotopes in which the isotopic mixing and fractionation response to the carbon cycle can be assessed. The results showed that the δ34S-SO42− and δ18O-SO42− values in groundwater inherited the isotopic characteristics of four sulfate sources and shifted from acid mine drainage (AMD) derived from pyrite oxidation to atmospheric sulfates and agricultural fertilizers as well as domestic sewage along the groundwater flow path. In particular, the evolution of sulfate sources was accompanied by changes in the δ13C-DIC values in different seasons. During the dry season, the Rayleigh function described a possibility of enriched δ13C-DIC values in an AMD-affected aquifer, where a dropping water table or decreasing water content in unsaturated zones provided favorable acid buffering conditions. Bicarbonate neutralized protons to CO2 that was in turn spilled over the water surface. During the wet season, the depleted δ13C-DIC values in the AMD-affected aquifer reflected another possibility of the carbon cycle in which a rising water table caused a similar closed system to some extent and slowed down the replenishment of bicarbonate so that δ13C-DIC almost recorded the δ13C of soil CO2. In spite of the same AMD source, their influences on the carbon cycle in the dry and wet seasons were CO2 evasion and deficient CO2 uptake, respectively. By contrast, regardless of the season, the carbon cycle gradually shifted towards a net consumption of CO2 in the aquifer with atmospheric and agricultural sulfates as well as domestic sewage, and δ13C-DIC inherited the characteristic of HCO3−, indicating that these sources in anthropogenically-induced carbon processes hardly involved in isotopic fractionation of δ13C-DIC.

The groundwater fauna of the Classical Karst: hydrogeological indicators and descriptors

The Classical Karst (sensu stricto) represents a complex hydro-geomorphological unit as well as one of the best-known karstic systems in the world. For this reason, it was selected as a model area both for applied research and for the conservation of its highly diverse subterranean fauna. With this paper, we aim to provide an overview of groundwater fauna in the phreatic zone of the Classical Karst aquifer, based on all published material along with new surveys, and search for patterns of species distribution within hydrogeological subunits, with a focus on the potential use of species as tracers of hydrological connections and groundwater flowpaths. The Classical Karst, with an area of approximately 750 km2, and adjacent to alluvial and flysch areas provides habitats for 83 stygobionts (accompanied by 16 stygophiles) and is one of the richest hotspots for groundwater fauna across the globe. The rate of endemism within the area is very high (around 50%). Multivariate statistical analyses revealed the faunal turnover between hydrogeological subunits, as well as the effect of water flow regime. Our approach, using species turnover and the species indicator value, allowed us to recognise groups of indicator species, which can be considered as descriptors of aquifer type and habitat structure, as well as water flow regime and groundwater flowpaths.

The spatial variations of correlation between microbial diversity and groundwater quality derived from a riverbank filtration site, northeast China

To identify the relationship between microbial communities and groundwater quality parameters, especially typical groundwater contaminants including ammonium (NH4+), iron (Fe2+), and manganese (Mn2+), groundwater samples and aquifer media were collected from a riverbank filtration (RBF) site in Harbin city, northeast China. The structures of the microbial communities of aquifer media at different depths were illustrated through 16S rRNA sequencing using the Illumina MiSeq platform, and the correlation between microbial communities and water quality parameters was quantitatively demonstrated by redundancy analysis (RDA). The results revealed that the diversity of microbial communities decreased along the groundwater flow path at the south bank of Songhua river. The richness and diversity in the unsaturated zone (0–5 m) were significantly higher than those in the saturated zone (5–50 m) because of physiochemical properties of aquifer media and the redox environment in the RBF system. Core taxa, which were significantly related to the biotransformation behavior of iron, manganese, and ammonium, were Pseudomonas, Acinetobacter, Ochrobactrum, Sphingobium, and Sphingomonas. RDA results indicated that the critical physiochemical parameters that significantly influenced the composition of microbial communities were different in the saturated and unsaturated zones, and the total organic carbon (TOC), electrical conductivity (EC), nitrate (NO3−), and manganese (Mn2+) levels were the four principal factors affecting the function and composition of microbial communities in the whole RBF system. Proteobacteria, Sphingomonadales, and Sphingobium were the dominant bacteria at the phylum, order, and genus levels with 39.8%, 59.2%, and 65.3% contribution to the overall groundwater quality, respectively. The results obtained from this study should improve the understanding of the relationship between the structure of microbial communities (especially, bacteria related to the biotransformation of NH4+, Fe2+, and Mn2+) and physiochemical parameters.