Groundwater Flow Paths Research Articles

Integration of photogrammetry from unmanned aerial vehicles, field measurements and discrete fracture network modeling to understand groundwater flow in remote settings: test and comparison with geochemical markers in an Alpine catchment

Fast and cost-effective techniques for hydrogeological modeling are of broad interest for water resources exploitation, especially in remote settings, where hydrogeological measurements are difficult to perform. Unmanned aerial vehicles (UAV)-based techniques are potentially useful for these aims, but their application is still limited. In this study, a field-based approach and UAV-based approach are integrated for the computation of a discrete fracture network model of a fractured aquifer in the Central Alps. Then, calculated directions of the hydraulic conductivity components were compared with a geostatistical analysis of geochemical markers from sampled spring waters, to infer a conceptual model of groundwater flow. The comparison of field-based and UAV-based fracture measurements confirmed a good matching for fracture orientations and recognized a more reliable estimation of fracture dimensions for the UAV-based dataset. Nonetheless, an important variable for hydrogeological modeling—fracture aperture—is not measurable using UAV, as this requires field measurements. The calculated directions of the main conductivities fit well with the analyzed geochemical markers, indicating the presence of two partially separated fractured aquifers and describing their possible groundwater flow paths. The adopted integrated approach confirms UAV-based measurements as a potential tool for characterization of fracture sets as the input for hydrogeological modeling and for a fast and effective surveying tool, reducing time and cost for other following measurements.

Integrating current and historical water chemistry data with long-term piezometric records to develop a regional-scale conceptual flow model: Las Salinas spring, Medina del Campo, Spain

Study regionOld Las Salinas spring in Medina del Campo, Duero river basin, central Spain. Study focusMedina del Campo groundwater body (MCGWB) is a multilayer semiconfined aquifer subject to intensive pumping since the 1970’s, where the current existence of spas where there used to be traditional baths could confirm the existence of deep groundwater flow paths. The old spring of Las Salinas (OSLS) is a saline anomaly in an aquifer with predominance of CaCO3H waters whose occurrence has not yet been formally explained. Long-term geological, geophysical, hydrogeological and hydrochemical records were integrated and complemented with field work to clarify its existence. New hydrological insights for the regionOutcomes led to the conclusion that the hydrochemistry of the Olmedo and Palacio de las Salinas salt baths is associated with the existence of a major threshold in the impervious basement of the aquifer, which intercepted deep regional groundwater flow and caused upwelling to the surface under unperturbed conditions. These results allow for the development of a conceptual flow model at the regional scale that explains the changes in natural water chemistry that have been identified in recent decades.

Molecular Traits of Dissolved Organic Matter in the Subterranean Estuary of a High-Energy Beach: Indications of Sources and Sinks

Advective flows of seawater and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary (STE). Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon (OC) from terrestrial sources. Along the groundwater flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to disentangle, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from freshwater lens groundwater, seawater, and beach porewater samples collected over different seasons. The samples were analyzed for physico-chemistry (e.g., salinity, temperature, dissolved silicate), (reduced) electron acceptors (e.g., oxygen, nitrate, and iron), and dissolved organic carbon (DOC). DOM was isolated and molecularly characterized via soft-ionization ultra-high-resolution mass spectrometry, and molecular formulae were identified in each sample. We found that the islands’ freshwater lens harbors a surprisingly high DOM molecular diversity and heterogeneity, possibly due to patchy distributions of buried peat lenses. Furthermore, a comparison of DOM composition of the endmembers indicated that the Spiekeroog high-energy beach STE conveys chemically modified, terrestrial DOM from the inland freshwater lens to the coastal ocean. In the beach intertidal zone, porewater DOC concentrations, lability of DOM and oxygen concentrations, decreased while dissolved (reduced) iron and dissolved silicate concentrations increased. This observation is consistent with the assumption of a continuous degradation of labile DOM along a cross-shore gradient, even in this dynamic environment. Accordingly, molecular properties of DOM indicated enhanced degradation, and “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial consumption. Our data indicate that the high-energy beach STE is likely a net sink of OC from the terrestrial and marine realm, and that barrier islands such as Spiekeroog may act as efficient “digestors” of organic matter.

Permafrost Promotes Shallow Groundwater Flow and Warmer Headwater Streams

AbstractThe presence of permafrost influences the flow paths of water through Arctic landscapes and thereby has the potential to impact stream discharge and thermal regimes. Observations from 11 headwater streams in Alaska showed that July water temperatures were higher in catchments with more near‐surface permafrost. We apply a fully coupled cryohydrology model to investigate if the impact of permafrost on flow path depth could cause the same pattern in temperatures of groundwater discharging from hillslopes to streams. The model simulates surface energy and water balances, snow, and subsurface water and energy balances for two‐dimensional hillslope model cases with varying permafrost extent. We find that hillslopes with continuous permafrost have more shallow flow paths and twice as high rates of evapotranspiration, compared to hillslopes with no permafrost. For our simulated cases, 6.7% of the horizontal water flux moves through the top organic soil layers when there is continuous permafrost, while only 0.5% moves through organic layers without permafrost. The deeper flow paths in permafrost‐free simulations buffer seasonal temperature extremes, so that summer groundwater discharge temperatures are highest with continuous permafrost. Our results suggest that permafrost thawing alters groundwater flow paths and can lead to decreases in summer stream temperatures and reductions in evapotranspiration in headwater catchments. These changes are of potential importance for stream biotic components of ecosystems, however, the full impact remains unknown.

DNA Tracer Transport Through Porous Media—The Effect of DNA Length and Adsorption

AbstractArtificial tracer testing is an effective technique to identify fluid flow pathways and characterize subsurface hydrological properties. Synthetic DNA tracers, available in virtually unlimited number of unique variations, enable multiwell tracer testing and have the potential to improve the characterization of groundwater flowpaths enormously. This study investigated the effect of DNA length (i.e., number of base pairs) and adsorption on DNA tracer transport via well‐controlled laboratory experiments. Nine unique synthetic double‐stranded DNA tracers were designed with varying lengths (90–200 base pairs) and flowed with reference solute tracers (bromide and lithium) through columns packed with glass beads or Ottawa sand. The peak arrivals of DNA tracers were earlier than bromide in the glass‐bead column test, yet significantly retarded in the sand column test. In a given porous medium, DNA length did not have noticeable effects on peak arrival or tracer dispersion. However, DNA tracer recovery increased with increasing DNA length in the glass‐bead column test but decreased with increasing DNA length in the sand column test, likely resulting from the corresponding trends in partition coefficients as revealed by tracer transport modeling. Such reversed trends in different porous media likely resulted from the different likelihoods for longer DNA to have multisegment attachment during transport. Our findings suggest that both DNA length and the surface properties of the porous medium have notable effects on DNA tracer transport and hence need to be considered carefully in future tracer test designs in terms of site selection, DNA sequence design and data interpretation.

Deciphering groundwater flow-paths in fault-controlled semiarid mountain front zones (Central Chile)

The Mountain-Block Recharge (MBR), also referred to as the hidden recharge, consists of groundwater inflows from the mountain block into adjacent alluvial aquifers. This is a significant recharge process in arid environments, but frequently discarded since it is imperceptible from the ground surface. In fault-controlled Mountain Front Zones (MFZs), the hydrogeological limit between the mountain-block and adjacent alluvial basins is complex and, consequently, the groundwater flow-paths reflect that setting. To cope with the typical low density of boreholes in MFZs hindering a proper assessment of MBR, a combined geoelectrical-gravity approach was proposed to decipher groundwater flow-paths in fault-controlled MFZs. The study took place in the semiarid Western Andean Front separating the Central Depression from the Principal Cordillera at the Aconcagua Basin (Central Chile). Our results, corroborated by field observations and compared with worldwide literature, indicate that: (i) The limit between the two domains consists of N-S-oriented faults with clay-rich core (several tens of meters width low electrical-resistivity subvertical bands) that impede the diffuse MBR. The “hidden recharge” along the Western Andean Front occurs through (ii) focused MBR processes by (ii.a) open and discrete basement faults (mass defect and springs) oblique to the MFZ that cross-cut the N-S-oriented faults, and (ii.b) high-hydraulic transmissivity alluvial corridors in canyons. Alluvial corridors host narrow unconfined mountain aquifers, which are recharged by indirect infiltration along ephemeral streams and focused inflows from oblique basement faults. This study also revealed seepage from irrigation canals highlighting their key role in the recharge of alluvial aquifers in the Central Depression. The proposed combined geophysical approach successfully incorporated (hydro)geological features and geophysical forward/inverse modelling into a robust hydrogeological conceptual model to decipher groundwater flow-paths in fault-controlled MFZs, even in the absence of direct observation points.

Analysis of mining effects on the geochemical evolution of groundwater, Huaibei coalfield, China

Investigations were undertaken at the Xutuan and Renlou coal mines in Huaibei coalfield, Anhui Province, China to determine the effects of mining on the geochemical evolution of groundwater in the area. A total of 77 samples were collected between 1999 and 2017 from Neogene, Permian, and Carboniferous aquifers in two coal mines for hydrogeochemical analysis. The variation of hydrochemical types and the difference of Cl− and TDS suggest that the groundwater in the Neogene aquifer flow from the Xutuan coal mine to the Renlou coal mine. The high concentrations of chloride in groundwater in the Permian aquifer may be associated with recharge from the Neogene aquifer under the effects of mining. Principal component analysis and the results of chemical analysis of water samples were used to explore the water–rock processes in the three aquifers. The results suggest that the main controlling process of the Xutuan coal mine is ion exchange between Na+ and Ca2+, while the principal chemical processes in the Renlou coal mine are likely to be ion exchange and reverse ion exchange between Na+ and Ca2+. Mining will lead to the decline of the Neogene aquifer potentiometric heads and the compaction of aquifer sediments. This, in turn, could lead to the development of fissures caused by mining which could increase the hydraulic connection between the Neogene and the Permian aquifer, and increase the ion-exchange intensity of the Permian aquifer. The inverse geochemical modeling results of groundwater flow-paths in the Neogene and Carboniferous aquifers suggest that reverse ion-exchange process is taking place in these aquifers, which provides additional evidence for geochemical evolution. This combined method of using various lines of evidences from hydrogeology, multivariate statistical analysis, hydrogeochemical research, and geochemical models can explain the hydrogeochemical evolution processes of groundwater at a deeper level.

Hydrogeological controls on the flow regime of an ephemeral temperate stream flowing across an alluvial fan

Alluvial fans often support ephemeral streams whose flow regimes and sediment dynamics are strongly controlled by fan sedimentary characteristics and interactions with shallow groundwater aquifers. The hydrogeology of such fans has most often been documented for large fans located within arid climate zones. This research focuses on a small (0.075 km2) alluvial fan situated in a temperate, high rainfall climate (the Lake District, North West England). An ephemeral stream flowing across this alluvial fan plays a key role in supplying water and sediment to the River Ehen, which is the focus of a habitat restoration initiative. This study combines high spatial resolution, near surface geophysics and outcrop data with hydrological data to characterise the hydrogeological properties of the alluvial fan. A conceptual hydrogeological model was developed to understand the hydrology across the alluvial fan and how that affects water and sediment supply to the River Ehen. The alluvial fan is composed predominantly of permeable debris flow deposits and numerous palaeochannels which may provide preferential groundwater flow paths. During streamflows, partial to full stream-aquifer connectivity occurs, especially in the distal fan where groundwater discharges back into the stream. Streamflows occur when the fan apex infiltration rate/capacity threshold of approx. 60 l/s−1 is exceeded, typically following rainfall events >9–11 mm. Understanding interactions between ephemeral streams and their underlying aquifers enables better predictions of the timing and magnitude of future flows, and in-turn, their likely impacts on the water courses into which they discharge.

Conversion relationship between groundwater and surface water in the Taizi River Basin in China based on geochemical and isotopic characteristics.

Stable isotopes (δDVSMOW and δ18OVSMOW) and hydrochemical characteristics were analyzed to determine the groundwater and surface water characteristics of the Taizi River Basin in Northeast China. Total dissolved solids and major ion concentrations gradually increase along the Taizi River, and only parts of the middle and lower reaches were slightly reduced by replenishment from the Hun River and the tributaries of the eastern mountainous areas. The hydrochemical evolution of the surface water and groundwater may be due to a combination of mineral dissolution, mixing processes, cation exchange, and evaporation along the groundwater flow path. The geochemical and isotopic characteristics of the waters suggest that the surface water from the piedmont recharges the groundwater through faults and that precipitation near the mountain reservoirs contributes immensely to both water types. Lateral groundwater movement from the piedmont may be an effective source for replenishing river water in the middle and lower reaches. The geochemical and isotopic characteristics of the groundwater and surface water interaction in the Taizi River Basin contribute toward their comprehensive management.

Coupling Mineralogical Analyses, Leaching Tests and Kinetic Modelling to Unravel Groundwater Flow-Paths in a Complex Landslide: An Attempt from the Vedriano Case Study (Northern Italian Apennines)

As the rise in pore water pressure is one of the main factors in triggering landslides, the understanding of groundwater processes taking place at the hillslope scale is a crucial issue in slope stability analysis. However, identifying flow-paths travelled by water molecules from their infiltration is still a complex task. Hydrochemistry is recognized as a powerful tool that can help to gain useful hydrogeological information and has gradually become increasingly used in addition to conventional study methods. This manuscript presents a comprehensive geochemical investigation consisting of leaching tests and quantitative mineralogical analyses on soil samples, chemical analyses on groundwater samples and modelling. Our results highlighted the usefulness of coupling, even in hydrogeological studies focusing on landslides, geochemical surveys on both water and the soil matrix to constrain the interactions between host-rocks and groundwater. Moreover, it demonstrated that kinetic-based geochemical models, if properly calibrated on leaching tests, can provide valuable information on groundwater dynamics, allowing us to elucidate water-mixing processes beneath the soil surface.

Hydrogeological functioning of the Trías de Antequera. Hypothesis on the general groundwater flow model

The Trías de Antequera (in the north of the province of Malaga), belonging to the so-called Chaotic Subbetic Complexes, is formed by diverse materials (clays, evaporites, sandstone, ophites, dolostones, etc.). This complex has traditionally been considered to have low permeable behavior as a whole. Nevertheless, some studies evidence high hydrogeological complexity, with predominantly aquitard functioning and higher permeability in the evaporitic outcrops as a result of dissolution/karstification processes and halokinetic movements. Recharge is associated with direct rainfall infiltration and runoff entry through karst swallow holes, whilst discharge mainly takes place towards the Meliones Strait, in the Guadalhorce River. Hydrogeochemical data reveal a general increase of water temperature and mineralization from outlets located at higher places towards those at a lower altitude. This provokes a progressive transition of hydrochemical facies: from calcium-bicarbonate to sodium-chloride type, passing through calcium-sulphated waters. All these results suggest the existence of a hierarchized groundwater flowpath system, with different residence times in the media, similar to that proposed by Tóth for sedimentary basins. Thus, low mineralization groundwater of recent infiltration (flowing through karst conduits within the unsaturated zone) is mixed with older groundwater related to the deep ascending slow flow paths of greater residence time in the media.

A conceptual framework of groundwater flowpath and recharge in Ziban aquifer: south of Algeria

The Zyban aquifers (southern Algeria) are an important source of groundwater and are mainly found in limestone and dolomite from the Senono-Eocene. To be able to assess the effects of overuse of the Zeban groundwater, it is necessary to understand the origin of the recharge and determine its path out of the recharge zones. This study aims to better understand the flow path and replenishment areas using hydrology. For this purpose, 145 water samples were collected and analyzed for the determination of major ions and isotopes (18O, 2H and 3H). The isotope analysis of hydrogen, oxygen and tritium allowed a certain distinction between the origin and the mixture of water. The study area is located in the fault corridor of the South Atlas, a network of migration and folding defects that leads to a complex structure characterized by the subdivision of the formations and the variety of facies. The regional flow pattern is mainly influenced by the topography, and this is through the distribution of supply and flow, which is produced by transport at many depths and levels of distribution (local or regional). The rocky nature and the severe fractures are responsible for the division of the hydrogeological units and play an important role in the dynamics and transport of the groundwater. The hydrodynamic analysis shows the dominance of regional hydrogeology in terms of geological nature and structure. This is supported by a hydrochemical signal showing the distribution and evolution of the elements according to the structural and geological scheme (evolution from calcium bicarbonate facies to calcium sulphate medium facies).

Long-term redox conditions in a landfill-leachate-contaminated groundwater

Indicators of redox conditions; oxygen, sulphate, nitrate, ammonium, iron and manganese, and in addition, bicarbonate and total organic carbon were studied in groundwater samples contaminated by leachate emanating from Revdalen Landfill (Norway). Based on these variables, the study aimed to deduce the redox conditions in the aquifer. Literature on landfill leachate contamination of confined aquifers is scarce and to the best of our knowledge, this study, which describes long-term analysis of redox chemistry, is the first of its kind in such an environment. Groundwater samples were monitored for a period of 24 years, enabling us to describe redox conditions on both short-term and long-term bases. Levels of measured parameters in the contaminated aquifer varied spatially and with time, but were generally elevated except oxygen; pH (4.9–8.8), oxygen (0–11.3 mg/L), sulphate (0–28 mg/L), nitrate (0–16 mg N/L), ammonium (0.02–40 mg/L), iron (0–99 mg/L), manganese (0.06–16 mg/L), bicarbonate (22–616 mg/L) and total organic carbon (1.3–47 mg/L). From the result, levels of iron, manganese, nitrate and ammonium violated the Norwegian drinking water norms. However, iron, ammonium, total organic carbon and bicarbonate showed strong attenuation along the groundwater flow path. By contrast, oxygen, nitrate and sulphate increased farther out in the plume. The redox conditions that developed in the aquifer were similar to those previously reported for phreatic aquifers, structuring by proximity to the landfill as sulphate-reducing, iron-reducing, manganese-reducing, nitrate-reducing, and finally aerobic condition. Eventually, there was an apparent breakdown of this system due to ecosystem shift in the landfill when leachable reduced ions were depleted and the landfill became aerobic. Overall, the redox framework provided remarkable attenuation to contaminants, and thus prevented potential degradation of ecological health due to the landfill leachate.

Base flow in the Yarlungzangbo River, Tibet, maintained by the isotopically-depleted precipitation and groundwater discharge

The extension-induced rift systems on the Tibetan Plateau (TP) may convey large amount of groundwater to rivers, but sources and flow paths of such groundwater are unknown. The Yarlungzangbo River (YR) is the only large river that traverses the southern Tibetan plateau from west to east, following one major suture zone that is cut by extensional normal faults. The faults could influence the flow paths of groundwater discharging to the river. In this study, O and H isotopes, major ions and 222Rn concentrations are analyzed along the YR, and interpreted in relation to structural geology and tectonics. The YR exhibits an abrupt change of isotopic and chemical compositions along with a large increase in flow where the middle reach intersects NE-SW-trending rifts. Low values of δD and δ18O and high concentrations of major ions and 222Rn in the middle reach show that waters are modified isotopically and chemically by a variety of possible water origins, such as recharge of high-altitude glacier melt and discharge from groundwater. Groundwater contributes 27 to 40% of the river flow in the middle reach. Isotopically-light meltwater from high-altitude glacier melt cannot account for the isotope composition of the present outflow of groundwater. The O and H isotope data in the YR and discharging groundwater can be well explained by the groundwater originated as paleo-precipitation during a cooler time, such as the late Pleistocene to early Holocene. The paleo-groundwater discharge can account for about 36 × 108 m3 water budget unbalance in the middle reach. The study provides the first clear isotope evidence for the source of groundwater discharge into a large river through favorable conduits in large-scale active tensile fault zones and confirms the regional scale of groundwater flow on the Tibetan Plateau. Understanding the characteristics and changes of streamflow and surface-groundwater circulation on the Tibetan Plateau will help to manage water resources under a changing environment.

Hydrochemical characterization and quality assessment of groundwater in Meki River Basin, Ethiopian Rift

The present study was carried out in Meki River Basin (Ethiopian Rift) aiming at characterizing the hydrochemical composition of groundwater and evaluating the water quality status for drinking and irrigation uses. For this purpose, 63 primary groundwater samples from springs, hand-dug wells, and boreholes were collected and subjected to major ions analysis besides the in situ tests. The analysis result illustrates that Na+ and HCO3− are the principal cation and anion in the study area. The spatial investigation of the physiochemical parameters revealed that temperature, pH, TDS, EC, Na+, K+, HCO3−, Cl−, SO42−, and F− show a positive trend from the highland towards the rift floor along the groundwater flow direction. On the contrary, Ca2+ and Mg2+ show a negative tendency along the groundwater flow path. The groundwater of the study area generally evolves from predominantly fresh (TDS 750 mg/l) Na–HCO3 water type in the rift floor following the regional groundwater flow path. Gibbs diagram supported by ionic ratio (Na+ + K+/Ca2+ + Mg2+ vs total cations (TZ+) and Chloro-Alkaline Indices (CAIs) suggests that rock–water interaction (silicate hydrolysis and cation exchange) is the major hydrochemical processes controlling the evolution of the groundwater in the study area. 19% of TDS, 11% of EC, 83% of K+, 84% of HCO3−, and 52% of F− of the total water samples were unsuitable for the drinking uses compared with WHO water quality guideline. Spatial distribution maps of major ions (Ca2+, Mg2+, Na+, K+, Cl−, HCO3−, SO42−, and F−) and binary plots of physical parameters (temperature, pH, TDS, and EC) with elevation were produced. Both the spatial maps and binary plots generally show a water quality drop rift wards. Similarly, using multiple agriculture water quality criteria such as EC, SAR, RSC, PI, and Na%, the suitability of the groundwater for agricultural use was evaluated. Except a few samples in the rift floor, the majority of the water samples are found to be suitable for irrigation uses. As the study area is located where active geothermal activity and fluid circulation through fault systems, it is highly advised to carry out further toxic trace elements analysis to investigate the possible influence on the water quality.

Three-dimensional hydrostratigraphical modelling supporting the evaluation of fluoride enrichment in groundwater: Lakes basin (Central Ethiopia)

Study regionThe Lakes Basin is located in the Main Ethiopian Rift. It covers the northern part of the rift valley basin, the Upper Awash River basin, and some sub-basins from the Omo River basin. Due to the presence of high fluoride (F−) content, natural contamination of groundwater has long been recognized as a water-related health issue in the area. Study focusA multidisciplinary research effort, including geological, hydrogeological, hydro-chemical, and geophysical investigations, was adopted to understand the 3D hydrogeological conceptual model and to evaluate F− enrichment in groundwater. New hydrological insights for the regionThe 3D hydrogeological conceptual model shows a complex hydrogeological environment and a clear hydraulic interconnection between different aquifers. The geological setting has deeply influenced the geometry of the aquifers, recharge and discharge areas, and F− enrichment in groundwater. Two hydrogeological units, namely sedimentary and volcanic multi-aquifers, were identified. The analyses of groundwater circulation, flow paths, and distribution of F- concentrations in each aquifer were conducted. In groundwater, the concentration of fluoride varies from 0.1 to 68.9 mg L−1; in surface water, it ranges from 0.6 to 244.2 mg L−1. Fluoride concentration of 62 % of the water samples analyzed exceeded the 1.5 mg L−1 WHO threshold for fluoride concentration in drinking water. The proposed methodological approach has been demonstrated to be a powerful tool that could be applied in other similar areas.

Geochemistry of Groundwater and Naturally Occurring Biogenic Pyrite in the Holocene Fluvial Aquifers in Uphapee Watershed, Macon County, Alabama

Naturally occurring biogenic pyrite has been found in Holocene fluvial aquifers in the Uphapee watershed, Macon County, Alabama. The electron microprobe (EMP) analysis showed that the pyrite grains contain 0.20–0.92 weight% of arsenic (As). The scanning electron microscope and energy dispersive spectroscopy (SEM-EDS) analysis confirmed a similar concentration of As in the pyrite that was consistent with the EMP analysis. The SEM analysis also confirmed the presence of additional trace elements such as cobalt (0.19 wt.%), and nickel (0.15 wt.%), indicative of pyrite’s capacity to sequester As and other trace elements. Pyrite grains were naturally formed and developed as large (20–200 μm) euhedral (i.e., cube, octahedron) crystals and non-framboid aggregates. However, the inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the As concentration in the groundwater was not high, and it was within the EPA drinking water standard for As (10 µg/L). These results indicate that dissolved As is sequestered in naturally formed pyrite found in the fluvial sediments. The groundwater was moderately reducing to slightly oxidizing (Eh = 46 to173 mV), and nearly neutral to slightly acidic (pH = 5.53 to 6.51). Groundwater geochemistry data indicated a redox sequence of oxidation, denitrification, Mn(IV) reduction, Fe(III) reduction, and sulfate reduction along the flow path in the fluvial aquifer. The downgradient increases in dissolved Mn and then Fe concentrations reflect increased Mn(II) and Fe(II) production via microbial competition as the aquifer becomes progressively more reduced. Bacterial sulfate reduction seems to dominate near the end of the groundwater flow path, as the availability of Mn- and Fe-oxyhydroxides becomes limited in sediments rich in lignitic wood where increasing sulfate reduction leads to the formation of biogenic pyrite. The groundwater is a Ca-SO4 type and is not SO4 limited; thus, sulfate may serve as an electron acceptor for the bacterial sulfate-reducing reactions that sequester As into pyrite, which in turn results in very low groundwater As concentration (<2 µg/L).

Improved Prediction of Management‐Relevant Groundwater Discharge Characteristics Throughout River Networks

AbstractGroundwater discharge zones connect aquifers to surface water, generating baseflow and serving as ecosystem control points across aquatic ecosystems. The influence of groundwater discharge on surface flow connectivity, fate and transport of contaminants and nutrients, and thermal habitat depends strongly on hydrologic characteristics such as the spatial distribution, age, and depth of source groundwater flow paths. Groundwater models have the potential to predict spatial discharge characteristics within river networks, but models are often not evaluated against these critical characteristics and model equifinality with respect to discharge processes is a known challenge. We quantify discharge characteristics across a suite of groundwater models with commonly used frameworks and calibration data. We developed a base model (MODFLOW‐NWT) for a 1,570‐km2 watershed in the northeastern United States and varied the calibration data, control of river‐aquifer exchange directionality, and resolution. Most models (n = 11 of 12) fit similarly to calibration metrics, but patterns in discharge location, flow path depth, and subsurface travel time varied substantially. We found (1) a 15% difference in the percent of discharge going to first‐order streams, (2) threefold variations in flow path depth, and (3) sevenfold variations in the subsurface travel times among the models. We recalibrated three models using a synthetic discharge location data set. Calibration with discharge location data reduced differences in simulated discharge characteristics, suggesting an approach to improved equifinality based on widespread field‐based mapping of discharge zones. Our work quantifying variation across common modeling approaches is an important step toward characterizing and improving predictions of groundwater discharge characteristics.

Isotope hydrology and hydrogeochemical modeling of Troodos Fractured Aquifer, Cyprus: The development of hydrogeological descriptions of observed water types

The origin of groundwater recharge and subsequent flow paths are often difficult to establish in fractured, multi-lithological, and highly compartmentalized aquifers such as the Troodos Fractured Aquifer (TFA). As the conjunctive use of stable isotopes and hydrogeochemical data provides additional information, we established a monitoring network for stable isotopes in precipitation in Cyprus. The local meteoric water line, altitude effect and seasonal variation of stable isotopes in precipitation are derived from monitoring data. Stable isotopes and hydrogeochemical data are combined to model water-rock interactions and groundwater evolution along a complete ophiolite sequence. As a result a generic hydrogeologic description for the observed water types is developed. Isotope hydrology was applied in conjunction with hydrogeochemical modelling in Kargiotis Watershed, a major north-south transect of the TFA. PHREEQC was used for hydrogeochemical modelling to establish generic descriptions for observed water types. Mean precipitation-weighted values from 16 monitoring stations were used to calculate the Local Meteoric Water Line (LMWL), which was found to be equal to δ2H = (6.58 ± 0.13)*δ18O + (12.64 ± 0.91). A general decrease of 1.22‰ for δ2H and 0.20‰ for δ18O in precipitation was calculated per 100 m altitude. A generic groundwater evolution path was established: 1. Na/MgClHCO3, 2. MgHCO3, 3. Ca/MgHCO3, 4. Ca/MgNaHCO3, 4a. MgNa/CaHCO3/Cl, 5. NaMg/CaHCO3/Cl, 6. NaHCO3, 7. Na/MgHCO3SO4, 8. NaSO4Cl/HCO3. Hydrogeologic descriptions, consisting of groundwater origin, flow path and possible active water-rock processes, have been realised for the observed water types. The first two water types occur in serpentine and ultramafic-gabbro springs. Type 3 waters represent early stages of recharge and/or short flow paths, in gabbro whereas types 4 and 5 are typical for further percolating waters in gabbro and diabase. Water types 6 and 7 occur both in diabase and in the basal group and represent the regional flow. Water type 8 is the end member of regional, upwelling groundwater in the basal group. The presented descriptions and methods have practical applications in groundwater exploration, characterization, and protection. The methodology can be applied in other complex aquifer systems.

Response of redox zonation to recharge in a riverbank filtration system: a case study of the Second Songhua river, NE China

Abstract Bank filtration induced by groundwater pumping results in redox zonation along the groundwater flow path. Besides the river water, recharge from other sources can change local redox conditions; therefore, redox zonation is likely to be complex within the riverbank filtration (RBF) system. In this study, hydrodynamics, hydrogeochemistry, and environmental stable isotopes were combined together to identify the redox conditions at an RBF site. The recharge characteristics and redox processes were revealed by monitoring the variations of water level, δ2H and δ18O, and redox indexes along shallow and deep flow paths. The results show that local groundwater is recharged from river, regional groundwater, and precipitation. The responses of redox zonation are sensitive to different sources. In the river water recharge zone near shore, O2, , Mn(IV), Fe(III), and are reduced in sequence, the ranges of each reaction are wider in deep groundwater because of the high-velocity deep flow. In the precipitation vertical recharge zone, precipitation intermittently drives O2, , and organic carbon to migrate through vadose zone, thereby decreasing the groundwater reducibility. In the regional groundwater lateral recharge zone in the depression cone, the reductive regional groundwater is continuously recharging local groundwater, leading to the cyclic reduction of Mn(IV) and Fe(III).