Groundwater Flow Paths Research Articles

River Corridor Sources Dominate CO2 Emissions From a Lowland River Network

AbstractRivers and streams are control points for CO2 emission to the air (fCO2), with emission rates often exceeding internal metabolism (net ecosystem production, NEP). The difference is usually attributed to CO2‐supersaturated groundwater inputs from upland soil respiration and rock weathering, but this implies a terrestrial‐to‐aquatic C transfer greater than estimated by terrestrial mass balance. One explanation is that riparian zones—rich in organic and inorganic C but mostly neglected in terrestrial mass balances—contribute disproportionately to fCO2. To test this hypothesis, we measured fCO2, NEP, and the lateral CO2 contributions from both terrestrial uplands (TER) and riparian wetlands (RIP) for seven reaches in a lowland river network in Florida, USA. NEP contributed about half of fCO2, but the remaining CO2 emission was generally much larger than measured TER. The relative importance of RIP versus TER varied markedly between contrasting hydrogeologic settings: RIP contributed 49% of fCO2 where geologic confinement forced lateral drainage through riparian soils, but only 12% where unconfined karst allowed deeper groundwater flowpaths that bypassed riparian zones. On a land area basis, the narrow riparian corridor yielded far more CO2 than the terrestrial uplands (33.1 vs. 1.4 g‐C m−2 yr−1), resulting in river corridors (i.e., stream channel plus adjacent wetlands, NEP + RIP) sourcing 87% of fCO2 to streams. Our findings imply that true terrestrial CO2 subsidies to streams may be smaller than previously estimated by aquatic mass balance and highlight the importance of explicitly integrating riparian zones into the conceptual model for terrestrial‐to‐aquatic C transfer.

Temperature-induced arsenic accumulation in groundwater from Pliocene aquifers of a semiarid continental basin

Contributions of geothermal reservoir and aquifer rock weathering to dissolved arsenic (As) concentrations remain poorly understood. This study investigated geochemical and isotopic characteristics of groundwater from the Quaternary aquifer (Quaternary-GW) and groundwater from the middle-Pliocene aquifer (Pliocene-GW) to reveal As enrichment mechanisms in geothermal groundwater. The results showed that As concentrations in Pliocene-GW were higher than in Quaternary-GW. δ7Li values varied in a wider range in Quaternary-GW than those in Pliocene-GW, and δ11B values were generally lower in Pliocene-GW than those in Quaternary-GW. Normally, δ7Li values in Pliocene-GW showed a negative correlation with the ratio of Li to Na+, and increased along the flow path, which indicated that the prolonged residence time was responsible for high δ7Li. Negative correlations between δ11B and HCO3–, CO32– and B in Pliocene-GW revealed the processes of B desorption and silicate weathering in the middle-Pliocene aquifer. The processes of B desorption and silicate weathering were enhanced by geothermal reservoir, which was evidenced from the negative correlation between δ11B and calculated reservoir temperature. Arsenic concentration increased with increasing δ7Li along the groundwater flow path in the middle-Pliocene aquifer, indicating that prolonged residence time of Pliocene-GW promoted As enrichment. Meanwhile, As concentrations were negatively correlated with δ11B in Pliocene-GW, illustrating that silicate weathering and desorption driven by geothermal conduction were important processes for As mobilization. In samples of Pliocene-GW with similar δ7Li and relatively low δ11B, As was mainly released from the original groundwater-rock interactions, especially desorption and silicate weathering facilitated by geothermal conduction. This study provides quantitative insights into As enrichment mechanisms in geothermal groundwater.

Effects of structure and volcanic stratigraphy on groundwater and surface water flow: Hat Creek basin, California, USA

Hydrogeologic systems in the southern Cascade Range in California (USA) develop in volcanic rocks where morphology, stratigraphy, extensional structures, and attendant basin geometry play a central role in groundwater flow paths, groundwater/surface-water interactions, and spring discharge locations. High-volume springs (greater than 3 m3/s) flow from basin-filling (<800 ka) volcanic rocks in the Hat Creek and Fall River tributaries and contribute approximately half of the average annual flow of the Pit River, the largest tributary to Shasta Lake. A hydrogeologic conceptual framework is constructed for the Hat Creek basin combining new geologic mapping, water-well lithologic logs, a database of active faults, LiDAR mapping of faults and volcanic landforms, streamflow measurements and airborne thermal infrared remote sensing of stream temperature. These data are used to integrate the geologic structure and the volcanic and volcaniclastic stratigraphy to create a three-dimensional interpretation of the hydrogeology in the basin. Two large streamflow gains from focused groundwater discharge near Big Spring and north of Sugarloaf Peak result from geologic barriers that restrict lateral groundwater flow and force water into Hat Creek. The inferred groundwater-flow barriers divide the aquifer system into at least three leaky compartments. The two downstream compartments lose streamflow in the upstream reaches (immediately downstream of the groundwater-flow barriers) and gain in downstream reaches with the greatest inflows immediately upstream of the barriers.

Characterizing seasonal differences in hydrological flow paths and source water contributions to alpine tundra streamflow

AbstractAlpine headwaters collect larger volumes of precipitation per unit area than neighbouring lowlands, recharge regional aquifers, and generate a greater proportion of river discharge than their limited extent would suggest. Despite the importance of alpine headwaters, field observations and assessments of source water contributions to streamflow in alpine tundra settings are sparse throughout the subarctic and absent in the Taiga Cordillera ecozone specifically. As such, it remains uncertain how changes to seasonally specific hydrological processes control discharge of the larger rivers to which these alpine headwaters contribute. This study quantified variability in source water contributions and flow paths during the 2019 open water season within a Mackenzie Mountain alpine tundra basin based on measurements of stable water isotopes, specific conductivity (SPC), and water volumes during runoff generating events. During the freshet, large daily snowmelt volumes resulted in the greatest volume of streamflow, which was composed mainly of pre‐event water (~92%). As the summer progressed, evapotranspiration increased, and groundwater flow paths extended, resulting in reduced event water fractions and hydrograph amplitude, and an extended duration of streamflow response. A headwater subbasin within the larger study basin was both hydrologically and isotopically unresponsive to summer rains, and instead was characterized by a delayed hydrograph response and reduced event water fraction. Results indicate this portion of the catchment was regulated by discharge from groundwater springs capable of sustaining streamflow before snowmelt commenced and during the dry summer months. As climate change continues, greater precipitation volumes and a longer open water season will likely result in reduced runoff and stream discharge from alpine basins as greater evapotranspiration and channel bed infiltration occur. This study provides a valuable data set and observations of seasonally distinct runoff generation processes to inform prediction of changes in northern alpine tundra hydrology in response to a warming climate.

Mobilization of subsurface carbon pools driven by permafrost thaw and reactivation of groundwater flow: a virtual experiment

Permafrost thaw leads to an increase in groundwater circulation and potential mobilization of organic carbon sequestered in deep Arctic sediments (e.g. 3–25 m below surface). Upon thaw, a portion of this carbon may be transported along new groundwater flow paths to surface waters or be microbially transformed or immobilized by in-situ biogeochemical reactions. The fate of thaw-mobilized carbon impacts surface water productivity and global climate. We developed a numerical model to investigate the effects of subsurface warming, permafrost thaw, and resultant increased groundwater flow on the mobilization and reactive transport of dissolved organic carbon (DOC). Synthetic simulations demonstrate that mobilization and groundwater-borne DOC export are determined by subsurface thermo-chemical conditions that control the interplay of DOC production (organic matter degradation), mineralization, and sorption. Results suggest that peak carbon mobilization from these depths precedes complete permafrost loss, occurring within two centuries of thaw initiation with the development of supra-permafrost groundwater flow systems. Additionally, this study highlights the lack of field data needed to constrain these new models and apply them in real-word site-specific applications, specifically the amount and spatial variability of organic carbon in deep sediments and data to constrain DOC production rates for groundwater systems in degrading permafrost. Modeling results point to key biogeochemical parameters related to organic matter and carbon bioavailability to be measured in the field to bridge the gap between models and observations. This study provides a foundation for further developing a physics-based modeling framework to incorporate the influence of groundwater flow and permafrost thaw on permafrost DOC dynamics and export, which is imperative for advancing understanding and prediction of carbon release and terrestrial-aquatic carbon exchange in warming Artic landscapes in the coming decades.

Integrated Hydrogeochemical Groundwater Flow Path Modelling in an Arid Environment

In this study, water-rock interaction, salinity sources, evolution, and the mixing of groundwater were modelled. The objectives of this research are to understand the hydrogeochemical factors that govern groundwater composition in a shallow aquifer system, Jazan Province, Saudi Arabia. The study aquifer is called a Quaternary aquifer, which is composed of gravel, sand, sandstone, and intercalated with some shale. In this study, 80 groundwater samples have been collected and analyzed for major ions and 30 representative samples were analyzed for Oxygen-18 (δ18O) and Deuterium (δD). NETPATH and environmental isotopes were integrated and applied to study the overall geochemical processes and to identify the salinity source in the groundwater. Saturation indices calculated for carbonates minerals indicates that 49%, 74%, and 61% of groundwater samples are undersaturated in terms of calcite, aragonite, and dolomite minerals, respectively. The remaining groundwater samples (51%, 39%, and 26%) are close to saturation with calcite, dolomite, and aragonite minerals, respectively. The saturation indices of gypsum, anhydrite, silica, strontionite, and sepiolite minerals show undersaturation in all groundwater samples, which is likely due to the dilution through the groundwater recharge from the surface runoff. In this study, water-rock interaction models were employed with the concentration of major ions of all selected groundwater samples, in addition to reference waters such as rain and sea waters, to evaluate the chemistry of groundwater in the flow path. Mixing calculations suggested that there is a variable contribution of rainwater (5% to 53%) in groundwater samples. The results indicate that evaporation and infiltration have a major impact on water chemistry in the study site. The intrusion of seawater at the coastal zone is well identified in some wells. Stable isotope data (δ18O and δD) support the results and underline the impact of evaporation processes on the groundwater and infiltration of evaporated water.

Genesis of the exotic chrysocolla — “copper pitch/wad” — atacamite/brochantite ore at the Exótica (Mina Sur) deposit, Chuquicamata, Chile

Detailed mineralogical and textural studies, combined with sequential X-ray diffraction and geochemical modeling, helped to solve the “copper pitch/wad” enigma in the Exótica deposit located downstream of the Chuquicamata porphyry copper deposit. Copper pitch and copper wad are essentially chrysocolla with co-precipitated Mn oxides, mainly birnessite, as well as pseudo-amorphous Mn oxide/oxyhydroxides. Linking the mineralogical, geochemical, and textural evidences with the geological, tectonic, and climatic evolution of the Chuquicamata–Calama area, a four-step genetic model for the evolution of the Exótica deposit is presented: (A) formation of a mature supergene enrichment profile at Chuquicamata (~ 30–25 Ma to ~ 15 Ma) during an erosion-dominated regime (∼900 m of erosion) which was accompanied by acidic (pH ∼2–4) Cu-Mn-Si-dominated rock drainage (ARD) with fluid flow southwards through the Exótica valley towards the Calama Basin, resulting in a strongly kaolinized and chrysocolla/copper wad-impregnated bedrock of the Exótica deposit; (B) deposition of the Fortuna gravels in the Exótica valley (starting ∼19 Ma) intercepted the Cu-Mn-Si-dominated ARD, triggering the main chrysocolla, copper pitch/wad mineralization as syn-sedimentary mineralization by chiefly surficial flow in strongly altered gravels; (C) tectonic freezing and onset of hyper-aridity (∼15–11 Ma) exposed the enriched chalcocite blanket of Chuquicamata to oxidation, resulting in acidic (pH ~ 2–4) and Cu-Si-dominated solutions with less Mn. These solutions percolated in a slightly more reducing groundwater flow path and mineralized relatively unaltered gravels with pure chrysocolla; and (D) ingression of confined chloride-rich groundwater in the upper oxidation zone of Chuquicamata, most likely between 6 and 3 Ma, is responsible for the atacamite/brochantite mineralization (pH ~ 5.5–7) of mainly unaltered gravels in the northern and central part of the Exótica deposit.

Residence times of groundwater along a flow path in the Great Artesian Basin determined by 81Kr, 36Cl and 4He: Implications for palaeo hydrogeology

Understanding the age distribution of groundwater can provide information on both the recharge history as well as the geochemical evolution of groundwater flow systems. Of the few candidates available that can be used to date old groundwater, 81Kr shows the most promise because its input function is constant through time and there are less sources and sinks to complicate the dating procedure in comparison to traditional tracers such as 36Cl and 4He. In this paper we use 81Kr in a large groundwater basin to obtain a better understanding of the residence time distribution of an unconfined-confined aquifer system. A suite of environmental tracers along a groundwater flow path in the south–west Great Artesian Basin of Australia have been sampled. All age tracers (85Kr, 39Ar 14C, 81Kr, 36Cl and 4He) display a consistent increase in groundwater age with distance from the recharge area indicating the presence of a connected flow path. Assuming that 81Kr is the most accurate dating technique the 36Cl/Cl systematics was unravelled to reveal information on recharge mechanism and chloride concentration at the time of recharge. Current-day recharge occurs via ephemeral river recharge beneath the Finke River, while diffuse recharge is minor in the young groundwaters. Towards the end of the transect the influence of ephemeral recharge is less while diffuse recharge and the initial chloride concentration at recharge were higher.

Predicted uranium and radon concentrations in New Hampshire (USA) groundwater—Using Multi Order Hydrologic Position as predictors

AbstractTwo radioactive elements, uranium (U) and radon (Rn), which are of potential concern in New Hampshire (NH) groundwater, are investigated. Exceedance probability maps are tools to highlight locations where the concentrations of undesirable substances in the groundwater may be elevated. Two forms of statistical analysis are used to create exceedance probability maps for U and Rn in NH groundwater. The first, Boosted Regression Tree (BRT), was selected for estimating U exceedance values. It computes exceedance values directly using the Bernoulli distribution function. The second method of statistical analysis used for Rn to determine exceedance probabilities is ordinary least squares (OLS) regression. In the process of determining exceedance probabilities for U and Rn, the utility of a new dataset is investigated. That new predictor dataset is the Multi‐Order Hydrologic Position (MOHP) dataset. MOHP raster datasets have been produced nationally for the conterminous United States at a 30‐m resolution. The concept behind MOHP is that, for any given point on the earth's surface, there is the potential for a longer groundwater flow path as one goes deeper beneath the land surface. MOHP predictors were tested in both models. Three MOHP predictors were found useful in the BRT model and two in the OLS model. MOHP data were found useful as predictors along with other site characteristics in predicting U and Rn exceedance probabilities in New Hampshire groundwater.

Hard-rock investigation using a non-invasive geophysical approach

Investigation of a hard-rock terrain is essential for the development of engineering infrastructures and groundwater assessment. In such investigations, evaluation of rock mass quality and faults/fractures is a challenging task. Conventionally, borehole tests are conducted to acquire subsurface geological information, and rock mass quality parameters such as rock quality designation (RQD). However, such tests are expensive, slow, limited to some specific locations, require more equipment, provide point measurements, and are hardly conducted in a steep topographic terrain. Generally, the non-invasive geophysical methods, such as geoelectrical methods are performed to assess the hard-rock sites. However, such parameters alone provide ambiguity in interpreting the geological units. In this work, a joint approach of four geophysical parameters (e.g., electrical resistivity (ER), self-potential (SP), magnetic, and induced polarization (IP)) is proposed to assess a highly heterogeneous terrain for the installation of the Accelerator Driven System (ADS) facility in South China. Geophysical data were obtained via a variety of surveyed parameters along different profiles. Electrical resistivity tomography (ERT) was integrated with borehole tests data to determine RQD for rock mass quality evaluation over the entire area. The results of magnetic interpretation were calibrated with well data to constrain the subsurface into distinct layers. ERT and magnetic coupled with Joint Profile Method (JPM) detected several localized faults/fractures. IP was integrated with ERT to distinguish between water and clay. Groundwater flow path was marked using SP. Compared with the past investigations, this novel approach deciphers functional analogous relation of RQD with ERT, magnetic, IP and SP in the hard-rock, provides thorough insights into the subsurface, removes the ambiguity caused by single geophysical parameter or the inadequate well data, and bridges the gaps between accurate geological model and the limited data for the development of engineering structures and groundwater resources.

Integrated application of a Bayesian mixing model, numerical model, and environmental tracers to characterize groundwater recharge sources in a mountainous area

In this study, the combined use of a Bayesian mixing model (BMM), numerical model (random walk particle tracking—RWPT), and environmental tracers (δ18O–δD, 3H, and CFC) was applied to elucidate the probabilistic contribution of the recharge sources, flow path, and residence time of groundwater across the mountainous area of Jeju Island, South Korea. Especially, the BMM ability to estimate the variable recharge contributions to the aquifer by different elevations and seasons was investigated. The δ18O–δD isotopes showed that groundwater in the study area was primarily fed by precipitation during the wet season, and the BMM estimated that wet season recharge contributed to approximately 64% of the total. The BMM-based probabilistic estimation of recharge sources revealed a mixed contribution of source waters from different elevations. A notable difference in recharge flow path was observed between highland (>450 masl) and lowland (<400 masl) wells, where the inflow of source water from the regional flow was dominant in the former and both regional and local recharges served as significant groundwater sources in the latter. Evidence from age tracers (3H and CFC-12) also supported different recharge mechanisms between highland and lowland wells. A reasonable match between the BMM- and RWPT-derived recharge contributions (RMSE 0.02–0.06) was achieved within the uncertainty ranges, with RWPT being particularly useful for capturing different flow paths between highland and lowland wells. The dynamics revealed here provide important information for establishing an improved and informed groundwater management plan for the mountainous area of Jeju Island. Ultimately, this study highlights the advantageous integrated analysis of BMM, RWPT, and environmental tracer analyses to enhance the reliability of recharge area estimation and increase the collective understanding of complex hydrogeological systems in mountainous areas.

Distribution, dynamic and influence factors of groundwater arsenic in the Manas River Basin in Xinjiang, P.R.China

High arsenic (As) groundwater in Xinjiang is mainly found in deep aquifers in the Manas River Basin (MRB) and the Kuitun River Basin (KRB) in the southern Junggar Basin. Groundwater samples (n = 21; including two phreatic water samples and 19 deep confined groundwater samples), were collected along hydrogeological profiles (north-south and west-east) in a high As groundwater area in the MRB in 2021. Inorganic As species, DOC and DIC were determined. As (V) was the major inorganic As species in 95% of the groundwater samples. High As groundwater was mostly found in reducing/sub-oxidizing and alkaline environments. Along the south-north groundwater flow path, a significant positive correlation was found between groundwater As, phosphate and carbonate, suggesting that competitive desorption may have been an important arsenic mobilizing process. Further, a significant negative correlation between groundwater As and the sulfate/chloride ratio and between DOC and the sulfate/chloride were observed - this suggests that sulfate reduction mediated by microorganisms may also have been an important mobilizing process. The lack of correlation between groundwater As and DOC suggests that the composition of organic matter as well as its concentration may have been an important controlling factor in arsenic mobilization.

Groundwater flow path and travel time using numerical models to understand radium transport in deep bedrock aquifers

Groundwater flow path and travel time using numerical models to understand radium transport in deep bedrock aquifers

Hydrogeological processes and hydrochemical effects in the Manas river catchment, Northwest China, over the past 60 years

The arid inland regions around the world are suffering from extreme water shortages and ecological crises due to the impacts of climate change and enhanced human activities. This study used multiple methods including trend and abrupt analysis, correlation analysis, water-balance analysis, and hydrochemical analysis to evaluate the hydrogeological processes and hydrochemical effects in 5 periods in the Manas River catchment, a typical arid inland catchment in China. The results show that groundwater storage in the catchment decreased by 5.45 × 108 m3 over the past 60 years, while glacial melt, snowmelt, and precipitation showed an increasing trend since the 1980 s. A drop in the groundwater level in the alluvial fan and a 28 % reduction in spring discharge in the lower alluvial fan since 1959 were caused by reduced river infiltration owing to the water diversion projects. Meanwhile, the concentrations of SO42- and Cl- in the alluvial fan groundwater have more than doubled over the period 1957–2018 as a result of the decrease of both groundwater level and flow rate. Long-term over-exploitation of groundwater associated with irrigation after 1978 strongly depleted the groundwater storage and altered the flow path of groundwater in the oasis zone. In addition, the expansion of arable land and increased temperatures enhanced evapotranspiration and salinization of shallow groundwater in the oasis and desert zones. Based on the results, we propose making full use of natural underground reservoirs to increase infiltration of river water and flood water in the alluvial fan zone, and reducing the arable land area to ensure the sustainability of water resources and ecology in the middle and lower reaches of the catchment. The results will provide reference for the sustainable development of water resources and environmental protection in arid inland regions and similar catchments worldwide.

Convergence of Groundwater Discharge through the Hyporheic Zone of Streams.

Significant attention has been given to hyporheic water fluxes induced by hydromorphologic processes in streambeds and the effects they have on stream ecology. However, the impact of hyporheic fluxes on regional groundwater flow discharge zones as well as the interaction of these flows are much less investigated. The groundwater-hyporheic interactive flow not only governs solute mass and heat transport in streams but also controls the retention of solute and contamination following the discharge of deep groundwater, such as naturally occurring solutes and leakage from geological waste disposal facilities. Here, we applied a physically based modeling approach combined with extensive hydrologic, geologic and geographical data to investigate the effect of hyporheic flow on groundwater discharge in the Krycklan catchment, located in a boreal landscape in Sweden. Regional groundwater modeling was conducted using COMSOL Multiphysics by considering geologic heterogeneity and infiltration constraint of the groundwater circulation intensity. Moreover, the hyporheic flow was analyzed using an exact spectral solution accounting for the fluctuating streambed topography and superimposed with the regional groundwater flow. By comparing the discharge flow fields with and without consideration of hyporheic flows, we found that the divergence of the discharge was substantially enhanced and the distribution of the travel times of groundwater was significantly shifted toward shorter times due to the presence of hyporheic flow. Particularly important is that the groundwater flow paths contract near the streambed interface due to the hyporheic flow, which leads to a phenomenon that we name "fragmentation" of coherent areas of groundwater upwelling in pinhole-shaped stream tubes.

Incorporating streambank wells in stream mass loading studies to more effectively identify sources of solutes in stream water

Stream synoptic sampling studies that include flow estimates derived from the stream tracer dilution method are now commonly performed to identify sources and processes controlling solute transport to streams. However, a limitation of this mass-loading approach is its inability to identify the side of the stream on which a source is located in the common case where loading is largely from groundwater discharge. Such bank-specific loading information can be particularly valuable at mining-affected sites where both mining-related and natural background metal sources are often closely intermingled. Here we combine the stream mass-loading approach with data from pairs of shallow hand-installed streambank wells located on opposite sides of a gaining metal-impacted headwater stream to estimate bank-specific metal loading rates. Study results successfully identify a right-bank zone in the upper half of the primary study reach as the dominant source of loading, where groundwater discharge is elevated in metals due to natural weathering of sulfide-rich bedrock. A left-bank zone in the lower half of the primary study reach was also identified as a secondary, yet still substantial, loading source, where groundwater is likely impacted by portal discharge and/or waste piles associated with an adjacent abandoned mine. Determining the dominance of the left-bank mining-related source compared to other potential right-bank natural sulfide weathering sources would not have been possible without the streambank wells. Streambank wells also enabled collection of dissolved gas samples for groundwater dating. Computed piston-flow 3H/3He groundwater ages show little to no correlation with either metal concentrations or loading rates, suggesting that groundwater residence time and flow path variations exert little influence on groundwater discharge chemistry compared to variations in bedrock and soil composition. This study thus provides proof of concept that the proposed method of combining streambank well data with stream tracer injection and synoptic sampling can provide useful information on bank-specific mass loading rates and sources of contaminants affecting stream water quality.

Radiocarbon Dating and Stable Isotopes Content in the Assessment of Groundwater Recharge at Santiago Island, Republic of Cape Verde

The over-exploitation of coastal aquifers has led to seawater intrusion issues in many parts of the globe; this problem, which is associated with water recharge deficit and anthropogenic pollution, represents the main source of groundwater degradation in Santiago Island in the Republic of Cape Verde’s archipelago. Brackish groundwater for agriculture and human consumption is being provided to several areas on Santiago Island as the only type of available water. Chemical and isotopic data obtained in three main groundwater systems were used in the characterization of the groundwater resources and in the identification of the main source responsible for their degradation. The obtained results indicate water–rock interaction as the major process responsible for the groundwater quality reflecting its lithological composition. Carbonatite dissolution can be partially responsible for the calcium increase along the groundwater flow path. Isotopic data (δ2H, δ18O; 3H and 14C) combined with the water chemistry provided a wide characterization of the groundwater recharge and identification of salinization processes (like seawater intrusion and marine aerosols dissolution in different sectors of the island). In the eastern part of Santiago Island, a different isotopic pattern (2H-18O) was observed in the groundwater samples, which was likely ascribed to different climate conditions. Carbon-14 determinations indicate apparent groundwater ages between 3.5 and 5.1 ka BP.

Isotope method to identify and quantify organic pollutant biodegradation during natural attenuation monitoring

AbstractBACKGROUNDOrganic pollution poses a huge threat to the environment and human health. It is necessary to evaluate the degree of the physical and chemical reactions of organic pollutants and to clarify the time and space laws of their degradation, so as to guide the pollution remediation work. This study aimed to evaluate the biodegradation process of typical pollutant components (benzene (BZ) and dichloroethylene (DCE)) in groundwater in a study area using the 13C compound specific isotope analysis, and to evaluate the differences in calculated degradation rates by using theoretical results.RESULTSThe field‐measured spatio‐temporal concentrations of BZ and DCE and the isotope data of 13C and 14C in groundwater proved the leading role of microorganisms in the degradation process. The BZ degradation extent (92.11%) was higher than that of DCE (83.67%) within 1 year, and indoor experiments verified this result. Microbial degradation extents calculated by 13C isotope were 85% and 79%, respectively. The first‐order degradation kinetic model verified the reasonability of the degradation results calculated by 13C isotope; the deviations of the degradation extents calculated by field‐measured data and 13C isotope are both &lt;10%. In addition, BZ was more likely to degrade upstream of the groundwater flow path.CONCLUSIONBiodegradation was the most important attenuation method of BZ and DCE, but there were differences in the degradation abilities of BZ and DCE by biological action, and BZ was preferentially degraded over DCE in the groundwater flow path. © 2022 Society of Chemical Industry (SCI).

Characteristics of hydraulic conductivity in mountain block systems and its effects on mountain block recharge: Insights from field investigation and numerical modeling

The process wherein groundwater flowing from mountain bedrock into lowland and adjacent alluvial aquifers, known as mountain-block recharge (MBR), is found across various climatic and geological settings. An understanding of the potential groundwater flow paths in mountain block systems is necessary for comprehending MBR spatial distribution. However, poorly characterized mountain block hydraulic properties, and especially a lack of direct measurements of hydraulic conductivity (K) at depths > 200 m, limit the characterization and quantifications of the MBR processes. In this study, we analyze hydraulic data set, namely 555 in-situ K measurements at various depths from two borehole sections extending from mountain block to mountain front in a potential disposal site for high-level radioactive waste. The K dataset was categorized into two groups: one for bedrock and another for fault zones, which was further classified into fault core K, damage zone K, and general fault zone K. Using a permeability conceptual model and multiple scenarios numerical modelling, this study examined the potential flow paths of MBR processes, mainly focusing on the characteristics of K in bedrocks and the hydraulic role of fault zones in mountain block systems. The distribution of Bedrock K supports the assumption of decreased trend with depth. A logarithmic fit through Bedrock K and depth pairs resulted in Log(K) = −1.62*Log(z) − 6.52, with low predictive power. This study illustrated the localized effects and spatially variable roles of fault zones in MBR within this particular hydrogeological configuration in Beishan, China. Our results provide insights into the MBR process in crystalline mountain block systems. Additionally, the hydraulic conductivity presented here provides data on the subsurface properties of mountain block systems in a crystalline area, and further facilitates the characterization and quantification of mountain-block recharge.

Identification of hydrogeological features using remote sensing and electromagnetic methods in the hard- rock formations of the Cameroon coastal plain (Central Africa): implications for water borehole location

Sustainable access to groundwater in rural areas such as the coastal plain of Cameroon (Sub-Saharan Africa) is an ongoing challenge due to the lack of knowledge on hydrogeological features favorable to the identification of potential groundwater targets. In this study, remote sensing imagery and electromagnetic surveys were used to assess the hydrogeological potential of the Ngwei locality hard-rock. Automatic extraction of lineaments from Landsat-8 images indicated 200 lineaments with a total length of 293 km corresponding to 0.6 km of lineament per square km, suggesting a median groundwater potential. The main directions are NNW-SSE, N-S and NNE-SSW while the secondary directions are NW-SE; NE-SW and E-W. Surface geophysical investigations showed an electromagnetic field of 0 to 0.57 mV. These surface and borehole investigations confirmed basement fracturing and spatial variations in weathering profiles whose aquifer structure and yields (4.4 m3/h on average) are similar to what is commonly observed in the hard rock aquifer. The correlation between borehole productivity and the nearest lineament suggests that these lineaments are not (at least not all) surface traces of regional discontinuities that may act as major groundwater flow paths. This implies that the productivity of the Ngwei aquifers appears to be dependent on the fissured/fractured horizon, so exploration should be based on a contextual rather than general approach.Analysis of the consistency of the results with the field data allowed to propose a preliminary electromagnetic log model and also shows the value of using this approach to assess the availability of potential groundwater zones in hard rock areas. However, the methods used should be supported by further investigations to allow a better understanding of the hydrogeological characteristics of this part of the Central African basement so that no one is left behind in the achievement of the SDGs.