Riverine groundwater discharge estimation in a dynamic river corridor using 222Rn

Groundwater discharge flux into rivers (riverine groundwater discharge or RGD) is essential information for the conservation and management of aquatic ecosystems and resources. One way to estimate area-integrated groundwater discharge into surface water bodies is to measure the concentration of a groundwater tracer within the water body. We assessed groundwater discharge using ²²²Rn, a tracer common in many surface water studies, through field measurements, surface water ²²²Rn mass balance model, and groundwater flow simulation, for the seldom studied but ubiquitous setting of a flooding river corridor. The investigation was conducted at the dam-regulated Lower Colorado River (LCR) in Austin, Texas, USA. We found that ²²²Rn in both the river water and groundwater in the river bank changed synchronously over a 12-hour flood cycle. A ²²²Rn mass balance model allowed for estimation of groundwater discharge into a 500-m long reach of the LCR over the flood. The groundwater discharge ranged between negative values (indicating recharge) to 1570 m³/h; groundwater discharge from groundwater flow simulations corroborated these estimates. However, for the dynamic groundwater discharge estimated by the ²²²Rn box model, assuming whether the groundwater ²²²Rn endmember was constant or dynamic led to notably different results. The resultant groundwater discharge estimates are also highly sensitive to river ²²²Rn values. We thus recommend that when using this approach to accurately characterize dynamic groundwater discharge, the ²²²Rn in near-stream groundwater should be monitored at the same frequency as river ²²²Rn. If this is not possible, the ²²²Rn method can still provide reasonable but approximate groundwater discharge given background information on surface water-groundwater exchange time scales.     

Sub‐surface water contribution to recession flow in a mountain headwater stream system based on single monitoring campaign

Discharge in mountain streams may be a mixture of snowmelt, water from surface runoff, and deep return flow through valley bottom alluvia. We used δ¹⁸O and δ²H, solute concentrations, and ²²²Rn to determine water sources of a headwater stream located at the McDonald Creek watershed, Glacier National Park, USA, during summer recession flow period. We analysed minimal water isotope ranges of ?17.6‰ to ?16.5‰ and ?133‰ to ?121‰ for δ¹⁸O and δ²H, respectively, potentially due to dominance of snow‐derived water in the stream. Likewise, solute concentrations measured in the stream through the watershed showed minimal variation with little indication of subsurface water input into the stream. However, we observed ²²²Rn activities in the stream that ranged from 39 to 2646 Bq/m³ with the highest value measured in middle of the watershed associated with channel constriction corresponding to changes in local orientation of underlying rocks. Downstream from this point, ²²²Rn activity decreased from 581 to 117 Bq/m³ in a series of punctuated steps associated with small rapids and waterfalls that we hypothesized to cause radon degassing with a maximum predicted loss of 427 Bq/m³ along a 400 m distance. Based on mass balance calculations using ²²²Rn activity values, streamflow, and channel characteristics, we estimated that groundwater contributed between 0.3% and 29% of total flow. Overall, we estimated a 5.9% of groundwater contribution integrated for stream reach measured at McDonald Creek during recession flow period. Finally, a lower mean hyporheic flux of 14 m³/day was estimated compared to the groundwater flux of 70 710 m³/day. These assessments highlight the potential for radon as a conservative tracer that can be used to estimate subsurface water contribution in mountain streams within a complex geologic setting. Copyright © 2015 John Wiley & Sons, Ltd.     

Combining isotopic tracers (222Rn and δ13C) for improved modelling of groundwater discharge to small rivers

In regions where aquifers sustain rivers, the location and quantification of groundwater discharge to surface water are important to prevent pollution hazards, to quantify and predict low flows and to manage water supplies. ²²²Rn is commonly used to determine groundwater discharge to rivers. However, using this isotopic tracer is challenging because of the high diffusion capacity of ²²²Rn in open water. This study illustrates how a combination of isotopic tracers can contribute to an enhanced understanding of groundwater discharge patterns in small rivers. The aim of this paper is to combine ²²²Rn and δ¹³C_DIC to better constrain the physical parameters related to the degassing process of these tracers in rivers. The Hallue River (northern France) was targeted for this study because it is sustained almost exclusively by a fractured chalk aquifer. The isotopes ²²²Rn, δ¹³C_DIC, δ²H and δ¹⁸O were analysed along with other natural geochemical tracers. A mass balance model was used to simulate ²²²Rn and δ¹³C_DIC. The results of δ²H and δ¹⁸O analyses prove that evaporation did not occur in the river. The calibration of a numerical model to reproduce ²²²Rn and δ¹³C_DIC provides a best‐fit diffusive layer thickness of 3.21 × 10^?5 m. This approach is particularly useful for small rivers flowing over carbonate aquifers with high groundwater DIC where the evolution of river DIC reflects the competing processes of groundwater inflow and CO₂ degassing. This approach provides a means to evaluate groundwater discharge in small ungauged rivers. Copyright © 2014 John Wiley & Sons, Ltd.     

Large seasonal changes in the recharge of seawater in a subterranean estuary revealed by a radon tracer

Activities of radon (²²²Rn) in groundwater were continuously monitored in a saline aquifer from September 2010 to July 2011. The activities of ²²²Rn ranged from 200 to 4300 Bq m^?3, with a large seasonal variation. Because the activity of ²²²Rn in seawater is low, ²²²Rn in saline groundwater must be produced in the aquifer from radium (²²⁶Ra) in rocks and sediments. The ²²²Rn activities were higher in the warm‐dry seasons (September–November 2010 and April–May 2011) when the saline aquifer was stable. In contrast, the lowest ²²²Rn activities were observed in the cold‐dry season (December 2010 and January–March 2011), because of the effective exchange between groundwater and seawater. In addition, sudden decreases of ²²²Rn activities coincided with episodic drops in groundwater temperatures. These results reveal that lower seawater temperature in winter may result in density‐driven seawater intrusion. During the wet season (June–July 2011), ²²²Rn activities were more clearly affected by semi‐monthly and diurnal tidal pumping, showing higher ²²²Rn activities during low and spring tides. Such a tidal effect was not clearly observed during the warm‐dry and cold‐dry seasons. This result implies that the residence time of SGD in coastal zones is significantly affected by seasonal changes in driving forces such as tidal pumping and seawater intrusion. Copyright © 2016 John Wiley & Sons, Ltd.     

Groundwater–surface water connectivity in a chain-of-ponds semiarid river

Semiarid rivers are often characterized by chains of small pools connected by riffles and wet meadows. The pools can be maintained by wet season surface runoff, groundwater discharge, or some combination thereof. Using synoptic surveys for several environmental tracers (δD and δ¹⁸O of H₂O, specific electrical conductance at 25°C [EC], chloride and ²²²Rn), we evaluated the groundwater—surface water connectivity of the Light River (South Australia) along an 8 km section in the vicinity of a proposed mining development. In all three surveys (representing spring, summer and winter conditions), the pools were maintained by regional groundwater discharge based on an elevated surface water EC (9–12 dS m⁻¹) similar to regional groundwater, elevated radon-222 activities (0.09–3.0 Bq L⁻¹) and low rainfall. Most pools were perennial, either because they directly received groundwater discharge or, indirectly, had an inflow originating from upstream groundwater-fed pools. The elevated salinity of regional groundwater is a key factor for the maintenance of perennial pools in the Light River because the potential for baseflow depletion by groundwater pumping is more limited.     

Mapping groundwater discharge to a coastal lagoon using combined spatial airborne thermal imaging,radon (222Rn) and multiple physicochemical variables

Coastal lagoons are significant wetland environments found on coastlines throughout the world. Groundwater seepage may be a key component of lagoon water balances, though only a few studies have investigated large (>100 km²) coastal lagoons. In this study, we combined airborne thermal infrared imagery with continuous measurements of radon (²²²Rn—a natural groundwater tracer), conductivity, water temperature and dissolved oxygen to map groundwater seepage to a large coastal lagoon in New Zealand. We found evidence of seepage along the margins of the lagoon but not away from the margins. Our findings confirmed previously known seepage zones and identified new potential locations of groundwater inflow. Both point source and diffuse seepage occurred on the western and northwestern margins of the lagoon and parallel to the barrier between the lagoon and sea. These observations imply geologic controls on seepage. The combination of remote sensing and in-situ radon measurements allowed us to effectively map groundwater discharge areas across the entire lagoon. Combined, broad-scale qualitative methods built confidence in our interpretation of groundwater discharge locations in a large, dynamic coastal lagoon.     

222Rn as Natural Tracer for LNAPL Recovery in a Crude Oil‐Contaminated Aquifer

The objective of this study was to investigate whether ²²²Rn in groundwater can be used as a tracer for light non‐aqueous phase liquid (LNAPL) quantification at a field site treated by dual‐phase LNAPL removal. After the break of a pipeline, 5 ha of soil in the nature reserve Coussouls de Crau in southern France was contaminated by 5100 m³ of crude oil. Part of this oil seeped into the underlying gravel aquifer and formed a floating oil body of about 3.9 ha. The remediation consists of plume management by hydraulic groundwater barriers and LNAPL extraction in the source zone. ²²²Rn measurements were performed in 21 wells in and outside the source zone during 15 months. In uncontaminated groundwater, the radon activity was relatively constant and remained always >11 Bq/L. The variability of radon activity measurements in wells affected by the pump‐and‐skim system was consistent with the measurements in wells that were not impacted by the system. The mean activities in wells in the source zone were, in general, significantly lower than in wells upgradient of the source zone, owing to partitioning of ²²²Rn into the oil phase. The lowest activities were found in zones with high non‐aqueous phase liquid (NAPL) recovery. LNAPL saturations around each recovery well were furthermore calculated during a period of high groundwater level, using a laboratory‐determined crude oil–water partitioning coefficient of 38.5 ± 2.9. This yielded an estimated volume of residual crude oil of 309 ± 93 m³ below the capillary fringe. We find that ²²²Rn is a useful and cheap groundwater tracer for finding zones of good LNAPL recovery in an aquifer treated by dual‐phase LNAPL removal, but that quantification of NAPL saturation using Rn is highly uncertain.     

Determination of groundwater discharge rates and water residence time of groundwater‐fed lakes by stable isotopes of water (18O, 2H) and radon (222Rn) mass balances

Lacustrine groundwater discharge (LGD) and the related water residence time are crucial parameters for quantifying lake matter budgets and assessing its vulnerability to contaminant input. Our approach utilizes the stable isotopes of water (δ¹⁸O, δ²H) and the radioisotope radon (²²²Rn) for determining long‐term average and short‐term snapshots in LGD. We conducted isotope balances for the 0.5‐km² Lake Ammelshainer See (Germany) based on measurements of lake isotope inventories and groundwater composition accompanied by good quality and comprehensive long‐term meteorological and isotopic data (precipitation) from nearby monitoring stations. The results from the steady‐state annual isotope balances that rely on only two sampling campaigns are consistent for both δ¹⁸O and δ²H and suggested an overall long‐term average LGD rate that was used to infer the water residence time of the lake. These findings were supported by the good agreement of the simulated LGD‐driven annual cycles of δ¹⁸O and δ²H lake inventories with the observed lake isotope inventories. However, radon mass balances revealed lower values that might be the result of seasonal LGD variability. For obtaining further insights into possible seasonal variability of groundwater–lake interaction, stable water isotope and radon mass balances could be conducted more frequently (e.g., monthly) in order to use the derived groundwater discharge rates as input for time‐variant isotope balances.     

Using new mass balance methods to estimate gross surface water and groundwater exchange with naturally occurring tracer 222Rn in data poor regions: a case study in northwest China

Given that the concentration of ²²²Rn in groundwater is much higher than that in surface water and that its radioactive half‐life (3.83 d) is short, ²²²Rn is an effective tracer of groundwater–surface water interactions. In this study, a new mass balance method is presented, which can be used to estimate specific groundwater–surface water interactions within a river reach. Three possible situations of interaction between groundwater and surface water are considered, and equations based on the mass conservation of ²²²Rn are formulated for judging specific groundwater–surface water interaction processes and for calculating water flux. A case study was conducted for the Nalenggele River, Northwest China, to demonstrate the usefulness of this method. Samples of river water and groundwater containing ²²²Rn were collected from the study area to estimate the interactions between groundwater and surface water. The amount of water exchanged during these interactions was estimated and the results show that transformations between groundwater and surface water are frequent along the stream. The ²²²Rn mass balance method is highly sensitive for studying such interactions, even in areas for which conventional hydrologic data are sparse. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamic river–groundwater exchange in the presence of a saline,semi‐confined aquifer

Understanding groundwater–surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealized aquifer systems. This paper presents a field‐based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater (<500 μS/cm and <100 years old) and a semi‐confined artesian aquifer containing old and saline groundwater (electrical conductivity > 2500 μS/cm and >10 000 years old). Continuous groundwater elevation and electrical conductivity monitoring within the different aquifers and the river suggest that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re‐established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 m from the river in response to flood events approximately 10 m in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low‐salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's law were 7 m³/m/day compared with 26 and 3 m³/m/day for the same periods via mass balance using Radon (²²²Rn) and chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater–surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. Copyright © 2015 John Wiley & Sons, Ltd.     

Estimation of submarine groundwater discharge in the Il‐Gwang watershed using water budget analysis and 222Rn mass balance

Submarine groundwater discharges (SGD) were investigated in a marine watershed in south‐eastern Korea using water budget analysis and a ²²²Rn mass balance model. Multi‐layered TOPMODEL added hydrological assumption was used to estimate groundwater components in the water budget analysis. Field observations of soil moisture, rainfall, runoff and groundwater fluctuations were used for calibration and validation of the hydrologic model. Based on observed hydrological data and terrain analyses, parameters for the hydrologic model were delineated and used to describe several hydrologic responses in the watershed. SGD estimations by ²²²Rn mass balance method were also performed at Il‐Gwang bay in July, 2010, and May, June, July and Nov. 2011. The estimated groundwater through hydrologic modeling and water balance analysis was 1.3x10⁶ m³/year, which rapidly increased during typhoon season due to heavy rainfall and permeable geologic structure. The estimated groundwater was approximately 3.7–27.1% of SGD as evaluated by ²²²Rn mass balance method ranges 3.44 and 17.45 m³m^?2year^?1. Even though SGD is predominantly influenced by tide fluctuation, the head gradient (difference) from hydrologic processes associated with heavy rainfalls can also have extra significant influences. Comprehensive understanding of SGD evaluation can be improved through a simultaneous application of both these approaches. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluating the source and seasonality of submarine groundwater discharge using a radon-222 pore water transport model

Pore water radon (²²²Rn) distributions from Indian River Lagoon, Florida, are characterized by three zones: a lower zone where pore water ²²²Rn and sediment-bound radium (²²⁶Ra) are in equilibrium and concentration gradients are vertical; a middle zone where ²²²Rn is in excess of sediment-bound ²²⁶Ra and concentration gradients are concave-downward; and an upper zone where ²²²Rn concentration gradients are nearly vertical. These ²²²Rn data are simulated in a one-dimensional numerical model including advection, diffusion, and non-local exchange to estimate magnitudes of submarine groundwater discharge components (fresh or marine). The numerical model estimates three parameters, fresh groundwater seepage velocity, irrigation intensity, and irrigation attenuation, using two Monte Carlo (MC) simulations that (1) ensure the minimization algorithm converges on a global minimum of the merit function and the parameter estimates are consistent within this global minimum, and (2) provide 90% confidence intervals on the parameter estimates using the measured ²²²Rn activity variance. Model estimates of seepage velocities and discharge agree with previous estimates obtained from numerical groundwater flow models and seepage meter measurements and show the fresh water component decreases offshore and varies seasonally by a factor of nine or less. Comparison between the discharge estimates and precipitation patterns suggests a mean residence time in unsaturated and saturated zones on the order of 5 to 7 months. Irrigation rates generally decrease offshore for all sampling periods. The mean irrigation rate is approximately three times greater than the mean seepage velocity although the ranges of irrigation rates and seepage velocities are the same. Possible mechanisms for irrigation include density-driven convection, wave pumping, and bio-irrigation. Simulation of both advection and irrigation allows the separation of submarine groundwater discharge into fresh groundwater and (re)circulated lagoon water.     

What is the source of baseflow in agriculturally fragmented catchments? Complex groundwater/surface‐water interactions in three tributary catchments of the Wabash River,Indiana, USA

Some conceptual models suggest that baseflow in agriculturally fragmented watersheds may contain little, if any, groundwater. This has critical implications for stream quality and ecosystem functioning. Here, we (a) identify the sources and flowpaths contributing to baseflow using ²²²Rn and ⁸⁷Sr/⁸⁶Sr and (b) quantify mean apparent ages of groundwater and baseflow using multiple isotopic tracers (CFC, SF₆, ³⁶Cl, and ³H) in 4 small (0.08 to 0.64 km²) tributary catchments to the Wabash River in Indiana, USA. ²²²Rn activities and ⁸⁷Sr/⁸⁶Sr ratios indicate that baseflow in 3 catchments is sourced primarily from groundwater; baseflow in the fourth is dominated by a source similar to agricultural run‐off. CFC‐12 data indicate that springs in 1 catchment are discharging significant proportions of water that recharged between 1974 (42 ± 2 years) and 1961 (55 ± 2 years). Those same springs have ³⁶Cl/Cl ratios between 1,381.08 ± 29.37 (×10^?15) and 1,530.64 ± 27.65 (×10^?15) indicating that a substantial proportion of the discharge likely recharged between 1975 (41 years) and 1950 (66 years). Groundwater samples collected from streambed mini‐piezometers in a separate catchment have CFC‐12 concentrations indicating that a large proportion of the recharge occurred between 1948 (68 ± 2 years) and 1950 (66 ± 2 years). Repeat sampling conducted in September 2015 after above‐average summer rainfall did not show significant decreases in mean apparent age. The relatively old ages observed in 3 of the catchments can be explained by geological complexities that are likely present in all 4 catchments, but overwhelmed by flow from the shallow phreatic aquifer in the fourth catchment.     

Natural Radioactivity at the Sin Quyen Iron-Oxide-Copper-Gold Deposit in North Vietnam

The field radiometric and laboratory measurements were performed at the Sin Quyen copper deposit in North Vietnam. The field gamma-ray spectrometry indicated the concentration of uranium ranging from 5.5 to 87 ppm, thorium from 5.6 to 33.2 ppm, and potassium from 0.3 to 4.7%. The measured dose rates ranged from 115 to 582 nGy/h, the highest doses being at the copper ore. Concentrations in the solid samples were in the range of 20–1700 Bq/kg for uranium, 20–92.7 Bq/kg for thorium, and 7–1345 Bq/kg for potassium. The calculated doses were from 22 to 896 nGy/h; both measured and calculated dose rates are mostly related to uranium. Concentrations of radium in water samples were below 0.17 Bq/L. Uranium in water samples was significantly higher than the hydrogeological background; the maximum of 13 Bq/L was at the waste zone pool, but neither radium nor uranium were present in tap water. Radon concentration in the dwelling air was from 42 to 278 Bq/m³ for ²²²Rn and from 8 to 193 Bq/m³ for ²²⁰Rn. The estimated committed dose rates were principally related to ²²²Rn concentration and ranged from 1.1 to 8.1 mSv/y.     

Localizing and quantifying groundwater-surface water interactions at different scales: A tracer approach at the River Moselle,Germany

Groundwater-surface water interactions (GSI) connect rivers and streams with riparian areas and the adjacent aquifer. Although these interactions exert a substantial control of quantity and quality of both groundwater and surface water, knowledge on GSI along rivers at the regional scale, particularly for inland waterways, is still limited. We investigated GSI along the river Moselle, an important federal inland waterway in Germany, by using radon and tritium to identify gaining (water flux from the aquifer to the surface water) and losing (water flux from the surface water to the aquifer) stream conditions, respectively. Gaining stream conditions were identified by continuously measuring radon along the river during boat surveys with a high spatial resolution (every 2 km) during intermediate (October 2020) and near low flow conditions (August/September 2021). The tritium concentrations in surface water and groundwater and the resulting tritium inventories were used to characterize losing stream conditions Monthly tritium inventories from 2017 to 2022 revealed a mean loss for the whole period of 20.3 % and a mean gain of 21.8%. Both were probably triggered by a combination of losing stream conditions and flood-induced mass transfer of water from the aquifer back into the river as well as discharge fluctuations. At the investigated site Lehmen there were direct indications of an influence of surface water due to elevated tritium concentrations in the groundwater (up to 13.3 Bq L⁻¹). Using radon mass balance modelling, good agreements of simulated versus measured radon data with respect to two groundwater end-member scenarios were obtained during intermediate flow (Spearman's ρ: 0.97 and 0.99; MAE: 10.1 and 3.4 Bq L⁻¹) and near low flow (Spearman's ρ: 0.97 and 0.99; MAE: 11 and 6.5 Bq L⁻¹). Considerable groundwater inflow was limited to the meander of Detzem, where cumulated groundwater inflow of about 19 m³ s⁻¹ (9.5% of total discharge) and 4.2 m³ s⁻¹ (3.8% of total discharge) was simulated during intermediate and near low flow, respectively. However, the groundwater inflow was relatively low compared to alpine streams, for example. The study will help to better identify and quantify GSI at the regional scale and provide methodological guidance for future studies focusing on inland waterways.     

Tracking Groundwater Discharge to a Large River using Tracers and Geophysics

Few studies have investigated large reaches of rivers in which multiple sources of groundwater are responsible for maintaining baseflow. This paper builds upon previous work undertaken along the Fitzroy River, one of the largest perennial river systems in north‐western Australia. Synoptic regional‐scale sampling of both river water and groundwater for a suite of environmental tracers (⁴He, ⁸⁷Sr/⁸⁶Sr, ²²²Rn and major ions), and subsequent modeling of tracer behavior in the river, has enabled definition and quantification of groundwater input from at least three different sources. We show unambiguous evidence of both shallow “local” groundwater, possibly recharged to alluvial aquifers beneath the adjacent floodplain during recent high‐flow events, and old “regional” groundwater introduced via artesian flow from deep confined aquifers. We also invoke hyporheic exchange and either bank return flow or parafluvial flow to account for background ²²²Rn activities and anomalous chloride trends along river reaches where there is no evidence of the local or regional groundwater inputs. Vertical conductivity sections acquired through an airborne electromagnetic (AEM) survey provide insights to the architecture of the aquifers associated with these sources and general groundwater quality characteristics. These data indicate fresh groundwater from about 300 m below ground preferentially discharging to the river, at locations consistent with those inferred from tracer data. The results demonstrate how sampling rivers for multiple environmental tracers of different types—including stable and radioactive isotopes, dissolved gases and major ions—can significantly improve conceptualization of groundwater—surface water interaction processes, particularly when coupled with geophysical techniques in complex hydrogeological settings.     

Delineating coastal groundwater discharge processes in a wetland area by means of electrical resistivity imaging, 224Ra and 222Rn

Coastal groundwater discharge (CGD) plays an important role in coastal hydrogeological systems as they are a water resource that needs to be managed, particularly in wetland areas. Despite its importance, identifying and monitoring CGD often presents physical and logistical constraints, restraining the application of more traditional submarine groundwater discharge surveying techniques. Here we investigate the capability of electrical resistivity imaging (ERI) in the Peníscola wetland (Mediterranean coast, Spain). ERI surveying made it possible to identify and delineate an ascending regional groundwater flow of thermal and Ra‐enriched groundwater converging with local flows and seawater intrusion. The continuous inputs of Ra‐rich groundwater have induced high activities of Ra isotopes and ²²²Rn into the marsh area, becoming among the highest previously reported in wetlands and coastal lagoons. Geoelectrical imaging enabled inferring focused upward discharging areas, leaking from the aquifer roof through a confining unit and culminating as spring pools nourishing the wetland system. Forward modelling over idealized subsurface configurations, borehole datasets, potentiometric records from standpipe piezometers, petrophysical analysis, and four natural and independent tracers (²²⁴Ra, ²²²Rn, temperature and salinity) permitted assessing the geoelectrical model and a derived hydrogeological pattern. The research highlights the potential of ERI to improve hydrogeological characterization of subsurface processes in complex contexts, with different converging flows. Additionally, a hydrogeological conceptual model for a groundwater‐fed coastal wetland was proposed, based on the integration of surveying datasets. Copyright © 2013 John Wiley & Sons, Ltd.     

Comparison of environmental tracers including organic micropollutants as groundwater exfiltration indicators into a small river of a karstic catchment

Understanding groundwater–surface water (GW–SW) interactions is vital for water management in karstic catchments due to its impact on water quality. The objective of this study was to evaluate and compare the applicability of seven environmental tracers to quantify and localize groundwater exfiltration into a small, human-impacted karstic river system. Tracers were selected based on their emission source to the surface water either as (a) dissolved, predominantly geogenic compounds (radon-222, sulphate and electrical conductivity) or (b) anthropogenic compounds (predominantly) originating from wastewater treatment plant (WWTP) effluents (carbamazepine, tramadol, sodium, chloride). Two contrasting sampling approaches were compared (a) assuming steady-state flow conditions and (b) considering the travel time of the water parcels (Lagrangian sampling) through the catchment to account for diurnal changes in inflow from the WWTP. Spatial variability of the concentrations of all tracers indicated sections of preferential groundwater inflow. Lagrangian sampling techniques seem highly relevant for capturing dynamic concentration patterns of WWTP-derived compounds. Quantification of GW inflow with the finite element model FINIFLUX, based on observed in-stream Rn activities led to plausible fluxes along the investigated river reaches (0.265 m³ s⁻¹), while observations of other natural or anthropogenic environmental tracers produced less plausible water fluxes. Important point sources of groundwater exfiltration can be ascribed to locations where the river crosses geological fault lines. This indicates that commonly applied concepts describing groundwater–surface water interactions assuming diffuse flow in porous media are difficult to transfer to karstic river systems whereas concepts from fractured aquifers may be more applicable. In general, this study helps selecting the best suited hydrological tracer for GW exfiltration and leads to a better understanding of processes controlling groundwater inflow into karstic river systems.     

Hydrogeological and hydrogeochemical control of groundwater salinity in an arid inland basin: Dunhuang Basin,northwestern China

High groundwater salinity has become a major concern in the arid alluvial plain of the Dunhuang Basin in northwestern China because it poses a significant challenge to water resource management. Isotopic and geochemical analyses were conducted on 55 water samples from springs, boreholes and surface water to identify potential sources of groundwater salinity and analyse the processes that control increasing salinity. The total dissolved solid (TDS) content in the groundwater ranged from 400 to 41 000 mg/l, and high TDS values were commonly associated with shallow water tables and flow‐through and discharge zones in unconfined aquifers. Various groundwater contributions from rainwater, agricultural irrigation, river water infiltration and lateral inflows from mountains were identified by major ions and δD and δ¹⁸O. In general, HCO₃^? and SO₄^2? were the dominant anions in groundwater with a salinity of <2500 mg/l, whereas Cl^? and SO₄^2? were the dominant anions in groundwater with a salinity of >2500 mg/l. The major ion concentrations indicated that mineral weathering, including carbonate and evaporite dissolution, primarily affected groundwater salinity in recharge areas. Evapotranspiration controlled the major ion concentration evolution and salinity distribution in the unconfined groundwaters in the flow‐through and discharge areas, although it had a limited effect on groundwater in the recharge areas and confined aquifers. Agricultural irrigation increased the water table and enhanced evapotranspiration in the oasis areas of the basin. TDS and Cl became more concentrated, but H and O isotopes were not enriched in the irrigation district, indicating that transpiration dominated the increasing salinity. For other places in the basin, as indicated by TDS, Cl, δD and δ¹⁸O characteristics, evaporation, transpiration and water–rock interactions dominated at different hydrogeological zones, depending on the plant coverage and hydrogeological conditions. Groundwater ages of ³H, and δD and δ¹⁸O compositions and distributions suggest that most of the groundwaters in Dunhuang Basin have a paleometeoric origin and experienced a long residence time. These results can contribute to groundwater management and future water allocation programmes in the Dunhuang Basin. Copyright © 2015 John Wiley & Sons, Ltd.     

Investigation of submarine groundwater discharge to tidal rivers: Evidence for regional and local scale seepage

Fluxes of submarine groundwater discharge (SGD) were investigated into two tidal rivers on the north and south shore of Long Island, NY, during July 2015. Ground‐based handheld thermal infrared (TIR) imagery, combined with direct push‐point piezometer sampling, documented spatially heterogeneous small‐scale intertidal seepage zones. Pore waters were relatively fresh and enriched in nitrogen (N) within these small‐scale seeps. Pore waters sampled just 20 cm away, outside the boundary of the ground‐based TIR‐located seepage zone, were more saline and lower in N. These ground‐based TIR‐identified seeps geochemically represented the terrestrial fresh groundwater endmember, whereas N in pore waters sampled outside of the TIR‐identified seeps was derived from the remineralization of organic matter introduced into the sediment by tidal seawater infiltration. A ²²²Rn (radon‐222) time‐series was used to quantify fresh SGD‐associated N fluxes using the N endmembers sampled from the ground‐based TIR pore water profiles. N fluxes were up‐scaled to groundwater seepage zones identified from high‐resolution airborne TIR imagery using the two‐dimensional size of the airborne TIR surface water anomalies, relative to the N flux from the time‐series sampling location. Results suggest that the N load from the north‐shore tidal river to Long Island Sound is underrepresented by at least 1.6–3.6%, whereas the N load from SGD to a south‐shore tidal river may be up to 9% higher than previous estimates. These results demonstrate the importance of SGD in supplying nutrients to the lower reaches of tidal rivers and suggest that N loads in other tidal river environments may be underestimated if SGD is not accounted for.