Groundwater Discharge Rates Research Articles

Submarine groundwater discharge as a major nutrient source in river‐fed vs. tidally dominated estuaries

AbstractThe tidal tributaries of the lower Chesapeake Bay experience seasonally recurring harmful algal blooms and the significance of submarine groundwater discharge (SGD) as a nutrient vector is largely unknown. Here, we determined seasonal SGD nutrient loads in two tributaries with contrasting hydrodynamic conditions, river‐fed (York River) vs. tidally dominated (Lafayette River). Radon surveys were performed in each river to quantify SGD at the embayment‐scale during spring and fall 2021. Total SGD was determined from a 222Rn mass balance and Monte Carlo simulations. Submarine groundwater discharge rates differed by a factor of two during spring (Lafayette = 11 ± 17 cm d−1; York = 6 ± 10 cm d−1) and a factor of six during fall (Lafayette = 19 ± 27 cm d−1; York = 3 ± 7 cm d−1). Groundwater N concentrations and fluxes varied seasonally in the York (4–7 mmol N m−2 d−1). In the Lafayette River, seasonal N fluxes (22–37 mmol N m−2 d−1) were driven by seasonal water exchange rates, likely due to recurrent saltwater intrusion. Submarine groundwater discharge–derived nutrient fluxes were orders of magnitude greater than riverine inputs and runoff in each system. Additionally, sediment N removal by denitrification and anaerobic ammonium oxidation would only remove ~ 1–11% of dissolved inorganic nitrogen supplied through SGD. The continued recurrence of harmful algal blooms in the Bay's tidal tributaries may be indicative of an under‐accounting of submarine groundwater‐borne nutrient sources. This study highlights the importance of including SGD in water quality models used to advise restoration efforts in the Chesapeake Bay region and beyond.

Seasonal dynamics of groundwater discharge: Unveiling the complex control over reservoir greenhouse gas emissions

The pronounced topographical differences, giving rise to numerous water bodies, also endow these formations with substantial hydraulic gradients, leading to pronounced groundwater discharge within their low-lying, natural reservoir settings. However, the dynamics of groundwater discharge in reservoirs and their impact on greenhouse gas (GHG) production and emission under different conditions remain unclear. This study focuses on a reservoir in southeastern China, where we conducted seasonal field observations alongside microcosm incubation experiments to elucidate the relationship between greenhouse gas emissions and groundwater discharge. Based on the radon (222Rn) mass balance model, groundwater discharge rates were estimated to be 2.14 ± 0.49 cm d−1 in autumn, 4.04 ± 2.09 cm d−1 in winter, 2.55 ± 1.32 cm d−1 in spring, and 2.61 ± 1.93 cm d−1 in summer. Groundwater discharge contributes on average to 31.23 % of CH4, 35.65 % of CO2, and 11.26 % of N2O emissions across all seasons in the reservoir. Groundwater primarily influences GHG emissions by directly inputting carbon and nitrogen, as well as by altering aquatic chemical conditions and the environment of dissolved organic matter (DOM), exerting significant effects particularly during spring and autumn seasons. Especially, in winter, higher groundwater discharge rates influence microbial activity and environmental conditions in the water body, including the C/N ratio, which somewhat reduces its enhancement of greenhouse gas emissions. This study provides an in-depth exploration of greenhouse gas emissions from reservoirs and examines the impact of groundwater on these emissions, aiming to reduce uncertainties in understanding greenhouse gas emission mechanisms and carbon and nitrogen cycling.

Vulnerability indicators of seawater intrusion in offshore aquifers

This study represents the first attempt to define vulnerability indicators for offshore fresh groundwater, extending prior analyses of the key threats to onshore coastal aquifers. The method applies an existing steady-state sharp-interface coastal aquifer model with semi-confined offshore extension to characterise the sensitivities of the tip and toe locations (top and bottom of the freshwater–seawater interface, respectively) and the freshwater and seawater volumes to environmental changes that threaten offshore freshwater resources. Sensitivity analysis applied to seven case studies quantifies their vulnerability to seawater intrusion from sea level rise, recharge change and a regional change in the onshore groundwater heads. The analysis demonstrates that the tip-to-toe distance in offshore aquifers is constant for different fresh groundwater discharge rates (under certain conditions). Otherwise, the interface is usually longer (flatter slope) in offshore aquifers than in onshore aquifers, except where the offshore limit of the aquifer is reached. The offshore extension of the aquifer leads to more seaward toe positions, increases the interface length and reduces the onshore seawater volume. As the freshwater discharge increases, the offshore freshwater volume becomes more vulnerable to impacts from changes in the freshwater discharge until the tip reaches the offshore limit or the toe crosses the coastline. For high values of freshwater discharge, the volume of freshwater stored offshore is more vulnerable to losses from changes in the freshwater discharge rate than is the onshore freshwater storage. For lower values of freshwater discharge, however, offshore freshwater storage is generally less vulnerable than onshore freshwater storage. Contrasts in the behaviour of onshore and offshore freshwater require that both need to be considered in coastal aquifer vulnerability assessments.

Evidence of Groundwater Seepage and Mixing at the Vicinity of a Knickpoint in a Mountain Stream

AbstractStreamflow generation and biochemical hotspots are significantly influenced by groundwater contributions distributed along the drainage network. However, identifying the geomorphic landscape features that drive groundwater‐surface water interactions remains challenging. In this study, we investigate the role of knickpoints in controlling these interactions in a mountainous stream in Switzerland. We employ a combination of synoptic sampling of environmental tracers, endmember mixing calculations, and groundwater flow simulations. Our findings reveal substantial groundwater seepage concentrated near the knickpoint of the main river stem. Using parsimonious groundwater flow modeling, we validate the hypothesis that the topographical shape of the knickpoint enhances local groundwater discharge rates. We quantify that approximately 20% of the total catchment streamflow originates from around the knickpoint. These results indicate that knickpoints are significant hotspots for groundwater seepage and physicochemical mixing, providing a clear method for identifying major localized sources of streamflow generation.

High resolution identification and quantification of diffuse deep groundwater discharge in mountain rivers using continuous boat-mounted helium measurements

Discharge of deeply sourced groundwater to streams is difficult to locate and quantify, particularly where both discrete and diffuse discharge points exist, but diffuse discharge is one of the primary controls on solute budgets in mountainous watersheds. The noble gas helium is a unique identifier of deep groundwater discharge because groundwater with long residence times is commonly enriched in helium. In this study, a portable mass spectrometer was used to measure longitudinal variation in dissolved helium concentrations in two mountainous rivers at high spatial resolution not feasible with traditional sampling techniques. Helium profiles were then simulated using a mass-balance model to quantify longitudinal variation in groundwater discharge to the receiving rivers. Results indicate helium concentrations were enriched by multiple orders of magnitude above atmospheric equilibrium in both rivers and that this persisted for up to 18 km below observed pulse inputs in the Colorado River. Helium mass-balance models match observed longitudinal patterns with the exception of sharp initial increases in helium observed in the rivers. Increased longitudinal groundwater discharge rates correspond to mapped geologic structures in both watersheds that likely transport deep geothermal water. Models show variable sensitivity to spatial assignment of input variables representing the groundwater source, illustrating the importance of collecting data from discrete groundwater discharges where possible. The methodology shows promise for field experiments designed to assess air–water exchange rates and to quantify total groundwater discharge from a combination of discrete and diffuse sources.

Is the impact of groundwater on lake greenhouse gas dynamics underestimated? A comparative analysis of subsurface and ecological factors

Lakes are recognized as important sources of greenhouse gases (GHGs) emissions to the atmosphere, influenced by various ecological processes. Groundwater discharge into lakes, despite its small volume, has high concentrations of dissolved carbon and nitrogen that significantly affect the production and emission of GHGs from lakes. Therefore, a comprehensive investigation of the mechanisms behind lake GHGs emissions under the influence of groundwater discharge and ecological processes, holds substantial scientific importance. However, due to uncertainties and quantification challenges associated with groundwater discharge, related research is currently limited. Here, we conducted year-round field observations on Honghu Lake, a large shallow eutrophic lake in Hubei Province, China. We analyzed the seasonal variation of GHGs emissions and quantified the groundwater discharge flux using radon (222Rn). The fluxes of methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) at the water–air interface were estimated to be 31.1 ± 4.88 mg m−2 d−1, 386 ± 90.7 mg m−2 d−1, and 0.327 ± 0.072 mg m−2 d−1, respectively. CH4 is the primary greenhouse gas in Honghu Lake, contributing to 82.2 % and 62.2 % of the lake’s total emissions over 20-year and 100-year frames. The average rate of groundwater discharge was 8.19 ± 0.471 mm d−1, with the highest discharge rate in winter and the lowest in spring. The daily groundwater discharge volume accounts for 0.649 % of the lake’s total water volume. The daily contributions of groundwater discharge to the lake’s total emissions of CH4, CO2, and N2O were 0.318 %, 12.1 %, and 2.59 %, respectively. The facilitative role of groundwater discharge in CO2 emissions primarily manifests through the transport of dissolved organic carbon (DOC) and high concentrations of CO2 into the lake. Meanwhile, CH4 emissions depend on the activity of methanogenic bacteria, substrate availability, and anaerobic conditions, whereas N2O emissions are influenced by temperature and nutrient levels. Our study reveals that in the short term, the effect of groundwater on lakes is relatively minimal. Yet, its long-term role as a steady supplier of carbon and nutrients to lakes should not be overlooked. This study reveals the ways in which groundwater discharge and internal lake dynamics collectively fuel GHGs emissions. Our findings offer a new perspective and critical theoretical support for the assessment of lake greenhouse gas emissions.

Conceptualizing Controlling Factors for PFAS Salting Out in Groundwater Discharge Zones Along Sandy Beaches.

Understanding fate and transport processes for per- and poly-fluoroalkyl substances (PFAS) is critical for managing impacted sites. "PFAS Salting Out" in groundwater, defined herein, is an understudied process where PFAS in fresh groundwater mixes with saline groundwater near marine shorelines, which increases sorption of PFAS to aquifer solids. While sorption reduces PFAS mass discharge to marine surface water, the fraction that sorbs to beach sediments may be mobilized under future salinity changes. The objective of this study was to conceptually explore the potential for PFAS Salting Out in sandy beach environments and to perform a preliminary broad-scale characterization of sandy shoreline areas in the continental U.S. While no site-specific PFAS data were collected, our conceptual approach involved developing a multivariate regression model that assessed how tidal amplitude and freshwater submarine groundwater discharge affect the mixing of fresh and saline groundwater in sandy coastal aquifers. We then applied this model to 143 U.S. shoreline areas with sandy beaches (21% of total beaches in the USA), indirectly mapping potential salinity increases in shallow freshwater PFAS plumes as low (<10 ppt), medium (10-20 ppt), or high (>20 ppt) along groundwater flow paths before reaching the ocean. Higher potential salinity increases were observed in West Coast bays and the North Atlantic coastline, due to the combination of moderate to large tides and large fresh groundwater discharge rates, while lower increases occurred along the Gulf of Mexico and the southern Florida Atlantic coast. The salinity increases were used to estimate potential perfluorooctane sulfonic acid (PFOS) sorption in groundwater due to salting out processes. Low-category shorelines may see a 1- to 2.5-fold increase in sorption of PFOS, medium-category a 2.0- to 6.4-fold increase, and high-category a 3.8- to 25-fold increase in PFOS sorption. The analysis presented provides a first critical step in developing a large-scale approach to classify the PFAS Salting Out potential along shorelines and the limitations of the approach adopted highlights important areas for further research.

Flow and Transport in Coastal Aquifer‐Aquitard Systems: Experimental and Numerical Analysis

AbstractCoastal aquifers are commonly layered, and thus, a clear understanding of groundwater flow and salt transport in layered coastal aquifers is important for managing fresh groundwater. However, the influence of leakage between adjacent aquifers on flow and transport processes remains largely unknown where the influence of tides is considered. This study used laboratory experiments and numerical simulation to examine the processes of flow and transport within a tidal aquifer‐aquitard system (i.e., an unconfined aquifer underlain by a semi‐confined aquifer, with an intervening thin aquitard). The laboratory‐scale observations of the current study are the first observations of offshore fresh groundwater within a semi‐confined coastal aquifer. The numerical and laboratory results are in close agreement, revealing that upward leakage from the semi‐confined aquifer into the saltwater wedge of the overlying unconfined aquifer caused buoyant instabilities to form. The development of freshwater fingers created complex saltwater‐freshwater mixing, leading to mixed saltwater influx‐efflux patterns across the sloping aquifer‐ocean interface. Compared with non‐tidal conditions, tidal forces reduced the net upward leakage from the semi‐confined aquifer to the overlying unconfined aquifer. This increased the horizontal flow toward the sea, which in turn reduced the extent of the saltwater wedge in the semi‐confined aquifer. The higher rates of both fresh and saline submarine groundwater discharge (SGD), caused by tides, led to lower groundwater ages in the semi‐confined aquifer. These findings have important implications for unveiling the complex characteristics of seawater intrusion, SGD and geochemical hotspots within layered coastal aquifers.

Seasonal characteristics of groundwater discharge controlled by precipitation and its environmental effects in a coal mining subsidence lake, eastern China

Many regions have formed subsidence lakes due to underground mining in the world. However, seasonal variations of lacustrine groundwater discharge (LGD) rate and solute fluxes in the coal mining subsidence were rarely reported. In this study, we conducted four seasonal samplings in a coal mining subsidence, during which samples for stable water (δ18O) and radioactive (222Rn) isotopes were collected to quantify the seasonal dynamics of LGD rates. The LGD rates estimated from the 222Rn mass balance model were 10.2 ± 8.7, 5.5 ± 3.2, 11.5 ± 7.8, and 7.8 ± 4.5 mm d−1 in summer, autumn, winter and spring, respectively. According to the 18O mass balance model, the corresponding LGD rates were 15.1, 7.3, 15.6, and 11.3 mm d−1 in summer, autumn, winter and spring, respectively. We found a significant correlation between precipitation and LGD rates, suggesting precipitation was recognized as the main control factor for seasonal variations of LGD rates. Based on this correlation, the extrapolated LGD rates over a year ranged from 3.1 to 12.7 mm d−1 with an average of 8.8 mm d−1. Moreover, the fluxes of dissolved silicon (DSi), iron (Fe), and manganese (Mn) from LGD in autumn were (1.6 ± 0.9) × 105, (1.9 ± 1.1) × 104, and (1.1 ± 0.6) × 104 mol a−1, respectively. Correspondingly, in winter they were (3.5 ± 2.4) × 105, (4.1 ± 2.8) × 103, and (2.8 ± 1.9) × 103 mol a−1, respectively. This study demonstrated significantly seasonal variations of LGD, with precipitation being the main control factor of LGD in the coal mining subsidence lake. The fluxes of dissolved substance (DSi, Fe, Mn) from LGD need to be emphasized because they may have important impacts on the ecological stability in coal mining subsidence lakes.

A multi-tracer approach to quantifying groundwater discharge in flat areas

Identifying of groundwater discharge in surface waters is of outstanding interest in order to complete water balance models, understand the dynamics of surface hydrology, or quantify solute contribution, including nutrients and pollutants. This study aims to evaluate groundwater discharge into two small streams in the Atlantic coastal area of the Argentine Pampa Plain and ascertaining the relationship between groundwater age and 222Rn as a discharge indicator. In August 2017, samples were collected from two streams, La Ballenera and Vivoratá, and 222Rn activities, CFCs concentration, ionic composition, EC, and stable isotopes (δ18O, δ2H) were measured. The chemical composition and EC of the two streams was similar. The isotopic composition showed some difference in the Vivoratá stream among the samples close to the headwaters, which are more depleted and have a deuterium excess value similar to the ordinate of the local meteoric line. This behavior is consistent with the dominance of baseflow, with values most depleted appearing upstream and the most enriched being downstream as a result of slight evaporation from the course. The three samples in La Ballenera stream are closer to the global meteoric line, although greater enrichment is observed upstream. 222Rn concentrations were in the range of 1–2.5 Bq.L−1 in both. In the Vivoratá stream, 222Rn showed a tendency to increase in the downstream direction of the flow, while in the La Ballenera stream this trend was observed in the first two sections. CFCs values were significantly homogeneous at all sites. For most of the samples, the recharge year was set between 1987 and 1990, resulting in transit times in the order of 30 years. Combining field measurements of streamflow and EC, laboratory analyses of stable isotopes, CFCs and 222Rn, provides an efficient method for estimating groundwater discharge rates, which is ascertained by water apparent ages.

The influence of the 2022 extreme drought on groundwater hydrodynamics in the floodplain wetland of Poyang Lake using a modeling assessment

The 2022 extreme drought in Poyang Lake (China) caused severe damage to a wide range of sectors. It also emphasized the fact that, even in regions with abundant surface water resources, groundwater resource is still a crucial component of drought management strategies. However, providing up-to-date assessments of groundwater response remains challenging due to the limited data in complex floodplain settings. This study uses a groundwater flow modeling FEFLOW to investigate the influences of the most extreme drought in 2022 on groundwater hydrology in the floodplain wetland of Poyang Lake. Model simulation indicates that the extreme drought led to a significant decrease in floodplain groundwater levels, with a maximum drop of about 5 m. In addition, the extent of the groundwater level drop varies significantly across the floodplain wetland, particularly for the strongest impact (drop > 4 m) occurring in the eastern area of the floodplain. It is expected that the extreme drought accelerates the groundwater discharge rate that is about 3 times than that of the normal year, and thus resulting in a losing condition of the floodplain wetland. On an annual scale, the groundwater discharge of the floodplain wetland in 2022 is estimated to be about 14.5 times than that of a normal year, demonstrating a significant attenuation of groundwater storage under drought environments. The reduction in rainfall combined with the associated low lake level is most likely to exert substantial influences on reduced floodplain groundwater storage. This study is the first attempt to interrogate the response of wetland groundwater to the unpredictable drought event in Poyang Lake, contributing to improved understanding of future water resources allocation and drought resistance strategy.

Vertical leaching of paleo-saltwater in a coastal aquifer–aquitard system of the Pearl River Delta

Marine transgressive and regressive processes have promoted the formation of deltas and estuaries from the postglacial period, and the coastal aquifer-aquitard system can be identified based on the complicated sedimentary environments in the river delta. The groundwater system is usually composed of freshwater and paleo saltwater, and the vertical leaching of paleo saltwater from aquitard to fresh aquifer occurs due to density difference and gravity effect. In this study, radium quartet and radon were used as the tracers for groundwater due to their different half-lives. We constructed a radium reactive transport model considering the vertical leaching of paleo saltwater in a deltaic aquifer-aquitard system and then applied it in the Pearl River Delta (PRD). The activity ratio of 224Ra to 228Ra was used to characterize whether the groundwater system was controlled by α-recoil or sufficient 232Th under fully weathered condition, and results showed that the deltaic system was well weathered and the vertical leaching of paleo saltwater inhibited the α-recoil input and increased the decay of the sorbed parent nuclide in the aquifer. After incorporating the vertical leaching of saltwater in the model, the value of retardation factor in the shallow aquitard evidently increased, meaning that the leaching of salty porewater into the aquifer greatly affected the adsorption–desorption process of radium isotopes in the aquitard, and the value of retardation factor reduced to the lower limit in the basal aquifer, indicating that the desorption process was determined by the vertical leaching of saltwater here. After considering the effect of vertical leaching, the estimated water transit time in the aquifer was decreased and the groundwater discharge rate was increased, and the converse results were obtained in the aquitard. The proposed radium reactive transport model advances the understanding of the salinization process in coastal aquifers through vertical migration and facilitates the understanding of freshening process of paleo saltwater in the river delta with similar hydrogeological settings.

Temporal and Spatial Variations in Subterranean Estuary Geochemical Gradients and Nutrient Cycling Rates: Impacts on Groundwater Nutrient Export to Estuaries

AbstractSubterranean estuaries (STEs) form at the land‐sea boundary where groundwater and seawater mix. These biogeochemically reactive zones influence groundwater‐borne nutrient concentrations and speciation prior to export via submarine groundwater discharge (SGD). We examined a STE located along the York River Estuary (YRE) to determine if SGD delivers dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to the overlying water. We assessed variations in STE geochemical profiles with depth across locations, times, and tidal stages, estimated N removal along the STE flow path, measured hydraulic gradients to estimate SGD, and calculated potential nutrient fluxes. Salinity, dissolved oxygen (DO), DIN, and DIP varied significantly with depth and season (p &lt; 0.05), but not location or tidal stage. Ammonium dominated the DIN pool deep in the STE. Moving toward the sediment surface, ammonium concentrations decreased as nitrate and DO concentrations increased, suggesting nitrification. Potential sediment N removal rates mediated by denitrification were &lt;8 mmoles N m−2 d−1. The total groundwater discharge rate was 38 ± 11 L m−2 d−1; discharge followed tidal and seasonal patterns. Net SGD nutrient fluxes were 0.065–3.2 and 0.019–0.093 mmoles m−2 d−1 for DIN and DIP, respectively. However, microbial N removal in the STE may attenuate 0.58% to &gt;100% of groundwater DIN. SGD fluxes were on the same order of magnitude as diffusive benthic fluxes but accounted for &lt;10% of the nutrients delivered by fluvial advection in the YRE. Our results indicate the importance of STE biogeochemical transformations to SGD flux estimations and their role in coastal eutrophication and nutrient dynamics.

Using 222Rn temporal and spatial distributions to estimate the groundwater discharge rate and associated nutrient fluxes into high salinity lakes in Badain Jaran Desert, Northwest China.

Groundwater is the main source of water and salt recharge for to lakes in arid regions. Quantifying the groundwater discharge and its nutrient input is important in the evolution of lake environments in the Badain Jaran Desert (BJD), Northwest China. In this study, ten BJD lakes were sampled for 222Rn in April and September 2021, and the 222Rn mass balance model was used to quantify the groundwater discharge rates and derived nutrient fluxes to these lakes. The results showed that the 222Rn activity and the groundwater recharge rate of lake water both present a positively correlated with lake water depth. The hot points of high 222Rn activity in the lake water were consistent with the locations of groundwater discharge areas. According to the 222Rn temporal and spatial distributions, the mean groundwater recharge rates for the ten lakes in April and September were 5.4 ± 0.6 and 7.7 ± 1 mm/d, respectively, and the annual mean groundwater discharge rates varied between 1.1 ± 0.2 and 14.6 ± 1.6 mm/d, with a mean of 7 ± 0.9 mm/d. Given that all the perennial lakes in the BJD have the same groundwater recharge rate as the mean recharge rate of the ten studied lakes, the annual mean groundwater recharge amount received by the lakes in the entire BJD is (5.6 ± 0.7) × 107 m3/a. According to the groundwater recharge amount, the inputs of dissolved inorganic nitrogen, dissolved inorganic phosphorus, dissolved inorganic silicon, total nitrogen, and total phosphorus to the BJD lakes from groundwater were (4.7 ± 0.6) × 105, (3.8 ± 0.5) × 104, (7.9 ± 1) × 105, (7.2 ± 0.9) × 105, (2.5 ± 0.3) × 104 kg/a, respectively. This study provides a reference for quantifying of groundwater discharge rates into salt lakes in other arid regions.

Groundwater discharge and streams drive spatial alkalinity and pCO2 dynamics in two contrasting tropical lagoons

Coral reef lagoons are areas of complex carbon cycling, however, regional (e.g. land use) and global (e.g. climate) factors, including land runoff and ocean acidification, are adversely affecting carbonate-building coral reef systems. Coupled with this, surplus nutrients entering coastal waters can prompt excess algae growth, which can stimulate further carbon dioxide (CO2) production in the water column, thus enhancing coral/sediment dissolution. However, new inputs of alkalinity into coastal systems can buffer against acidification. By combining the natural groundwater tracer radon (222Rn) with carbonate chemistry in two contrasting Cook Island lagoons (the fringing Muri Lagoon on Rarotonga and the comparatively larger Aitutaki near-atoll lagoon), we were able to identify multiple drivers of coral reef acidification and regulation. Despite the lagoons having similar rates of submarine groundwater discharge (3.1 to 3.3 cm d−1; although the rate in Aitutaki is a maximum rate based on an assumed 100 m seepage face), groundwater inputs of CO2 and alkalinity were primarily driven by different sources (discrete offshore seeps in Muri Lagoon and dredged channels in Aitutaki Lagoon). Streams delivered low alkalinity water to both lagoons, but high pCO2 waters to the Aitutaki Lagoon in contrast to Muri Lagoon. Aragonite saturation states (ΩAr) ranged between 2.2 and 5.2, with areas of low ΩAr corresponding to areas of high radon and excess algal growth in Muri Lagoon, and areas that receive low alkalinity surface water in Aitutaki. Time-series sampling indicated that tidal heights and the ability of seawater to overtop the fringing reef influenced groundwater dynamics, lagoon hydrodynamics and carbonate chemistry. Groundwater discharge and stream flows were a significant freshwater source of new geologic CO2 and alkalinity to each lagoon, while recirculated seawater is likely a significant source of biologic CO2 driven by microbial respiration in sediments. The study found that while groundwater inputs of alkalinity may reduce acidification, they do not fully counteract ongoing acidification and CO2 inputs. This study also highlighted the need for future studies to undertake detailed spatial measurements to accurately characterise tropical island carbon dynamics due to the heterogeneous nature of these environments.

From Recharge, to Groundwater, to Discharge Areas in Aquifer Systems in Quebec (Canada): Shaping of Microbial Diversity and Community Structure by Environmental Factors.

Groundwater recharge and discharge rates and zones are important hydrogeological characteristics of aquifer systems, yet their impact on the formation of both subterranean and surface microbiomes remains largely unknown. In this study, we used 16S rRNA gene sequencing to characterize and compare the microbial community of seven different aquifers, including the recharge and discharge areas of each system. The connectivity between subsurface and surface microbiomes was evaluated at each site, and the temporal succession of groundwater microbial communities was further assessed at one of the sites. Bacterial and archaeal community composition varied between the different sites, reflecting different geological characteristics, with communities from unconsolidated aquifers being distinct from those of consolidated aquifers. Our results also revealed very little to no contribution of surface recharge microbial communities to groundwater communities as well as little to no contribution of groundwater microbial communities to surface discharge communities. Temporal succession suggests seasonal shifts in composition for both bacterial and archaeal communities. This study demonstrates the highly diverse communities of prokaryotes living in aquifer systems, including zones of groundwater recharge and discharge, and highlights the need for further temporal studies with higher resolution to better understand the connectivity between surface and subsurface microbiomes.

A Proposed Approach towards Quantifying the Resilience of Water Systems to the Potential Climate Change in the Lali Region, Southwest Iran

Computing the resilience of water resources, especially groundwater, has hitherto presented difficulties. This study highlights the calculation of the resilience of water resources in the small-scale Lali region, southwest Iran, to potential climate change in the base (1961–1990) and future (2021–2050) time periods under two Representative Concentration Pathways, i.e., RCP4.5 and RCP8.5. The Lali region is eminently suitable for comparing the resilience of alluvial groundwater (Pali aquifer), karst groundwater (Bibitarkhoun spring and the observation wells W1, W2 and W3) and surface water (Taraz-Harkesh stream). The log-normal distribution of the mean annual groundwater level and discharge rate of the water resources was initially calculated. Subsequently, different conditions from extremely dry to extremely wet were assigned to the different years for every water system. Finally, the resilience values of the water systems were quantified as a number between zero and one, such that they can be explicitly compared. The Pali alluvial aquifer demonstrated the maximum resilience, i.e., 1, to the future climate change. The Taraz-Harkesh stream, which is fed by the alluvial aquifer and the Bibitarkhoun karst spring, which is the largest spring of the Lali region, depicted average resilience of 0.79 and 0.59, respectively. Regarding the karstic observation wells, W1 being located in the recharge zone had the lowest resilience (i.e., 0.52), W3 being located in the discharge zone had the most resilience (i.e., 1) and W2 being located between W1 and W3 had an intermediate resilience (i.e., 0.60) to future climate change.

A Coupled SWAT-AEM Modelling Framework for a Comprehensive Hydrologic Assessment

This study attempts to integrate a Surface Water (SW) model Soil and Water Assessment Tool (SWAT) with an existing steady-state, single layer, unconfined heterogeneous aquifer Analytic Element Method (AEM) based Ground Water (GW) model, named Bluebird AEM engine, for a comprehensive assessment of SW and GW resources and its management. The main reason for integrating SWAT with the GW model is that the SWAT model does not simulate the distribution and dynamics of GW levels and recharge rates. To overcome this issue, often the SWAT model is coupled with the numerical GW model (either using MODFLOW or FEFLOW), wherein the spatial and temporal patterns of the interactions are better captured and assessed. However, the major drawback in integrating the two models (SWAT with—MODFLOW/FEM) is its conversion from Hydrological Response Unit’s (HRU)/sub-basins to grid/elements. To couple them, a spatial translation system is necessary to move the inputs and outputs back and forth between the two models due to the difference in discretization. Hence, for effective coupling of SW and GW models, it may be desirable to have both models with a similar spatial discretization and reduce the need for rigorous numerical techniques for solving the PDEs. The objective of this paper is to test the proof of concept of integrating a distributed hydrologic model with an AEM model at the same spatial units, primarily focused on surface water and groundwater interaction with a shallow unconfined aquifer. Analytic Element Method (AEM) based GW models seem to be ideal for coupling with SWAT due to their innate character to consider the HRU, sub-basin, River, and lake boundaries as individual analytic elements directly without the need for any further discretization or modeling units. This study explores the spatio-temporal patterns of groundwater (GW) discharge rates to a river system in a moist-sub humid region with SWAT-AEM applied to the San Jacinto River basin (SJRB) in Texas. The SW-GW interactions are explored throughout the watershed from 2000–2017 using the integrated SWAT-AEM model, which is tested against stream flow and GW levels. The integrated SWAT-AEM model results show good improvement in predicting the stream flow (R2 = 0.65–0.80) and GW levels as compared to the standalone SWAT model. Further, the integrated model predicted the low flows better compared to the standalone SWAT model, thus accounting for the SW-GW interactions. Almost 80% of the stream network experiences an increase in groundwater discharge rate between 2000 and 2017 with an annual average GW discharge rate of 1853 Mm3/year. The result from the study seems promising for potential applications of SWAT-AEM coupling in regions with considerable SW-GW interactions.

Parametric study of fluidization conditions of bed sediments in the hyporheic zone

AbstractThe phenomenon of fluidization of the bed sediment of the hyporheic zone involves the production of dynamic suspension of solid particles in a stream of water flowing in the direction opposite to gravity. Fluidization may be permanent; then, it becomes a spring or a headwater zone, where groundwater discharge rate is almost unchanged within an entire hydrological year. But it can also occur periodically, depending on the changing hydrodynamic conditions. In such cases, the fluidization of the bed sediment of the hyporheic zone may disturb its functioning by changing the conditions of surface water and groundwater exchange. The disturbances affect hydrodynamic and hydrochemical processes connected with water exchange, but first of all, the living conditions of hyporheic zone flora and fauna. Therefore, the phenomenon of fluidization is significant for broadly understood ecology of the hyporheic zone. The paper presents the analysis of conditions of fluidization occurrence depending on the main hydrodynamic parameters that may affect this process. The authors analysed the influence of the difference in water level between the drained aquifer and the river, the hydraulic conductivity and anisotropy of the aquifer, the vertical hydraulic conductivity in the hyporheic zone, thickness of the hyporheic zone, and the width of the river on the possibility of fluidization of the bed sediment of the hyporheic zone. The range of variability of particular parameters corresponds to the actual variability of those parameters in natural conditions. It was proved that the fluidization of the bed sediment of the hyporheic zone mostly depends on: water level difference, hydraulic conductivity of the aquifer, and value of vertical hydraulic conductivity in the hyporheic zone. The other parameters have little influence on this process.

Sedimentary basins reduce stability of Antarctic ice streams through groundwater feedbacks

Antarctica preserves Earth’s largest ice-sheet, which in response to climate warming, may lose ice mass and raise sea level by several metres. The ice-sheet bed exerts critical controls on dynamic mass loss through feedbacks between water and heat fluxes, topographic forcing, till deformation and basal sliding. Here we show that sedimentary basins may amplify critical feedbacks that are known to impact ice-sheet retreat dynamics. We create a high-resolution subglacial geology classification for Antarctica by applying a supervised machine-learning method to geophysical data, revealing the distribution of sedimentary basins. Hydro-mechanical numerical modelling demonstrates that during glacial retreat, where sedimentary basins exist, the groundwater discharge rate scales with the rate of ice unloading. Antarctica’s most dynamic ice streams, including Thwaites and Pine Island glaciers, possess sedimentary basins in their upper catchments. Enhanced groundwater discharge and its associated feedbacks are likely to amplify basal sliding and increase the vulnerability of these catchments to rapid ice retreat and enhanced dynamic mass loss.