Estimates Of Groundwater Discharge Research Articles

Groundwater driven carbon fluxes in a restored coastal saltmarsh wetland: Implications for coastal wetland restoration

Coastal wetlands play a crucial role in the global carbon cycle, yet they have been extensively degraded over the past decade. Though restoration efforts are underway worldwide, there is limited understanding of the role groundwater plays in transporting dissolved carbon within restored wetlands. Here, we address this knowledge gap by investigating water and carbon fluxes in the restored Tomago Wetlands in coastal NSW, Australia. We aim to 1) quantify surface water exports of the main carbon components, 2) estimate greenhouse gas emissions from the aquatic parts of the wetland, and 3) determine the contribution of groundwater discharge to the surface water carbon export and greenhouse gas emissions from the wetland. Sampling took place following a wet period and a freshening event. A radon mass balance model estimated groundwater discharge contributing 10 % to the surface water flow in the restored wetland. The wetland was a source of four of the five main carbon components including DOC, DIC, alkalinity, and CO2, with most of the exported carbon being in the form of alkalinity. Surface water DOC, DIC, alkalinity, and CO2 exports were, 1.3 ± 0.5, 35.5 ± 13.1, 39.4 ± 14.6 and 6.5 ± 2.1 mmol/m2/d, respectively. The average water to atmosphere flux of CO2 and CH4 were 104 ± 209 (mmol/m2/d) and 21 ± 42 (µmol/m2/d). Groundwater-driven carbon fluxes contributed 100 % of the surface water DOC, 30 % of the DIC, 15 % of the alkalinity exports, and 5 % of the overall CO2 losses (lateral aqueous export + atmospheric emissions) from the wetland, with minor contributions to CH4. Carbon exports from both the wetland and groundwater observed in this restored wetland were found to be lower than those reported in the literature for natural or mature wetlands. This would have important implications when developing carbon accounting methods. Overall, our findings highlight the importance of groundwater in carbon transport within restored wetlands and emphasise the need for inclusion of shallow groundwater dynamics in carbon budgets and inventories of coastal wetlands that have or will undergo restoration.

Identification and quantitative estimates of groundwater transfers to formerly charophyte dominated marl lakes with radon-222

Interaction with groundwater determines many processes in marl lakes. Net transfer of inorganic carbon helps define their chemical characteristics and determines their unique benthic flora. Nutrient enrichment weakens the biogeochemical buffering mechanisms which help maintain a clear-water state and many small, shallow marl lakes are prone to siltation. Despite hydrological processes being recognised as important for the complex interactions between plants, nutrient availability and physical sediment properties which shape marl lake ecology, groundwater discharge to many of these lakes has never been quantified. The aim of this study was to locate and quantify groundwater transfers to degraded marl lakes in a Special Area of Conservation on the island of Ireland. A RAD7 radon detector identified and measured elevated concentrations of 222Rn in three lakes for quantifying their groundwater influx with a 222Rn mass-balance equation. Conservative estimates of mean daily groundwater discharge to Kilroosky Lough, Drumacrittin Lough, and Dummy's Lough were 143 m3, 502 m3, and 269 m3 respectively. With extrapolation to the entire hydrological year, annual groundwater recharge contributed approximately 47 %, 155 %, and 50 % of the respective lake volumes. The areas within the lakes which were found to have the highest groundwater influence also closely matched the locations where substantial charophyte communities persist suggesting that the two are linked. These findings underline the importance of groundwater transfers for the water budget in small marl lakes and will inform management efforts to mitigate their eutrophication.

Time-Lapse Geophysical Measurements for Monitoring Coastal Groundwater Dynamics in an Unconfined Aquifer.

The coastal zone, which is the interface between land and sea, is hydrodynamically very active due to the complex interactions of various hydrological controls and variable-density fluids. These forces vary over time, resulting in a state of dynamic equilibrium in the system. The major hydrological processes in coastal aquifer systems are salt water intrusion and submarine groundwater discharge, which are interdependent. Monitoring these complex processes is crucial for sustainable coastal zone management but poses a significant research challenge. In this study, we demonstrate the effectiveness of non-invasive geophysical techniques, specifically the time-lapse electrical resistivity imaging method, in conjunction with groundwater monitoring, for monitoring coastal groundwater dynamics in an unconfined aquifer at varying time scales and hydrogeological settings present at formerly glaciated sites worldwide. We generated two-dimensional baseline salt water intrusion maps for the test site, located on the coast of Rhode Island, USA. The time-lapse electrical resistivity survey method enables the rapid estimation of fresh groundwater discharge. Our approach offers insight into the mechanisms and seasonably variable salt water-freshwater interactions in unconfined heterogeneous aquifers. Although the results are site-specific, their implications are broad and may stimulate other studies related to sea to land pollution (sea water intrusion) and land to sea pollution (groundwater discharge) in heterogeneous coastal aquifer settings.

Intercomparison of marine tracer and catchment-based submarine groundwater discharge estimates for a karst aquifer

Submarine groundwater discharge (SGD) is a major source of solutes to the ocean. Consequently, the accuracy of estimates of the magnitude and seasonal variability of SGD is critical for the management of coastal marine ecosystems. However, very few intercomparison studies between different conceptual approaches have been made to assess the reliability and consistency of SGD flow estimates while simultaneously gauging the applicability of underlying models and budget assumptions in real-world scenarios. To date, intercomparison efforts have focused on homogeneous aquifers, and less so on highly heterogeneous aquifers such as karst systems. Here we compare the results of tracer-based (salt mass balance, tidal prism and radon mass balances) and catchment-based methods (water balance, semi-distributed karst network model) to assess SGD at a data-rich coastal location. All approaches tested here reveal a positive relationship between SGD fluxes and groundwater level measured close to the preferential groundwater flow pathways in the phreatic karst aquifer (e.g. water-saturated conduits or fracture network). We use this finding to derive site-specific relationships between groundwater level and SGD flow rates. We derive these relationships from least square fitting of Darcian and Non-Darcian models and regression analysis. The relationships validate the findings derived independently from hydraulic/hydrogeological modelling of the complex karst networks. By combining various estimates made with distinct assumptions, this method produces robust estimates of the SGD seasonal variability, essential to develop optimal management strategies of SGD resources in coastal karstic regions. Where direct measurements of SGD are not possible, monitoring of coastal groundwater level close to preferential phreatic groundwater pathways can be thus used as a useful indicator to compare past groundwater flow measurements and derive site-specific relationships, particularly in settings where large flow rate changes are present such as karst aquifers.

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.

Spatiotemporal estimation of fresh submarine groundwater discharge across the coastal shorelines of Oahu Island, Hawaii

Abstract The integrated hydrological model is a powerful tool that is used to assess the temporal distribution of fresh groundwater discharge especially in coastal areas. The coastal regions of Hawaii are examples of crucial natural resources for the Hawaiian economy and general ecological health. To fully comprehend the intricate interactions between coastal hydrology processes and ecosystems, it is necessary to evaluate the fresh submarine groundwater discharge (FSGD) at the Heeia shoreline using an integrated hydrological modeling technique. Under steady-state settings, the results showed that the present daily average of FSGD is around 0.43 m3/days across 1 m of the shoreline. However, we showed that the FSGD values were considerably impacted by climate change, groundwater head of the coastal aquifer, recharge rate, and sea level rise, particularly by the end of the 21st century. The post-development FSGD fluxes were 1.5–3.5 times greater than the freshwater transported by the Heeia stream, demonstrating the considerable contribution of the FSGD to the coastal zones of Heeia. The results also showed an exponential association between the FSGD and the groundwater level for the coastal unconfined aquifer.

Seasonality in groundwater recharge in Coastal Southwestern India and its hydrological implications based on stable isotopes (δ18O, δD)

This study observes and interprets the spatio-temporal variation in the oxygen and hydrogen isotopic composition (δ18O, δD) of shallow groundwater in the Southwestern Coastal state of India, Kerala, in terms of seasonally varying rainfall, physiography, and hydrogeological settings of this region. This study identifies effective groundwater recharge sources and their relative contributions and provides first-order estimates of submarine groundwater discharge by interpreting the spatio-temporal variations in the stable isotopes of oxygen and hydrogen in groundwater, collected from 225 locations for pre-monsoon and post-monsoon seasons in conjunction with the isotopic composition of rainfall. Spatio-temporal variations in the δ18O and d-excess, with other hydrogeological data, reveals that: (1) ∼4.9 Billion Cubic Meter (BCM) of fresh water from rainfall during May to October mixes with ∼5.9 BCM of residual pre-monsoon groundwater across Kerala. (2) ∼ 11% area of Kerala is additionally recharged by the NE monsoon rainfall. (3) The annual groundwater recharge varies physiographically from ∼2.2 BCM in the lowland to ∼1.5 BCM in the midland and ∼1.2 BCM in highland regions. (4) On an annual basis, 41% of replenishable groundwater is drained as SGD into the Arabian Sea.

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

AbstractGroundwater 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.

Voluntary metering of rural groundwater extractions: understanding and resolving the challenges

Understanding the rate of extraction from bores (or wells) can be essential in estimating groundwater discharge at a regional scale and understanding pressures on sustainable use. The challenges in doing so include the impracticality of directly measuring extractions from all, or even a large proportion of, operating bores using flow meters, especially in rural and remote areas. This challenge may be addressed by metering a representative sample of bores and generalising results to develop estimation methods; however, even achieving this presents considerable obstacles. While the benefits of metering a subset of bores to progress groundwater science and management are recognised, the obstacles to implementing metering and guidance on overcoming them are not well documented. In the Surat Basin, Australia, most groundwater bores are used for stock watering and domestic purposes, with less than 0.1% metered. As part of a research program to understand regional groundwater extraction in this area, a voluntary bore metering program has been undertaken. In this paper the challenges that arose when recruiting participants, installing and maintaining flow metering equipment, and interpreting and using data collected are described. Lessons learnt during implementation of the program that can guide other voluntary metering of rural groundwater extractions are discussed.

Nutrient delivery by groundwater discharge to headwater streams in agricultural catchments

AbstractNutrient concentrations in riparian groundwater are variable from site to site, making it challenging to use these data to quantify nutrient fluxes contributed by groundwater discharge to headwater streams. Instead, we define event‐excluded baseflow (BFEE), and use stream discharge data and stream samples collected during BFEEto estimate groundwater discharge and loading of nutrients to the streams. Based on this method, in two study catchments (Thames River basin, Ontario, Canada) groundwater discharge during BFEEcontributed about 30% of the total streamflow and 20%–30% of the total loading of soluble reactive phosphorus (SRP) and nitrate. Previous estimates of baseflow using hydrographic separation techniques indicate that groundwater discharge likely contributes even larger fractions, ~50%–60% of flow in these catchments. We infer that groundwater likely contributes ~40% of the annual SRP load and ~ 50%–60% of the nitrate‐N load. These results indicate that groundwater discharge plays an important role in the nutrient loading to headwater streams. This reinforces the findings of earlier studies that have inferred a dominant influence of legacy reservoirs of nutrients in the subsurface of similar agricultural catchments on the loads of these nutrients in headwater streams.

Present and future thermal regimes of intertidal groundwater springs in a threatened coastal ecosystem

Abstract. In inland settings, groundwater discharge thermally modulates receiving surface water bodies and provides localized thermal refuges; however, the thermal influence of intertidal springs on coastal waters and their thermal sensitivity to climate change are not well studied. We addressed this knowledge gap with a field- and model-based study of a threatened coastal lagoon ecosystem in southeastern Canada. We paired analyses of drone-based thermal imagery with in situ thermal and hydrologic monitoring to estimate discharge to the lagoon from intertidal springs and groundwater-dominated streams in summer 2020. Results, which were generally supported by independent radon-based groundwater discharge estimates, revealed that combined summertime spring inflows (0.047 m3 s−1) were comparable to combined stream inflows (0.050 m3 s−1). Net advection values for the streams and springs were also comparable to each other but were 2 orders of magnitude less than the downwelling shortwave radiation across the lagoon. Although lagoon-scale thermal effects of groundwater inflows were small compared to atmospheric forcing, spring discharge dominated heat transfer at a local scale, creating pronounced cold-water plumes along the shoreline. A numerical model was used to interpret measured groundwater temperature data and investigate seasonal and multi-decadal groundwater temperature patterns. Modelled seasonal temperatures were used to relate measured spring temperatures to their respective aquifer source depths, while multi-decadal simulations forced by historic and projected climate data were used to assess long-term groundwater warming. Based on the 2020–2100 climate scenarios (for which 5-year-averaged air temperature increased up to 4.32∘), modelled 5-year-averaged subsurface temperatures increased 0.08–2.23∘ in shallow groundwater (4.2 m depth) and 0.32–1.42∘ in the deeper portion of the aquifer (13.9 m), indicating the depth dependency of warming. This study presents the first analysis of the thermal sensitivity of groundwater-dependent coastal ecosystems to climate change and indicates that coastal ecosystem management should consider potential impacts of groundwater warming.

Groundwater discharge rates and uncertainties in a coastal lagoon using a radon mass balance

Coastal lagoons face anthropogenic pressures worldwide, and understanding groundwater discharge to these sensitive environments can enable better management of water budgets and nutrient inputs. The main objective of the study was to quantify groundwater seepage into a large and shallow coastal lagoon using a detailed radon (222Rn, a natural groundwater tracer) mass balance. To assess and reduce uncertainty, three wind-speed scenarios were modelled in two seasons using two 222Rn-in-groundwater endmembers. Groundwater discharge to the lagoon ranged between 5.2 ± 5.8 m3/s to 18.7 ± 19.6 m3/s during summer and 0.9 ± 2.2 m3/s to 8.1 ± 10.5 m3/s during winter. Wind-driven radon evasion was the most influential component of the radon mass balance. Higher summer wind speeds resulted in groundwater discharge estimates 3.5 times greater than those during winter. We carried out an in-depth uncertainty analysis of the radon mass balance results, which confirmed the importance of accounting for individual uncertainties, particularly the most sensitive parameters of the model. Overall, results from the radon mass balance revealed groundwater discharge to the lagoon was 1–2 orders of magnitude greater than previous localised seepage meter estimates. This is likely because radon tracer approaches quantify both one-directional discharge and bi-directional porewater exchange at a larger spatial scale than traditional seepage meter methods. In spite of large uncertainties, the possible range of reasonable estimates revealed groundwater seepage as an important component of the water budget at this site. This study highlights the benefits of broad-scale tracer-based mass balances for quantifying groundwater discharge to coastal lagoons and the value of in-depth uncertainty analysis to increase confidence in seepage flux estimates.

Submarine groundwater discharge and associated nutrient fluxes in the Greater Bay Area, China revealed by radium and stable isotopes

The estuary-bay system is a common and complex coastal environment. However, quantifying submarine groundwater discharge (SGD) and associated nutrient fluxes in the complex coastal environment is challenging due to more dynamic and complicated riverine discharge, ocean processes and human activities. In this study, SGD and SFGD (submarine fresh groundwater discharge) fluxes were evaluated by combining stable and radium isotopes in the Guangdong-Hong Kong-Macau Greater Bay Area (GBA), a typical estuary-bay system. We first built a spatially distributed radium mass balance model to quantify SGD fluxes in coastal areas of GBA integrating the Pearl River Estuary (PRE), bays and shelf. We then used the stable water isotope (δ2H and δ18O) end-member mixing model to distinguish submarine fresh groundwater discharge (SFGD) from SGD. Based on the 228Ra mass balance, the estimated SGD fluxes in the PRE, adjacent bay, and shelf areas were (6.14 ± 2.74) × 108 m3 d-1, (3.00 ± 1.11) × 107 m3 d-1, and (5.00 ± 5.64) × 108 m3 d-1, respectively. Results showed that the largest area-averaged SGD was in the PRE, followed by that in the adjacent shelf and the bay. These differences may be mainly influenced by ocean forces, urbanization and benthic topographies controlling the variability of groundwater pathways. Further, the three end-member mixing model of 228Ra and salinity was developed to confirm the validity of the estimated SGD using the Ra mass balance model. In the two models, groundwater end-member and water apparent age estimation were the main sources of uncertainty in SGD. The estimated SFGD flux was (1.39 ± 0.76) × 108 m3 d-1, which accounted for approximately 12% of the total SGD. Combining stable and radium isotopes was a useful method to estimate groundwater discharge. Moreover, the estimated SGD associated dissolved inorganic nitrogen (DIN) flux was one order of magnitude higher than other DIN sources. SGD was considered to be a significant contributor to the DIN loading to the GBA. The findings of this study are expected to provide valuable information on coastal groundwater management and environmental protection of the GBA and similar coastal areas elsewhere.

Fresh and saline submarine groundwater discharge in a large coastal inlet affected by seasonal upwelling

AbstractSubmarine groundwater discharge is recognized as a major source of chemicals to the global ocean, exerting large control over coastal water composition. Radon and 226Ra are used to evaluate, for the first time, the occurrence and magnitude of submarine groundwater discharge in the Ría de Vigo, a large, highly productive embayment affected by seasonal, wind‐driven upwelling. The system is naturally enriched in 222Rn due to the regional granitic basement geology: high 222Rn activities (up to 106 Bq m−3) are detected in wells and boreholes in the drainage basin of the embayment. High 222Rn activities (>400 Bq m−3) are also measured in certain areas of the embayment. Comparatively lower 226Ra activities (<4 103 Bq m−3) were measured in the freshwater sources to the bay. Mass balances obtained with a box model are used to perform a volumetric estimate of fresh and saline submarine groundwater discharge in the Ría de Vigo under contrasting circulation patterns. Fresh groundwater is shown to be a relevant hydrological component of the Ria de Vigo water balance, equivalent to 9% ± 4% and 23% ± 9% of the volume discharged by tributary rivers during winter and summer, respectively. On the other hand, recirculation of seawater through permeable sediments is capable of filtering the entire upper volume of the Ria de Vigo through its seafloor in <100 days and might thus be a previously overlooked major source of regenerated solutes to the system.

Evaluating Baseflow Simulation in the National Water Model: A Case Study in the Northern High Plains Region, USA

AbstractThe National Water Model (NWM) is a high‐resolution hydrological model capable of providing streamflow forecast at 2.7 million reaches across the conterminous United States (U.S.). It utilizes a conceptual (not physically explicit) module for estimating groundwater discharge (baseflow) to streams, and the baseflow module only allows for one‐way flux from the aquifer to the streams, with no interflow between surficial or groundwater catchments. This study evaluated the ability of the NWM to simulate baseflow in different geologic conditions with a case study in the Northern High Plains region. The study also evaluated two alternative formulations for the baseflow module in the NWM with the goal of improving simulated baseflow accuracy. The NWM simulated baseflow more accurately in a clayey catchment than in a sandy catchment. The model, however, over simulated baseflow during storm runoff events and under simulated during low flow events in both catchments. Analysis of groundwater recharge simulation showed that the NWM over simulated groundwater recharge for both catchment types, and reducing groundwater recharge using U.S. Geological Survey Baseflow Indexes improved the model performance for baseflow simulation, especially in the sandy catchment. Evaluation of the alteranative formulations showed that a modified Rorabaugh‐Rutledge formulation was able to improve the baseflow simulation with RMSE of 7.98 m3/s and R2 of 0.41 and also improved the timing of peak flows.

Groundwater discharge impacts marine isotope budgets of Li, Mg, Ca, Sr, and Ba

Groundwater-derived solute fluxes to the ocean have long been assumed static and subordinate to riverine fluxes, if not neglected entirely, in marine isotope budgets. Here we present concentration and isotope data for Li, Mg, Ca, Sr, and Ba in coastal groundwaters to constrain the importance of groundwater discharge in mediating the magnitude and isotopic composition of terrestrially derived solute fluxes to the ocean. Data were extrapolated globally using three independent volumetric estimates of groundwater discharge to coastal waters, from which we estimate that groundwater-derived solute fluxes represent, at a minimum, 5% of riverine fluxes for Li, Mg, Ca, Sr, and Ba. The isotopic compositions of the groundwater-derived Mg, Ca, and Sr fluxes are distinct from global riverine averages, while Li and Ba fluxes are isotopically indistinguishable from rivers. These differences reflect a strong dependence on coastal lithology that should be considered a priority for parameterization in Earth-system models.

Rethinking a groundwater flow system using a multiple-tracer geochemical approach: A case study in Moab-Spanish Valley, Utah

The Glen Canyon Group Aquifer (GCGA) is the sole source of public water supply for the city of Moab, Utah, a domestic and international tourist destination. Population and tourism growth are likely to target the GCGA for future water resources, but our analysis indicates that additional withdrawals would likely be sourced from groundwater storage and not be sustained by recharge. A quantitative estimate of groundwater discharge from the GCGA is problematic because the downgradient aquifer boundary is the Colorado River, and groundwater discharge to the river is very small compared to the river flow. A water budget based on a conceptual model of GCGA discharging into an adjacent alluvial Valley-Fill Aquifer (VFA) was reported by Sumsion (1971) and numerous subsequent studies have repeated and utilized this water budget. The GCGA contains stable isotopes, tritium, 3He/4He ratios, dissolved solids, and sulfate concentrations that contrast with the VFA, indicating it is instead recharged by local streams rather than from the GCGA. Water-budget calculations, based on: (1) measured spring discharge and streamflow gains, (2) horizontal gradients in VFA groundwater age, and (3) GCGA outcrop area vadose-zone pore waters are all less than previously thought. Using a lumped parameter model and 14C groundwater ages, we estimate recharge to the deeper GCGA (DGCGA) to be 4.2 ± 2.3 × 106 m3/yr, which is approximately equal to the measured discharge from wells and springs.

Quantification of submarine groundwater discharge (SGD) using radon, radium tracers and nutrient inputs in Punnakayal, south coast of India

The present study focused on the estimation of submarine groundwater discharge (SGD) and the effects of nutrient fluxes due to the SGD process. The parameters of SGD such as magnitude, character, and nutrient flux in Punnakayal region of South East coast of India were evaluated using multiple tracers of groundwater inputs in 2019. It was found that the elevated values for the tracers in the study area, displayed a gradational change in the values as move from estuarine part to the offshore. Simultaneous occurrence of fresh and saline SGD is observed on the study sites. Also, indicated that the SGD fluxes ranged from 0.04 to 0.12 ​m3 ​m−2 ​d−1 ​at the estuary and 0.03–0.15 ​m3 ​m−2 ​d−1 ​at the groundwater site. A substantially increased value for 222Rn activities is distinguished in the estuary to values over 312 dpm L−1. Nutrient embellishments were generally greatest at locations with substantial meteoric elements in groundwater; however, the recirculation of saltwater through the geological formation could provide a way of transferring terrestrially-derived nutrients to the coastal zone at many places.

Radium Mass Balance Sensitivity Analysis for Submarine Groundwater Discharge Estimation in Semi-Enclosed Basins: The Case Study of Long Island Sound

Estimation of submarine groundwater discharge (SGD) to semi-enclosed basins by Ra isotope mass balance is herein assessed. We evaluate 224Ra, 226Ra and 228Ra distributions in surface and bottom waters of Long Island Sound (CT-NY, USA) collected during spring 2009 and summer 2010. Surface water and bottom water Ra activities display an apparent seasonality, with greater activities during the summer. Long-lived Ra isotope mass balances are highly sensitive to boundary fluxes (water flux and Ra activity). Variation (50%) in the 224Ra, 226Ra and 228Ra offshore seawater activity results in a 63 – 74% change in the basin-wide 226Ra SGD flux and a 58 – 60% change in the 228Ra SGD flux, but only a 4 – 9% change in the 224Ra SGD flux. This highlights the need to accurately constrain long-lived Ra activities in the inflowing and outflowing water, as well as water fluxes across boundaries. Short-lived Ra isotope mass balances are sensitive to internal Ra fluxes, including desorption from resuspended particles and inputs from sediment diffusion and bioturbation. A 50% increase in the sediment diffusive flux of 224Ra, 226Ra and 228Ra results in a ~30% decrease in the 224Ra SGD flux, but only a ~6 – 10% decrease in the 226Ra and 228Ra SGD flux. When boundary mixing is uncertain, 224Ra is the preferred tracer of SGD if sediment contributions are adequately constrained. When boundary mixing is well-constrained, 226Ra and 228Ra are the preferred tracers of SGD, as sediment contributions become less important. A three-dimensional numerical model is used to constrain boundary mixing in Long Island Sound, with mean SGD fluxes of 1.2 ± 0.9 *1013 L y-1 during spring 2009 and 3.3 ± 0.7 *1013 L y-1 during summer 2010. The SGD flux to Long Island Sound during summer 2010 was one order of magnitude greater than the freshwater inflow from the Connecticut River. The maximum marine SGD-driven N flux is 14 ± 11 *108 mol N y-1 and rivals the N load of the Connecticut River.

Identifying locations and sources of groundwater discharge into Poyang Lake (eastern China) using radium and stable isotopes (deuterium and oxygen-18)

Knowledge of groundwater discharge (location and sources) into Poyang Lake is needed for water resources management and ecological security. In this study, hydrochemical and stable (δD and δ18O) and radium (223Ra, 224Ra, 226Ra, and 228Ra) isotopic approaches were employed to study the hydrochemical and isotopic characteristics of groundwater and surface water (river water and lake water) to identify the places where groundwater discharged into Poyang Lake and the groundwater discharge sources. The results showed that the groundwater discharge area was extensive during the dry season. The locations of predominant groundwater discharge were indicated by the evolution of radium and stable isotopes in lake water along two water flow profiles. At the confluence of Ganjiang and Xiushui rivers, groundwater with more negative δ18O value than that of the lake water discharged into this area, and the estimated groundwater discharge proportion in this area was close to that of the river water input. The main sources of groundwater input for Poyang Lake were inferred to originate from clastic rock pore-fissure aquifer and bedrock fissured aquifer around this lake. This study also found that groundwater affected by the anthropogenic activities may have discharged into Poyang Lake. Future studies are required to focus on groundwater discharge into Poyang Lake for its management and protection.