Coastal Groundwater Research Articles

Large submarine groundwater discharges to the Arabian Sea from tropical southwestern Indian Coast: Measurements from seepage meters deployed during the low tide

Submarine Groundwater Discharge (SGD) is the flow of fresh groundwater and recirculated seawater to the sea. SGD plays a crucial role in the transport of solute-rich terrestrial groundwater as well as recirculated seawater. We conducted seasonal investigations for two years on a tropical, high-rainfall occurring coastline in southwestern India to quantify the SGD from the nearshore. A combination of subsurface seepage meters and porewater samplers was used for an entire tidal cycle to estimate the seepage rates and to understand the processes controlling the discharge. The estimated seepage rates from this region are one of the highest reported in the world. This could be due to the large span of the highly porous coastal aquifer and the high annual rainfall (over 450 cm) in this region. The seepage rates are 754 cm/day, 572 cm/day and 296 cm/day for the pre-monsoon (February 2020), post-monsoon (December 2020) and pre-monsoon (February 2021) seasons, respectively and showed high spatial and temporal variability. The end-member concentrations of groundwater and seawater were used to delineate the fresh and recirculated SGD from the total seepage. The recirculated SGD (rSGD) dominates during all the seasons taking up to 99 % of the total SGD during the pre-monsoon seasons (February) and 70 % during the post-monsoon (December) season. The fresh SGD (fSGD) is low (∼1 %) during the pre-monsoon season and increases up to 30 % during the post-monsoon. A considerable amount (0.1–10 % of the total SGD) of salinity enrichment was observed at the upper saline plume. We suspect this could be due to the evaporation of recirculated seawater due to ambient weather conditions. The fSGD in the study area is mainly controlled by the high inland hydraulic head, and the rSGD is regulated by the tides. The subsurface seepage meters used in this study eliminate the instabilities in seepage measurements and can be replicated in less-studied tropical coastal zones to quantify the volume of SGD entering the world oceans. The inferences reported can benefit the public and decision-makers in managing coastal groundwater resources and are interesting to the researchers working on delineating the hydrological and geochemical processes in the nearshore.

Hydrogeochemical evaluation for human health risk assessment from contamination of coastal groundwater aquifers of Indo-Bangladesh Ramsar site

The suitability of groundwater (GW) and the corresponding health risk caused by heavy metals become prime concerns in the recent era; so, the determination of GW quality most important for drinking, agricultural, and domestic purpose is the foremost responsibility of the concurrent researchers. Our study intends to develop an innovative approach to delineate the health hazard risk zone (HHRZ) association to the hydro chemical condition of available GW of the most prominent Indo-Bangladesh Ramsar region by coupling the groundwater quality measurement (GWQM), degree of concentration (Cd), and health risk index (HRI) with the help of hazard quotient (HQ) method. A total of 199 GW samples were collected from the Ramsar sites of both countries and twelve (12) significant physiochemical properties, namely Sodium (Na+), Potassium (K+), Magnesium (Mg2+), Nitrate (NO3−), Bicarbonate (HCO3-), Phosphate (PO42−), Fluoride (F−), Chloride (Cl−), Sulphate (SO42−), pH, Arsenic (As) and Electoral conductivity (EC) considered as water quality and health hazard causative factors for this research work. This study also assesses the individual health of each parameter and anthropogenic pressure on this Ramsar region. The produced GWQM reveals that the GW of extensive areas in the eastern, north-east, central, and south-central zone is characterized by unfit for consumption (11.47%), very poor (14.27%), and poor (20.76%) categories; whereas, health hazard map also identified quite a similar zone for significant health risk and wide geographical area of Indo-Bangladesh Ramsar site fall under very high (17.49%), high (13.26%), moderate (7.91%) health risk zone. Therefore, our study will help the policymakers, and governments of both countries in ensuring the monitoring plans, strict water management policies, and adoption of health hazard mitigating measures to ensure the well beings of the exposed inhabitants of this coastal Ramsar site region.

Multi-Annual Dynamics of a Coastal Groundwater System with Soil-Aquifer Treatment and Its Impact on the Fate of Trace Organic Compounds

The combination of managed aquifer recharge (MAR) with soil-aquifer treatment (SAT) has clear advantages for the future sustainable quality and quantity management of groundwater, especially when using treated wastewater. We built a Marthe flow and transport model of an MAR–SAT system located in a near-shore sand aquifer, for quantifying the influence of environmental factors (climate, tides, and operational conditions) on the coastal hydrosystem with regard to the fate of trace organic compounds (TrOCs). The simulations show the impact of these factors on flow rates and dilution, and thus, on the potential reactivity of TrOCs. The dilution of secondary treated wastewater (STWW) is variable, depending on the operations (feeding from infiltration ponds) and on shore proximity (dilution by saltwater). We show that, close to the ponds and during infiltration, the attenuation of TrOC concentrations can be explained by reactivity. At the natural outlet of the aquifer, the simulated average residence times ranged from about 70 to 500 days, depending upon seasonal dynamics. It is important to study TrOCs at site scale in order to anticipate the effect of natural variations on the SAT and on the fate of TrOCs.

Fast desalinization of residual saltwater using subsurface dams combined with saltwater discharge or freshwater recharge

Countermeasures against seawater intrusion (SWI) are critical to prevent coastal groundwater resources from deterioration. Subsurface dams have been recognized as one of effective approaches to prevent SWI, but most likely to produce residual saltwater at the landside of the dam. Understanding dynamic behaviors and removal mechanism of residual saltwater can facilitate aquifer desalinization, especially under combined effect of subsurface dams and hydraulic controls. Little attention has been paid to synergistic impact on residual saltwater removal. This study performed laboratory-scale experiments and numerical simulations to investigate residual saltwater trapped in the storage area upon construction of a subsurface dam along with saltwater discharge/freshwater recharge. Effects of subsurface dam designs, aquifer properties as well as discharge and recharge rates on the flow and salinity transport process were explored and compared in a coastal aquifer. Two indexes including a reduction rate of residual saltwater length (RRSL*) and removal rate of residual salt mass (RRSM*) were introduced to quantify effectiveness of residual saltwater removal. Experimental and numerical results found that the subsurface dam needs a minimum effective height to achieve complete removal of residual saltwater. Moreover, a shorter subsurface dam (e.g., 9 cm in the experiment) achieves a faster removal rate compared to a taller dam (e.g., 15 cm). Particularly, attenuation of relatively low-concentration mixing zone between 10% and 50% isolines was a quite slow process. Saltwater discharge or freshwater recharge near the dam exhibits more effectiveness in desalinization than far from it. Furthermore, extraction-injection imposed more remarkable influences on saltwater removal than aquifer properties. Findings from the study have significant implications in better understanding of residual saltwater removal mechanism and timescale for nearshore groundwater systems.

Enrichment of geogenic phosphorus in a coastal groundwater system: New insights from dissolved organic matter characterization

High concentrations of geogenic phosphorus (P) in coastal aquifer systems pose a serious and continuous threat to the health of marine ecosystems. A major source for geogenic P enrichment in aquifer systems is the mineralization of P-containing organic matter. However, the mechanisms that drive the enrichment remain unclear. Therefore, our study sought to characterize the occurrence, sources, and enrichment mechanisms of geogenic P in a coastal confined aquifer system of the Pearl River Delta, southern China. To achieve this, we conducted Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and fluorescence excitation-emission-matrix spectra (EEMs) as well as hydrochemistry and stable carbon isotope analyses. Our findings indicated that intense degradation of P-containing organic matter produced up to 8.07 mg/L of geogenic P in a reducing environment with abundant organic matter. The dissolved organic matter (DOM) of high-P groundwater (P > 1 mg/L) contained more humic-like fluorophores and exhibited higher humification. Groundwater with high P concentrations contained more aliphatic compounds and highly unsaturated-low O compounds, and the enrichment of P was mostly associated with CHOP compounds in the region of aliphatic compounds and CHON2P compounds in the region of highly unsaturated-low O compounds. Different types of dissolved organic phosphorus (DOP) can be mineralized into P, and even the mineralization of phosphonates takes precedence over the more unstable phosphate esters. P produced by the metabolism of different types of DOP was assimilated by marine microorganisms (e.g., heterotrophic bacteria and archaea), and the newly synthesized organic P compounds by chemosynthesis were subsequently released into the groundwater. Over time, P continues to be enriched in the aquifer system. This study provides new insights into subsurface P cycling in coastal aquatic systems.

Geoelectrical Characterization of Coastal Aquifers in Agbado-Ijaye, Lagos, Southwestern Nigeria; Implications for Groundwater Resources Sustainability

Water is a natural resource; its availability depends on climatic and geological conditions, and it is invariably controlled by human activities. Agbado-Ijaye lies within a coastal area, where local communities have been facing incessant water shortages, especially during the dry season. This study investigated the groundwater-bearing geological unit(s) using hydrogeophysical techniques in the coastal environment. The electrical resistivity technique, involving vertical electrical sounding (VES) and two-dimensional (2D) electrical resistivity imaging via Wenner array electrode configuration, was used to characterize the geoelectric distribution. Twenty VES stations were investigated and current electrodes (AB/2 m) spacing expanded from 1–200 m; four 2D electrical resistivity imaging traverses having a length of 200 m each and interelectrode spacing of 10 m (level 1) to 60 m (level 6) was adopted. Four geoelectric units were delineated, namely: topsoil (15–251 Ωm), clayey (28–100 Ωm), clayey sand (125–190 Ωm) and sandy (205–876 Ωm) with thicknesses ranging from 0.7–1.3 m, 4.1–19.0 m, 2.6–15.6 m and undefined depth, respectively. The 2D imaging sections also detected similar geoelectric layers, corroborating the VES-derived sections. The inverted sections delineated two different aquifers: the shallower low-yield aquifer comprising sandy clay/clayey sand units with a maximum depth of about 5.5 m. This layer is adjudged to be the continental plain sand of the Benin Formation. The deeper high-yield aquifer with a maximum depth of 30.4 m is a beach sand unit that belongs to the Tertiary Alluvium of the Dahomey Basin. The study showed that hydrogeophysical investigation is vital in exploring, developing, and managing coastal groundwater resources.

Groundwater quality assessment along the West of New Damietta Coastal City of Egypt using an integrated geophysical and hydrochemical approaches

Recently, the groundwater became very important source for the Egyptian water balance. Therefore, assessing its quality and quantity is necessary before initiating any developmental plans using this resource. In this research, a new operated power plant, which is located in the coastal area of West New Damietta City, north-east of Egypt was designed to work using the coastal groundwater aquifer. Therefore, an integrated approaches of both geophysical methods and hydrochemical analysis were applied to identify the subsurface lithology, the water-bearing layers and determine the potential use of such coastal groundwater. Therefore, a total of seventeen vertical electrical soundings (VES) and five time-domain electromagnetic soundings (TEM) were performed. Moreover, groundwater samples were collected from seven existing wells to be analyzed for water quality assessment. The geophysical results identified five geoelectric layers. The first geoelectric layer has a resistivity of 1–7.9 Ω m and a thickness range of 7–9 m. The second layer has a resistivity of 0.9–4.4 Ω m and a thickness range of 7–8 m. The third layer is 10–21 m thick with a resistivity value of 0.5–2 Ω m. The fourth layer, which is thicker (29 to 42 m), has a resistivity of 0.7–3 Ω m, while the last layer has a resistivity of 1.7–9 Ω m. According to the hydrochemical analysis, the aquifer is dominated by Na–Cl water type. The brackish nature of the water is revealed by the TDS range of 7035 mg/l to 7735 mg/l. The results collected demonstrate the groundwater's quantity and quality availability for the power plant’s sustainable use.

Numerical modeling investigations of colloid facilitated chromium migration considering variable-density flow during the coastal groundwater table fluctuation.

Fluctuation of the groundwater table in the coastal zone influences the migration of colloids in vadose zone, which can further carry contaminants to transport. To capture the variable-density water flow and the migration processes, this study developed a colloid-facilitated migration model by adjusting the adsorption coefficient and considering the relationship between colloid and salinity based on the experimental observations. This model was further applied to explore the effects of freshwater and seawater fluctuations on the migration and transformation of colloids and Cr in coastal vadose zones. The greater the hydraulic conductivity of the saturated aquifer was, the more Cr were discharged into the ocean by submarine groundwater discharge. Furthermore, the increase in the freshwater fluctuation amplitude expanded the pollution ranges of colloidal Cr and dissolved Cr. The rise of the seawater fluctuation amplitude had a more obvious reduction effect on the total solid retained Cr in the contaminant source, compared with that of the freshwater fluctuation. As the seawater fluctuation amplitude increased from 0.1m to 0.8m, the ratio of total solid retained Cr reduction in the contaminant source to the initial value increased from 1.8 % to 7.8 %. The results obtained from this study deepens our understanding of how colloids and contaminants migrate across a coastal area from vadose zone induced by the groundwater table fluctuation.

Chemicals of emerging concern in coastal aquifers: Assessment along the land-ocean interface

Submarine Groundwater Discharge (SGD) is recognized as a relevant source of pollutants to the sea, but little is known about its relevance as a source of chemicals of emerging concern (CECs). Here, both the presence and distribution of a wide range of CECs have been evaluated in the most comprehensive manner to date, in a well-characterized Mediterranean coastal aquifer near Barcelona (Spain). Samples from coastal groundwater and seawater allowed for the unique spatial characterization of the pollutants present in the land-ocean interface, an outstanding research gap that required attention. The main goals were (1) to determine CECs in the aquifer, so as to evaluate the SGD as a relevant source of marine pollution, and (2) to identify new tracers to improve our understanding of SGD dynamics. To this end, 92 CECs were located in the aquifer by using wide-scope analytical target methodologies (>2000 chemicals). Among them, the perfluoroalkyl and polyfluoroalkyl substances (PFAS), along with the pharmaceuticals carbamazepine and topiramate, were revealed to be good markers for tracing anthropogenic contamination in ground- and seawater, in concrete situations (e.g., highly contaminated sites). Additionally, non-target analysis expanded the number of potential tracers, making it a promising tool for identifying both the source and the fate of pollutants.

Use of electrical resistivity tomography to reveal the shallow freshwater–saline interface in The Fens coastal groundwater, eastern England (UK)

The Fens is a region that contributes 11% of the agri-food economy from just 4% of the agricultural land covering England (UK). This region is vulnerable to soil salinisation from sea-level rise with estimated 100-year flood events projected to be observed up to every 2 years by 2100. Seawater intrusion and upwelling of saline groundwater can provide an additional pathway; however, the area’s groundwater has not been assessed and the risk is unknown. This study used data from the British Geological Survey’s stratigraphic core archive to produce the first stratigraphic map of the loosely consolidated Holocene deposits in the South Holland–Holbeach Marsh region. There is a sandy unconfined aquifer towards the coast, a semiconfined central region with a silty cap and a clay confining cap in the north region. Electrical resistivity tomography data indicate water level depths of 0.58 ± 0.37 m above mean sea level (msl) in February 2021 and 0.01 ± 0.72 m msl in August 2021. The saline–freshwater boundary was at 1.70 ± 0.82 m msl in February 2021, deepening to 2.00 ± 1.02 m msl in August 2021, but the only evidence of seasonal fluctuation was within 10 km of the coast. A potential, but unverified, freshwater lens up to 3.25 m thick may exist beneath the surface. These results suggest that freshwater–saline interface fluctuations may primarily be driven by surface hydrology and would be vulnerable to climate-change-induced future variations in factors that affect surface water.

Digital Twin Concept Utilizing Electrical Resistivity Tomography for Monitoring Seawater Intrusion

Electrical resistivity tomography (ERT) has been seen as an appropriate instrument in several works to monitor and aid in the control of seawater intrusion (SWI) in coastal groundwater systems. This study seeks to discuss the synthesis of a digital twin that couples information between the physical space through ERT as a monitoring sensor and the digital space using SWI simulations to accurately model the behavior of SWI in the present and future settings. To showcase the concept, a Python-based simulation was presented that shows (a) the joint forward modeling-simulation scheme for calculating expected ERT apparent resistivity values from simulated SWI and (b) the calibration of the digital coastal aquifer system through genetic algorithm to accurately match the outputs of the SWI simulations with the ERT measurements.

Seawater intrusion alters nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions: Based on Bayesian isotope mixing model and microbial functional network

Seawater intrusion is a global coastal environmental issue of great concern and significantly impacts the regional biogeochemical environment and material cycles, including nitrogen cycling. To reveal the mechanism of seawater intrusion altering nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions, the Bayesian mixing model (δ15N-NO3– and δ18O-NO3–) and 16S rDNA gene amplicon sequencing are used to establish nitrogen cycling pathways and microbial functional network. The results show that the nitrate in the coastal groundwater is from manure and septic waste (M&S, over 44 %), soil organic nitrogen (SON, over 20 %), and nitrogen fertilizer (FN, over 16 %). The hydrological interaction has promoted the coupling between material cycling and microbial community in the coastal groundwater systems. Among them, precipitation infiltration has caused the gradual decrease of specific microbes along the flow direction, such as Lactobacillus, Acinetobacter, Bifidobacterium, etc. And seawater intrusion has caused the mutations of specific microbes (Planktomarina, Clade_Ia, Wenyingzhuangia, Glaciecola, etc.) and convergence of microbial community at the salt-freshwater interface in the aquifer. In the coastal intruded aquifer systems, the nitrogen cycling pattern can be divided into oxidation and reduction processes. The oxidation process involves the enhancement of nitrification while the weakening of denitrification and anammox with the increase of aquifer depth. The reduction process consists of the enhancement of denitrification and anammox while the erosion of nitrification and ammonification with increased seawater intrusion. In addition, seawater intrusion can mitigate nitrate contamination by promoting denitrification and anammox in coastal areas.

Physicochemical coastal groundwater dynamics between Kauhakō Crater lake and Kalaupapa settlement, Moloka‘i, Hawai‘i

Land-based sources of groundwater pollution can be a critical threat to coral reefs, and a better understanding of “ridge-to-reef” water movement is required to advance management and coral survival in the Anthropocene. In this study a more complete understanding of the geological, atmospheric, and oceanic drivers behind coastal groundwater exchange on the Kalaupapa peninsula, on Moloka‘i, Hawai‘i, is obtained by analyzing high resolution geochemical and geophysical time-series data. In concert with multiyear water level analyses, a tidally and precipitation-driven groundwater connection between Kauhakō Crater lake and submarine groundwater discharge (SGD) fluxes are demonstrated. Results include an average discharge rate of 190 cm d−1 and the detection of water-flow pathways past cesspools that likely contribute to higher nutrient loading near the SGD sites. This underlines the importance of managing anthropogenic nutrients that enter the shallow freshwater lens such as through cesspools and are consequently discharged via SGD onto coral reef habitats.

Advance prediction of coastal groundwater levels with temporal convolutional and long short-term memory networks

Abstract. Prediction of groundwater level is of immense importance and challenges coastal aquifer management with rapidly increasing climatic change. With the development of artificial intelligence, data-driven models have been widely adopted in hydrological process management. However, due to the limitation of network framework and construction, they are mostly adopted to produce only 1 time step in advance. Here, the temporal convolutional network (TCN) and models based on long short-term memory (LSTM) were developed to predict groundwater levels with different leading periods in a coastal aquifer. The initial data of 10 months, monitored hourly in two monitoring wells, were used for model training and testing, and the data of the following 3 months were used as prediction with 24, 72, 180, and 360 time steps (1, 3, 7, and 15 d) in advance. The historical precipitation and tidal-level data were incorporated as input data. For the one-step prediction of the two wells, the calculated R2 of the TCN-based models' values were higher and the root mean square error (RMSE) values were lower than that of the LSTM-based model in the prediction stage with shorter running times. For the advanced prediction, the model accuracy decreased with the increase in the advancing period from 1 to 3, 7, and 15 d. By comparing the simulation accuracy and efficiency, the TCN-based model slightly outperformed the LSTM-based model but was less efficient in training time. Both models showed great ability to learn complex patterns in advance using historical data with different leading periods and had been proven to be valid localized groundwater-level prediction tools in the subsurface environment.

Hydrochemical evolution processes of multiple-water quality interfaces (fresh/saline water, saline water/brine) on muddy coast under pumping conditions.

A thorough understanding of the evolution of coastal brine-bearing groundwater systems under the influence of human activities contributes to the sustainable use of coastal groundwater resources. Therefore, this study comprehensively investigated the processes associated with hydrochemical changes in groundwater during brine exploitation based on long-term monitoring data. The dataset comprised 102 samples (including groundwater, seawater, rain and river samples) collected from 1966 to 2021 to capture the hydrochemical variability. Significant changes in the brine water table and concentration between the pre-overexploitation period (1965-2000) and the exploitation period (2000-2021) are observed. From the relationship between stable isotope (δ18O and δ2H) values and Cl- contents, shallow saline water (SSW) near the drawdown cone (with δ18O and δ2H values of -4.66 ‰ ~ -3.57 ‰ and - 42.1 ‰ ~ -32.8 ‰, respectively) is similar geochemically to the brine inside the drawdown cone (with values of -4.30 ‰ ~ -3.10 ‰ and - 39.0 ‰ ~ -32.3 ‰, respectively), indicating that the SSW has a recharge effect on the underground brine. The delta values of major cations were calculated to analyze the hydrochemical processes at different water quality interfaces (saline/freshwater interface, salinewater/brine interface). The results demonstrated that the SSW and deep saline water (DSW) at the offshore brine/saline water interface undergo seawater intrusion, where SSW undergoes a salinization process with cation exchange (Na+ is exchanged for Ca2+ and Mg2+ in clay), while DSW undergoes a freshening process. The SSW at the saline/freshwater interface is affected by freshwater pumping, and the DSW undergoes a salinization process influenced by brine intrusion presenting the common salinization process with cation exchange. The hydrochemical model shows that the brine concentration recovered during the restricted exploitation stage due to the dissolution of gypsum and halite. If no measures are taken to restrict exploitation, then brine will eventually be replaced by saline water.

Hydro-chemical assessment of coastal groundwater aquifers for human health risk from elevated arsenic and fluoride in West Bengal, India

The vulnerability of groundwater in the coastal regions in terms of As, F−, and NO3− exposure is growing rapidly. Hence, the present study focused on assessing groundwater quality, ecological richness, and HR in the coastal districts of West Bengal by applying field-based CD, GWQI, ERI, and HRI techniques. After assessing the GW vulnerability, it is stated that approximately 40–50 % area of the two selected coastal district's GW is poor to very poor in quality, the ecology of GW is threatened, and human health is faced serious risk for both dry and wet season. The Wilcox and USSL diagram verified that nearly 50 % GW aquifers of coastal district of West Bengal are not fit for irrigation and drinking. The findings of this study will be beneficial to manage and control groundwater vulnerability in the coastal regions for water scientists, policy makers, and researchers as well in sustainable way.

Data analysis and integrated modeling of compound flooding impacts on coastal drainage infrastructure under a changing climate

Low-lying coastal areas are susceptible to multiple types of flooding from marine, subsurface, and surface sources. The co-occurrence of a rainfall event with high sea level, which raises coastal groundwater tables conjointly, may result in a compound flooding event, which could be much more widespread and dangerous than that of an individual flooding source. Focusing on Imperial Beach as a low-elevation coastal community in California, this paper provides a generalized methodology to determine the vulnerability of coastal stormdrain systems to compound seawater, groundwater, and stormwater flooding under a changing climate. Although marine inundation by itself is expected to flood only 7% of the study area by 2100 under the most pessimistic scenario (i.e. 2 m rise), our results show that more than 20% of the study area's subterranean stormdrain system is already at the risk of subsurface flooding at current sea levels. While marine inundation is a concern near the coastline as sea level rises, the results show that seawater intrusion into the stormdrain system can impact areas kilometers away from the coastline. The consequences of sea-level rise (SLR) can be exacerbated by an under-resourced stormdrain system, caused by groundwater infiltration through system defects. As such, by a 2 m rise in current sea-level, the flooding volume may double in an ideal system (i.e., no defects) while this ratio can increase six fold in a slightly defective stormdrain system (with 0.25 % porosity systemwide). The continuous simulations of the stormdrain system performance indicate that flood events will be more destructive and frequent under these circumstances.

Managerial sustainability indices for improving the coastal groundwater decisions by a parallel simulation-optimization model.

Seawater intrusion is one of the causes of groundwater quality degradation in coastal zones. This phenomenon is intensified by overexploitation of coastal aquifers. In this paper, optimal management strategies have been determined to prevent the advance of seawater using a parallel simulation-optimization decision model. This model has been applied to a real case study of Ajabshir aquifer located in Urmia Lake basin, Iran, for a 20-year planning horizon (2015-2034). Four categorizes of new sustainability indices (indices of protection, reliability, vulnerability, and aquifer area with a groundwater problem) as the objective functions have been examined for the first time. The developed management problems based on these four categories have been solved under two different conditions of groundwater elevation and salinity concentration. The results of 20-year period simulations indicate that by changing the extraction pattern in different regions of the aquifer (as the decision variables) based on the solution of management problems, the largest decrease in net recharge (0.065 million cubic meters) occurs in the second half of the hydrologic year (October to March) compared to the continued condition in which all factors are similar to 2014. The contribution of using indices in this study can help the local water managers to identify the high-risk areas for better planning and other coastal settings.

Occurrence of ammonium in the acidic-circumneutral coastal groundwater of Beihai, Southern China: δ15N, δ13C, and hydrochemical constraints

High levels of ammonium (NH4+) in groundwater can threaten the health of surface ecosystems and the safety of water supplies. To ascertain the occurrence of NH4+ in the coastal acidic groundwater of Beihai, Southern China, a combined hydrochemical and isotopic (δ15N in nitrate (NO3−) and NH4+; δ13C in dissolved inorganic carbon (DIC)) study was performed. In addition to a chloride concentration-based fresh water-seawater mixing model, δ15NNO3 vs δ15NNH4, Δδ13CDIC,react (changes in δ13CDIC values caused by reaction), and ΔCSO4,react (changes in sulfate concentrations caused by reaction) were applied to interpret the genesis of NH4+. The results show groundwater NH4+ loadings range from < 0.04 to 6.2 mg/L, with typically higher loadings along the coastline than in the plains. Elevated NH4+ loadings (>0.2 mg/L) are commonly accompanied by enriched δ15NNO3 and depleted δ15NNH4 values in relation to the anthropogenic endmember, depleted or enriched Δδ13CDIC,react values, and ΔCSO4,react > 0.00 or < 0.00 mmol/L. Such isotopic fingerprints and hydrochemical features indicate dissimilatory nitrate reduction to ammonium (DNRA), glucose fermentation, methanogenesis (acetate fermentation), and sulfide oxidation or sulfate reduction. The findings suggest acidic rainwater, sulfide oxidation, and NO3− reduction by ferrous iron (Fe2+) could jointly account for the formation of acidic groundwater (pH as low as 4.0) that is of meteoric origin. Fermentative and sulfide-driven DNRA with anthropogenic NO3− is notably responsible for the enrichment of groundwater NH4+ concentrations, with a minor contribution from anthropogenic NH4+ infiltration and algae decomposition. As a result of silicate and carbonate weathering, seawater inputs, and fermentation, the addition of bicarbonate (HCO3−) results in circumneutral pH values. These results indicate DNRA plays a critical role in the enrichment of NH4+ and, hence, improve understanding of its genesis in the groundwater of Beihai and other similar areas.

Effect of Seawater Intrusion on the Formation of Chlorinated and Brominated Trihalomethanes in Coastal Groundwater

Around the world, coastal groundwater is increasingly subject to seawater intrusion (SWI). The quality and characteristics of such waters differ from those of surface and groundwater. In the current study, trihalomethane (THM) formation under varying levels of SWI, natural organic matter (NOM), and chloride-to-bromide (Cl/Br) ratio was evaluated. Different levels of SWI were simulated by mixing deionized water with real seawater (RSW) collected from the Indian Ocean or synthetic seawater (SSW) by varying seawater volumes from 0% to 3%. Humic acid (0 to 5 mg/L) was added to represent NOM at concentrations mimicking natural levels of dissolved organic carbon. The chlorine demand of the simulated water samples was significantly correlated to SWI levels and NOM concentrations. THM concentration in SSW increased from 12.64 µg/L to 105.34 µg/L after 24 h and to 115.8 µg/L after 48 h for an increase from 0% to 3% volume of seawater. For water samples simulated with RSW, maximum THMs after 24 h were 119.2 µg/L, and after 48 h were 126.4 µg/L. An increase in NOM concentration in seawater-intruded water samples resulted in increasing THMs, especially tribromomethane. However, the increment in THMs at higher NOM concentration was lower compared to that at low NOM concentration. Chlorine demand was positively correlated only to tribromomethane. An increase in bromide concentration resulted in lower Cl/Br ratio and a concomitant increase in brominated THMs. The bromine substitution factor corresponding to increasing SWI of 0.25–3% decreased from 2.67 to 1.81 over a reaction time of 24 to 48 h, indicating a shift from TBM dominance to chlorinated THMs.