Coastal Groundwater Research Articles

Intermittent freshwater extraction and cold-water recharge for mitigating seawater intrusion in coastal aquifers

Seawater intrusion seriously threatens the quality of coastal groundwater, affecting nearly 40% of the world's population in coastal areas. A study was conducted in the Kamini watershed situated in the Udupi district of Karnataka to assess the groundwater quality and extent of seawater intrusion. During the pre-monsoon period, 57 groundwater and 3 surface water samples were analyzed to understand the impact of seawater on the groundwater and surface water. The analysis revealed that the groundwater in the study area is slightly alkaline. The weighted overlay analysis map indicated that 11% of the study area is unsuitable for drinking water due to the influence of seawater. The Piper plot analysis revealed that the groundwater is predominantly CaMgCl facies. The hydrogeochemical facies evolution diagram (HFED) showed that 62% of the groundwater is affected by seawater. The HFED and Piper plots also indicate that the surface water is also affected by seawater. These results are also supported by various molar ratios such as Cl- vs. Cl⁻/HCO3⁻, Cl⁻ vs. Na⁺/Cl⁻, Cl- vs. SO42-/Cl-, and Cl⁻/HCO3- vs. Mg2+/Ca2+, suggesting that the majority of the water sample has been affected by seawater. The saturation indices indicated that mineral dissolution has significantly contributed to groundwater salinization. The correlation between sulfate concentration and calcite and dolomite dissolution suggested the influence of seawater intrusion in the coastal aquifer. The process of reverse ion exchange mainly influences the groundwater chemistry according to chloroalkali indices. The total hazard index (THI) values of nitrate and fluoride exceeded limits, posing health risks to adults and children. Studies suggest that with time and space, seawater intrusion is increasing in some pockets of the study area, especially along the west coast.

In coastal plains, saline water intrusion (SWI) and potentially hazardous pollutants are harmful to local human health. The southern Laizhou Bay has become a typical representative of the northern silty coast due to its extensive silt sedimentation and the significant impact of human activities. This research focuses on a portion of the southern Laizhou Bay, using GIS-based spatial analysis, water quality index methods and health risk assessments to evaluate the impact of saltwater intrusion and potential hazardous pollutants. The results show that the groundwater in the study area is significantly impacted by saline water intrusion, leading to major ion concentrations that far exceed World Health Organization (WHO) standards. The groundwater chemical types of brine and brackish water in the study area are mainly Cl-Na, and the main chemical types of fresh water are HCO3-Ca·Na. The average concentration sequence of the main ions in groundwater is K+ > HCO3− > Cl− > Na+ > SO42− > Ca2+ > Mg2+. The average hazard quotient (HQ) sequence in typical pollutants is Cl− > F− > NO3-N > Se > Mn > NO2-N > Cu > Pb > Zn > Fe, and the carcinogenic risk (CR) sequence caused by carcinogenic heavy metals is Cd > As > Cr. The noncarcinogenic health risk area is mainly distributed in the northwest of the study area, while the potential carcinogenic risk area is in the central region. The Cl is the greatest noncarcinogenic risk to adults and children. The mean HQ values for adults and children were 95.69 and 146.98, indicating a significant noncarcinogenic risk. The mean CR values for adults and children were 0.00037 and 0.00057, suggesting a relatively low carcinogenic risk. SWI is the main influencing factor on human health; therefore, it is necessary to prevent and control SWI. Moreover, potentially hazardous pollutants are carcinogenic and noncarcinogenic risks and are caused by agriculture, industry and other human activities. The findings of this research offer scientific insights for groundwater pollution control and saline water intrusion management in similar coastal areas.

Climate change and groundwater overexploitation endanger the sustainability of groundwater in India, particularly in the coastal region of Tamil Nadu. The increasing population, agricultural and industrial development, raised the demand for groundwater in the Coastal area of Pudukottai District, Tamil Nadu. Seawater intrusion establishes a challenging environment for freshwater management in coastal areas. A total of 64 groundwater samples were collected from different places, including borewells and open wells. The seawater intrusion is evaluated using the statistical method and physiochemical analysis such as water indices, ionic ratio and hydrochemical facies. As per the results of the groundwater quality index, 93% of the samples were not suitable for drinking as the examined parameter exceeded the Indian Standards. Piper’s diagram classify the Coastal groundwater into mixed Ca2+-Na+-HCO3-, Na+- HCO3- and Na+-Cl- type as the dominant. As per the seawater mixing index, 78% of the groundwater samples were affected by seawater intrusion. Further, the results of the ionic ratio (Na+/Cl-, Mg2+/Cl-, Ca2+/Mg2+, Ca2+/SO4, Ca2+/HCO3-) identify the salinization influence in the groundwater. The thematic maps created with the QGIS platform effectively to define the area of seawater intrusion. This study contributes to establishing a firm basis for decision-makers to enhance coastal groundwater management.

Coastal aquifers, the transition zone between freshwater and saltwater, show large salinity contrasts in the subsurface. Salinity is a key parameter to understand coastal groundwater flow dynamics and consequently also geochemical and microbial processes. For mapping porewater salinity, a variety of methods exists, mainly using electrical conductivity as a proxy. We investigate methods including hydrological/geochemical (well sampling, fluid logger) as well as geophysical method (direct push, geoelectrics) utilizing measurements near the high-water line of a high-energy beach at the North Sea island of Spiekeroog. We compare the methods, discuss their benefits and limitations and assess their spatial and temporal resolution. One key to enable a comparison is the estimation of formation factors transforming bulk conductivity measured by geophysical tools in to fluid conductivities obtained from direct measurements. We derive depth-dependent formation factors derived from time-series measurements of fluid loggers and a vertical electrode installation. Using these formation factors, the vertical electrode chain proves to provide reliable salinities at high spatial and temporal dimension. Direct-push profiling data provide the highest vertical resolution. However, a careful calibration is needed to allow for salinity quantification. On the other hand, electrical resistivity tomography (ERT) exhibits the lowest spatial resolution, but can image two-dimensional salinity distributions. We found ERT to fit very well to all other methods, but the data analysis should be aimed at salinities instead of bulk conductivities, i.e. including formation factors and temperature models into the inversion process.

This study investigated groundwater in the central Rif region of northern Morocco by analysing 55 water sampling points to assess its physicochemical and hydrogeochemical properties. Through hydrochemical analysis, GIS spatial exploration, and multivariate statistical analysis, a direct correlation was found between EC, TDS, and major ions, influencing overall water mineralization. The key findings included pH levels ranging from 6.10 to 8.52, EC from 828 to 4581 μS/cm, and varying concentrations of Ca2+, Mg2+, Na+, K+, HCO2–, Cl–, N–NO2–, and SO42–. Notably, TDS and TH ranged from 647.19–3609.36 mg/L and 64.23–1051.24 mg/L, respectively, with a significant portion of samples exceeding WHO guidelines, particularly chloride (61.81%), sulfate (92.72%), and nitrate (12.72%) samples. The Piper diagram highlights sodium chlorides (Na–Cl) as the predominant chemical facies (70.9%), while the Gibbs diagram emphasizes the impact of evaporation on water chemistry dynamics. This study revealed the complex influence of geological and anthropogenic factors on groundwater quality, potentially leading to seawater intrusion in coastal aquifers. The observed high mineralization and hardness levels, in addition to mild alkalinity, pose public health risks, underscoring the need for continuous monitoring and sustainable management practices in coastal groundwater management to protect human health and the environment.

Unraveling microbial community variation along a salinity gradient and indicative significance to groundwater salinization in the coastal aquifer

The permeability of aquifers strongly influences groundwater flow characteristics. Worldwide, coastal groundwater is often the primary freshwater source for coastal communities and ecosystems but is also particularly vulnerable to abstraction since saltwater intrusion may threaten its quality. Thus, understanding coastal permeability is crucial to the sustainable use of coastal groundwater. Here, we present the first global dataset of coastal permeability (CoPerm 1.0), which provides data on coasts’ landward, shoreline, and seaward permeability. CoPerm accounts for shoreline characteristics such as cliffs and beaches and contains information on four million segments representing more than two million kilometers of global coastline. Rocky Shores are the most abundant shoreline class, followed by mangroves, beaches, and muddy coasts. Permeability differs between the immediate shoreline (median permeability: 10−12.3 m2), the seaward (median: 10−13.3 m2), and the landward (median: 10−13 m2) sides of the coast. CoPerm provides input data for global coastal groundwater assessments and regional studies of submarine groundwater discharge or saltwater intrusion that can radiate into ecological and economic studies.

Combined effects of layered heterogeneity and cutoff wall on groundwater salinization caused by storm surge inundation: Numerical simulations

Submarine groundwater drainage (SGD) changes the elemental composition of the neighboring coastal ocean and impacts the biogeochemical cycles. To examine the seasonal and spatial variability in dissolved organic carbon (DOC) and labile organic compound biochemical compounds like dissolved carbohydrates (TDCHO), dissolved proteins (TDPRO), and dissolved free amino acid (TDFAA) concentrations during the dry and wet periods, groundwater samples were taken at 90 locations (180 samples) along the Indian coast. The mean DOC contents in Indian coastal groundwaters were more significant than the global mean values. DOC, TDCHO, TDPRO, and TDFAA concentrations are higher during wet than dry periods. The DOC and labile organic compound showed a substantial positive association with soil organic carbon, and respective labile compounds in soil, population, and land usage and poor relation with woodland territories, implying that soil organic compounds leaching is a source of DOC and other labile organic compounds into the groundwater. DOC and other labile compounds concentrations were linearly associated with population density, land usage, and sewage production, demonstrating that anthropogenic activities tightly regulate the formation of DOC in groundwater. During the wet and dry periods, total labile organic compounds (TDCHO, TDFAA, and TDPRO) constituted 21% and 10.5% of DOC, respectively. Compared to the wet time, more aromatic compounds accumulated during the dry season but were less bioavailable. SGD DOC flux contributed 2-7% of riverine DOC flux to the coastal ocean. The SGD flux from the Indian subcontinent to the nearby northern Indian Ocean accounts for approximately 2% of the worldwide SGD flux. The effect of DOC flux via SGD on coastal bacterial activity, the plankton food web, and the oxygen minimum zone must be studied.

Modelling approach integrating climate projections for coastal groundwater management

Abstract Fresh coastal groundwater is a valuable water resource of global significance, but its quality is threatened by saltwater intrusion. Excessive groundwater abstraction, sea‐level rise (SLR), land subsidence and other climate‐related factors are expected to accelerate this process in the future. The objective of this study is to (a) quantify the impact of projected climate change and (b) explore the role of individual hydrogeological boundaries on groundwater salinization of low‐lying coastal groundwater systems until 2100 CE. We employ numerical density‐dependent groundwater flow and salt transport modeling for this purpose, using Northwestern Germany as a case. Separate model variants are constructed and forced with climate data, that is, projected SLR and groundwater recharge, as well as likely ranges of other hydrogeological boundaries, including land subsidence, abstraction rates and drain levels. We find that autonomous salinization in the marsh areas, resulting from non‐equilibrium of the present‐day groundwater salinity distribution with current boundary conditions, is responsible for >50% of the salinization increase until 2100 CE. Sea‐level rise, land subsidence and drain levels are the other major factors controlling salinization. We further show that salinization of the water resources is a potential threat to coastal water users, including water suppliers and the agrarian sector, as well as coastal ecosystems. Regional‐scale uplifting of drain levels is identified as an efficient measure to mitigate salinization of deep and shallow groundwater in the future. The presented modeling approach highlights the consequences of climate change and anthropogenic impacts for coastal salinization, supporting the timely development of mitigation strategies.

Abstract Sea‐level rise (SLR) poses a severe threat to the coastal environment through seawater intrusion into freshwater aquifers. The rising groundwater table also exacerbates the risk of pluvial, fluvial, and groundwater flooding in coastal regions. However, current Earth system models (ESMs) commonly ignore the exchanges of water at the land‐ocean interface. To address this gap, we developed a novel land‐ocean hydrologic coupling scheme in a state‐of‐the‐science ESM, the Energy Exascale Earth System Model version 2 (E3SMv2). The new scheme includes the lateral exchange between seawater and groundwater and the vertical infiltration of seawater driven by the SLR‐induced inundation. Simulations were performed with the updated E3SMv2 for the global land‐ocean interface to assess the impacts of SLR on coastal groundwater under a high CO2 emission scenario. By the middle of this century, seawater infiltration on the inundated areas will be the dominant component in the land‐ocean coupling process, while the lateral subsurface flow exchange will be much smaller. The SLR‐induced seawater infiltration will raise the groundwater levels, enhance evapotranspiration, and increase runoff with distinct spatial patterns globally in the future. Although the coupling process is induced by SLR, we found topography and warming temperature have more control on the coupling impacts, probably due to the relatively modest magnitude of SLR during the selected future period. Overall, our study suggests significant groundwater and seawater exchange at the land‐ocean interface, which needs to be considered in ESMs.

Abstract Unconfined coastal aquifers are a main pathway for land‐sourced solutes to enter the oceans. The migration of these solutes in aquifers is highly affected by the groundwater flow and salt transport processes, which are, to a great extent, controlled by tides. While many studies have examined how tidal oscillations would influence the subsurface hydrodynamics in coastal aquifers, most of them ignored the potential impact of groundwater pumping, a common practice in coastal areas to satisfy the demand for freshwater. This study, by means of laboratory experiments and numerical simulations, explored the combined effects of tides and groundwater pumping on the pore water flow and salinity distributions in an unconfined coastal aquifer. The results show that, in a tide‐controlled aquifer, the addition of groundwater pumping would exacerbate the degree of seawater intrusion and lead to wider spreading and deeper penetration of the upper saline plume. Moreover, groundwater pumping would enhance the tide‐driven circulation in the upper saline plume and weaken the density‐driven circulation in the saltwater wedge, ultimately leading to the reduction in total submarine groundwater discharge. These findings may promote a deep insight into the complex coastal groundwater systems experiencing human activities, and provide guidance for better evaluating the environmental impact of groundwater pumping.

Abstract Rainfall and sea tides significantly affect the coastal groundwater. The effect of rainfall events and sea tides on groundwater is not fully understood. In this study, groundwater level and electrical conductivity (EC) were simultaneously measured in three monitoring wells to evaluate the behaviour of freshwater in the uplifted atoll island of Zhaoshu, China. We used the water level sensor monitoring the position and variability of freshwater in the island. In the monitoring period, 86 rainfall events (cumulative rainfall above 1 mm) were identified. The fresh groundwater periodically fluctuates with phase lags every 1–2 h following sea tides. The intermittent rainfall increases the volume of fresh groundwater, while groundwater fluctuation is controlled by tides. Multiple regression analysis and cross‐correlation analysis were used to analyse the response relationship of groundwater to rainfall and tides. Variation in the groundwater level lags the EC as the temporal fluctuation of the sea tides. Only in case of severe rainstorm (cumulative precipitation of an event above 300 mm), the contribution of rainfall on groundwater level fluctuation is greater than that of sea tide. Four response modes (RRR, RFF, RFR, RRF) decomposition of groundwater have been defined according to the tidal stage and threshold (70.3–301.1 mm) of rainfall. The tidal‐induced groundwater effect (TGE) is stronger than the rainfall‐induced groundwater effect (RGE) but it is the opposite in the third and fourth modes. These results and mechanisms could be applied to other atoll islands, for our understanding of rainfall infiltration processes with tidal effect, and could be instrumental in estimating groundwater resources.

Anchialine systems are coastal groundwater habitats around the world which host a unique community of cave adapted species (stygobionts). Such communities are expected to be separated by haloclines into either fresh or saline groundwater communities, hence climate changes (e.g., eustatic sea level shifts) and anthropic driven changes (e.g., salinization) may have a great impact on these stygobiont communities. Here we used cave-restricted species of Typhlatya from the Yucatan Peninsula as models to identify physiological capacities that enable the different species to thrive in marine groundwater (T. dzilamensis) or fresh groundwater (T. mitchelli and T. pearsei), and test if their distribution is limited by their salinity tolerance capacity. We used behavior, metabolic rates, indicators of the antioxidant system and cellular damage, and lactate content to evaluate the response of individuals to acute changes in salinity, as a recreation of crossing a halocline in the anchialine systems of the Yucatan Peninsula. Our results show that despite being sister species, some are restricted to the freshwater portion of the groundwater, while others appear to be euryhaline.

Anchialine systems are coastal groundwater habitats around the world which host a unique community of cave adapted species (stygobionts). Such communities are expected to be separated by haloclines into either fresh or saline groundwater communities, hence climate changes (e.g., eustatic sea level shifts) and anthropic driven changes (e.g., salinization) may have a great impact on these stygobiont communities. Here we used cave-restricted species of Typhlatya from the Yucatan Peninsula as models to identify physiological capacities that enable the different species to thrive in marine groundwater (T. dzilamensis) or fresh groundwater (T. mitchelli and T. pearsei), and test if their distribution is limited by their salinity tolerance capacity. We used behavior, metabolic rates, indicators of the antioxidant system and cellular damage, and lactate content to evaluate the response of individuals to acute changes in salinity, as a recreation of crossing a halocline in the anchialine systems of the Yucatan Peninsula. Our results show that despite being sister species, some are restricted to the freshwater portion of the groundwater, while others appear to be euryhaline.

Impact of ionic strength on goethite colloids co-transported with arsenite (As3+) through a saturated sand column under anoxic condition: Experiment and mathematical modeling

Seawater intrusions into coastal aquifers have been the object of study by hydrogeologists since long ago. A wide range of methods are used to predict the development of intrusions: analytical, numerical analytical and numerical. The purpose of this paper is to identify and evaluate seawater intrusion using a coastal groundwater intake as an example. The paper considers a coastal groundwater intake, where mineralization, total hardness and chlorides have increased due to intensive exploitation. Seawater intrusion is assumed to be the main reason for the groundwater quality deterioration. To prove the seawater intrusion motion, chemical sampling of water from the intake wells was carried out, which resulted in chlorine-bromine coefficient values indicating the presence of seawater admixture in fresh groundwater. The seawater intrusion was evaluated using numerical-analytical and numerical modelling. The estimation obtained by the analytical element method (GFLOW code) indicates the presence of seawater intrusion at the studied water intake. However, the analytical element method calculations provide the ultimate steady-state estimate of the intrusion development. Also, the method uses the assumption of a sharp freshwater-saltwater boundary. Numerical modelling provides a more realistic assessment. In building the three-dimensional numerical model, special attention was paid to the nature of the connection between the exploited aquifer and the sea. In the onshore part, the Quaternary aquifer is separated from the exploited aquifer by a clay layer with interbedded gravel and sand, but there is no data on the composition of sediments beneath the sea. To clarify their composition, profile numerical models were built in the FEFLOW program with different degrees of connection between the sea and the exploited aquifer. One of the models suggests a direct connection between the sea and groundwater through sand, while the other one is complicated by clay. According to the results of the profile models, the direct connection between the exploited aquifer and the sea was proved, which was taken into account in the construction of the three-dimensional numerical model. On the numerical model in the SEAWAT program, the change in the seawater intrusion position in different periods of water intake operation with different water withdrawal values was obtained, after which the calculation results according to numerical-analytical and numerical models were compared.

ABSTRACT Onsite wastewater treatment systems (OWTS) are a common wastewater treatment approach in coastal communities. Vertical separation distance (VSD) requirements between the drainfield and groundwater aim to ensure aerated soils for wastewater treatment. When the VSD declines, OWTS can fail. This study evaluated groundwater response to sea level rise (SLR) and the implications for OWTS. A groundwater monitoring network (13 wells) was used to evaluate groundwater depth in Dare County, North Carolina. Groundwater levels were measured with water level meters and pressure transducers. Trends in groundwater depth and SLR were analyzed to evaluate the influence of SLR on groundwater depth. From 1984–2022, mean groundwater levels have risen (∼7.6 mm/year) in response to SLR. Currently, sites at <2.7 m land elevation are most likely to have groundwater depths <1 m and inadequate VSD. Based on current precipitation and NOAA intermediate SLR projections, groundwater depth projections suggest that OWTS at lower elevations are more likely to experience groundwater inundation by 2040–2060. SLR has resulted in reduced VSD causing diminished wastewater treatment capacity in low-lying areas. OWTS VSD requirements are typically static due to regulatory constraints. Future management approaches should consider adapting to rising coastal groundwater levels because of increasing wastewater contamination risks.