Dynamics of major and trace elements during seawater intrusion in a coastal sedimentary aquifer impacted by anthropogenic activities

Seawater flooding of calcareous soils: Implications for trace and alkaline metals mobility

Developing management strategies to safeguard public health and environmental sustainability requires a comprehensive understanding of the solubility and mobility of trace and alkaline metals in the event of seawater flooding. This study investigated the effects of seawater flooding, along the duration of flooding, on the release of trace and alkaline metals (Mn, Fe, Cu, Zn, Ca, K, and Mg) in two calcareous soils (Krome and Biscayne) located in southern Florida. Seawater flooding experiments involved two soil types and four flooding durations (1, 7, 14, and 28 days) replicated three times. Freshwater flooding experiments were also conducted for comparison. After each flooding experiment, soil samples were collected at three depths (15, 30, and 45 cm), and analyzed for selected elements. Comparative analysis revealed significant releases of Mn, Fe, and Zn in both soils flooded by seawater compared to freshwater. In most cases, significant increments were evident as early as 1-day exposure to seawater flooding, which further increased with flooding duration. However, the impacts of seawater flooding had notable differences between the two soils. Seawater flooding in Krome soil for 28 days, resulted in higher Mn, Fe, and Zn contents by 58, 340, and 510% compared with freshwater flooding, while corresponding increases in Biscayne soil were 3.3, 130, and 180%, respectively. Comparable marginal increases in Cu content were observed for both soils. Similarly, seawater flooding increased K, Mg, and Na contents from single-day flooding. The interplay between soil type, column depth, flooding duration, and their interactions proved influential factors in determining Mn, Fe, Cu, and Zn releases, with peak levels typically observed on the 28th day of flooding and at bottom depths. Overall, these findings highlight the release of these elements, raising concerns about potential plant toxicity and groundwater or surface water contamination due to leaching and runoff.

Quantifying salinity in heterogeneous coastal aquifers through ERT and IP: Insights from laboratory and field investigations

The lithological and stratigraphical heterogeneity of coastal aquifers has a great influence on saltwater intrusion (SI). This makes it difficult to predict SI pathways and their persistence in time. In this context, electrical resistivity tomography (ERT) and induced polarization (IP) methods are receiving increasing attention regarding the discrimination between saltwater-bearing and clayey sediments. To simplify the interpretation of ERT data, it is commonly assumed that the bulk conductivity mostly depends on the conductivity of pore-filling fluids, while surface conductivity is generally disregarded in the spatial and temporal variability of the aquifers, particularly, once the aquifer is affected by the presence of saltwater. Quantifying salinities based on a simplified petrophysical relationship can lead to misinterpretation in aquifers constituted by clay-rich sediments. In this study, we rely on co-located data from drilled boreholes to formulate petrophysical relationships between bulk and fluid conductivity for clay-bearing and clay-free sediments. First, the sedimentary samples from the drilled wells were classified according to their particle size distribution and analyzed in the lab using spectral IP in controlled salinity conditions to derive their formation factors, surface conductivity, and normalized chargeability. Second, the deduced thresholds are applied on the field to distinguish clay-bearing sediments from brackish sandy sediments. The results are validated with logging data and direct salinity measurements on water samples. We applied the approach along the Luy River catchment and found that the formation factors and surface conductivity of the different unconsolidated sedimentary classifications vary from 4.0 to 8.9 for coarse-grained sand and clay-bearing mixtures, while normalized chargeability above 1.0 mS.m-1 indicates the presence of clay. The clay-bearing sediments are mostly distributed in discontinuous small lenses. The assumption of homogenous geological media is therefore leading to overestimating SI in the heterogeneous clay-bearing aquifers.

Impacts of climate change and coastal salinization on the environmental risk of heavy metal contamination along the odisha coast, India

Climate change-mediated rise in sea level and storm surges, along with indiscriminate exploitation of groundwater along populous coastal regions have led to seawater intrusion. Studies on groundwater salinization and heavy metal contamination trends are limited. Present study investigated the heavy metal contamination, associated risks and provided initial information on the impacts of groundwater salinization on heavy metals along the coastal plains of Odisha, India. Total 50 groundwater samples (25 each in post- and pre-monsoon) were collected and analysed. Concentrations of Fe (44%), Mn (44%), As (4%) and Al (4%) in post-monsoon and Fe (32%), Mn (32%), As (4%), B (8%) and Ni (16%) in pre-monsoon exceeded Bureau of Indian Standards (BIS) drinking water limits. High concentrations of heavy metals (Fe, Sr, Mn, B, Ba, Li, Ni and Co) and high EC (>3000 μS/cm) indicated that the groundwater-seawater mixing process has enhanced the leaching and ion exchange of metallic ions in central part of the study area. Multivariate statistical analysis suggested leaching process, seawater intrusion and agricultural practices as the main heavy metal sources in the groundwater. 4% of samples in post- and 16% in pre-monsoon represented high heavy metal pollution index (HPI). Pollution indices indicated the central and south-central regions are highly polluted due to saline water intrusion and high agricultural activities. Ecological risks in the groundwater systems found low (ERI <110) in both seasons. Children population found more susceptible to health risks than adults. Hazard index (HI > 1) has shown significant non-carcinogenic risks where Fe, Mn, As, B, Li and Co are the potential contributors. Incremental lifetime cancer risk (ILCR >1.0E-03) has suggested high carcinogenic risks, where As and Ni are the major contributors. The study concluded that groundwater salinization could increase the heavy metal content and associated risks. This would help policymakers to take appropriate measures for sustainable coastal groundwater management.

Integrated clustering analysis for delineating seawater intrusion and heavy metals in Arabian Gulf Coastal groundwater of Saudi Arabia

The intrusion of seawater (SWI) into coastal aquifers is a major concern worldwide, affecting the quantity and quality of groundwater resources. The region of Saudi Arabia that lies along the eastern coast has been affected by SWI, making it crucial to accurately identify and monitor the affected areas. This investigation aimed to map the degree of seawater intrusion in a complex aquifer system in the study area using an integrated clustering analysis approach. The study collected 41 groundwater samples from wells penetrating multi-layered aquifers, and the samples were analyzed for physicochemical properties and major ions. Clustering analysis methods, including Hierarchical Clustering Analysis (double-clustering) (HCA-DC), K-mean (KMC), and fuzzy k-mean clustering (FKM), were employed to evaluate the spatial distribution and association of the groundwater properties. The results revealed that the analyzed GW samples were divided into four clusters with varying degrees of SWI. Clusters A, B, C, and D contained GW samples with very low (fsea of 1.9%), high (fsea of 14.9%), intermediate (fsea of 7.9%), and low (fsea of 5.2%) degrees of SWI, respectively. FKM clustering exhibited superior performance with a silhouette score of 0.83. Additionally, the study found a direct correlation between the degree of SWI and increased concentrations of boron, strontium, and iron, demonstrating SWI's impact on heavy metal levels. Notably, the boron concentration in cluster B, which endured high SWI, exceeded WHO guidelines. The study demonstrates the value of clustering analysis for accurately monitoring SWI and associated heavy metals. The findings can guide policies to mitigate SWI impacts and benefit groundwater-dependent communities. Further research can help develop effective strategies to mitigate SWI effects on groundwater quality and availability.

Geochemistry of groundwater and metal(loid) behavior in the costal aquifers of the Maharlu Lake, Iran

Groundwater in coastal aquifers of the Maharlu Lake, southern Iran, has suffered from quality degradation and salinity increases in recent decades. These aquifers are important sources for drinking and agricultural uses in area around the lake. Hydrogeochemical analyses were carried out to identify the geochemical processes controlling groundwater chemistry and heavy metal concentrations in the coastal aquifer of the Maharlu Salt Lake. A total number of 54 water samples (45 samples from three coastal aquifers and 9 water samples from lake water) were collected for laboratory analysis of major ions and heavy metals three times from November-2014 to July-2015. In addition, physiochemical parameters of temperature, electrical conductivity, pH, and redox potential were measured in the field. The findings indicate three major controls of groundwater chemistry in the coastal aquifers of the Maharlu Lake including (1) lake water intrusions, (2) evaporites and carbonate dissolution-precipitation, and (3) input of organic matter probably linked to the seepage of domestic sewage with resulting sulfate reduction. In contrast, dissolution of minerals in surface sediments and direct evaporation of groundwater in the coastal aquifers seem to be insignificant based on geochemical ratios and water table depth. Concentrations of B, Li, Co, Cr, Cu, Ni, and U increase, but Zn concentrations decrease in groundwater due to the lake water intrusions into the costal aquifers. Gypsum dissolution and consequent carbonate reactions cause B, Co, Cr, Cu, Ni, Zn, and U enrichment in the groundwater samples. These findings can provide water managers and local authorities with a comprehensive framework of the coastal groundwater geochemistry, allowing a better understanding of the effects of current management practices and the implementation of mitigation approaches such as reduction of groundwater extraction to limit further lake saltwater intrusion and water resources deterioration.

Multi-isotopes revealing the coastal river anthropogenic pollutants and natural material flux to ocean: Sr, C, N, S, and O isotope study

Coastal river exports massive terrestrial materials to the adjacent marine environment with information about chemical weathering, providing critical insights on riverine flux and the potential impact on marine ecosystem. In this study, the preliminary data of dissolved strontium (Sr) and ⁸⁷Sr/⁸⁶Sr in a typical coastal river in southeastern China were collected along with hydrochemistry and C, N, S, and O isotopes to discriminate the source of terrestrial weathering and the riverine flux. Sr concentrations exhibited a range of 0.084 ~ 1.307 μmol L^-1, and ⁸⁷Sr/⁸⁶Sr values ranged 0.7089 ~ 0.7164. The total cationic charge (TZ⁺) ranged 0.2 ~ 11.7 meq L^-1 with the predominant Ca²⁺ which accounted for > 50% of TZ⁺, while the anions were dominated by HCO₃^-. The extremely high Na⁺ and Cl^- near the estuary indicated seawater mixing in such a coastal river. δ¹³C-DIC, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and δ³⁴S-SO₄^2- of river water ranged - 24.1‰ ~  - 9.2‰, 0.3‰ ~ 22.7‰, - 2.1‰ ~ 21.4‰, and - 9.3‰ ~ 18.0‰, respectively. δ¹³C enhanced correspondingly to decreased δ³⁴S, confirming the attendance of H₂SO₄ in carbonate weathering. Most δ¹⁸O values exhibited within ± 10‰, indicating the dominant nitrification process. δ¹⁵N presented slightly negative relationship with δ¹³C and no obvious correlation with δ³⁴S, indicating relatively limited impact of denitrification. The depleted δ¹³C and δ¹⁵N may be attributed to carbonate dissolution with nitric acids and the oxidation of organic matters into C and N pools. Quantitative analysis revealed that silicate weathering accounts for 79% of total dissolved Sr, indicating the dominant weathering process. The estimated monthly flux of dissolved Sr to the East China Sea was 138.1 tons, demonstrating an potential impact on seawater Sr isotope evolution. Overall, the investigations of multi-isotopes revealed the enhancement of weathering rates and the consequently depleted CO₂ consumption, which further proved the involvement of strong acids (H₂SO₄ and HNO₃). This study provides scientific insight in terrestrial weathering and anthropogenic impact of a typical coastal watershed and may orient the management of environmental issues related to coastal ecosystems.

Pollution assessment and sources of dissolved heavy metals in coastal water of a highly urbanized coastal area: The role of groundwater discharge

Heavy metal concentrations and physicochemical parameters in coastal waters were measured to analyze the spatial distribution characteristics, pollution degrees, and sources of heavy metals in the heavily urbanized Guangdong-Hong Kong-Macao Greater Bay Area (GBA) in China. Heavy metal concentrations in the eastern GBA were higher than those in the west, and the levels of Pb and Zn in seawater were higher than those in groundwater and river water. Both the pollution factors and comprehensive water quality index demonstrated that seawater was not contaminated with As, Cd, Cr, and Ni, whereas low to considerable levels of contamination of Pb and Zn were observed in the central and eastern sections of the GBA. Multiple statistical analyses suggested that the Pb and Zn contaminations in seawater were probably derived from atmospheric deposition and human activities, and the excess amounts of As, Cd, Cu, Ni, and Zn in groundwater were attributed to anthropogenic activities. The heavy metal fluxes from submarine groundwater discharge (SGD) were comparable to, or even greater than, those from local rivers. Therefore, SGD is a significant invisible contributor of heavy metals into the coastal ocean that has often been overlooked in comparison to other visible pollution sources. This study suggests that SGD should be considered in the assessment of heavy metal pollution and future water quality management protocols in marine ecosystems.

SWAT-SF: A flexible SWAT-based model for watershed-scale water and soil salinity modeling

Natural and anthropogenic complex processes release salts into the environment. These change the balances of natural systems in the watersheds. Models can assist study of salinity processes throughout watersheds. This study proposes a flexible water and soil salinity module for the Soil and Water Assessment Tool (SWAT) that can profitably incorporate expert scientific judgments into a deterministic salinity framework to adapt predicted salinity processes to actual salinity processes watershed-scale. The proposed flexible salinity model (SWAT-SF) was developed using new equations for salinity assessment and then successfully applied to the Dehram River Basin located south of Iran that endangers natural and anthropogenic salinity problems. The SWAT-SF model uses a general framework that can be utilized with/without the direct guidance of experts for the determination of salinity processes, the extent of the salinity, identification and investigation of the saline site, assessment and management of the salinity, and evaluation of salinity economics throughout the world.

The regular pattern and underlying mechanisms of seawater intrusion in the Modaomen channel in the Pearl River Estuary of China

In recent decades, the frequency and severity of inland river seawater intrusion has increased in the Pearl River Estuary (PRE) of China. Based on time series of water salinity data, river discharges, tides, and wind records for the period 2003-2015, this study aims to examine the regular pattern and underlying mechanisms of the seawater intrusion in the Modaomen channel of the PRE. Toward this end, the autocorrelation and spectral analysis are used to quantify the periodicity of runoff, tide, and water salinity and to infer the regular patterns of seawater intrusion. The Pearson correlation and the LMG metric are used to examine the effects of influencing factors on seawater intrusion. Results indicate that seawater intrusion in the Modaomen channel has distinct annual (369 days) and half-monthly (14.8 days) cycles, corresponding well with annual and half-monthly tidal cycles. Spatially, the frequency, severity, and periodicity of seawater intrusion decrease with increased landward distance away from the channel mouth. The prevention of seawater intrusion by incoming river flow becomes more remarkable as freshwater flows downstream. Shifts in wind speed and direction can strengthen or weaken seawater intrusion, depending on if wind is in the opposite or same direction of river flow. Further analysis using the LMG metric suggests that river discharge plays a dominant role in governing seawater intrusion. Overall, river discharges, tides, and wind are three major forces triggering the inland river seawater intrusion and account for, respectively, 46%, 15%, and 15% of the variation of water salinity in the Modaomen channel. The study is featured in not only identifying the major factors in driving seawater intrusion but also quantifying their relative contribution. Thus, the findings are important for water resources manager to predict and manage inland river seawater intrusion in the PRE of China.

Mapping salinization and trace element abundance (including As and other metalloids) in the groundwater of north-central Mexico using a double-clustering approach

This study aimed to determine the reliability of the double-clustering method to understand the spatial association and distribution of major and minor constituents in the groundwater of an arid endorheic basin in central Mexico (Comarca Lagunera Region). The results of the double-clustering approach were compared with well-known spatial statistics such as spatial autocorrelations (Moran index) and the local indicator of spatial association (LISA). Fifty-five groundwater samples were collected from diverse wells within the basin, and the major ions, metalloids, and trace elements were determined. Overall, the double-clustering analysis was an effective tool for identifying lithogenic/anthropogenic processes occurring in the basin and for establishing zones with high or low abundance of major ions and trace elements, even where processes affecting the groundwater quality were spatially dispersed. Although 89% of the samples showed As higher than the threshold value of 10 μg/L proposed by the World Health Organization for drinking water, both the double-clustering and LISA analyses identified As hotspots in the alluvial aquifer, where the extraction of deeper and warmer groundwater might promote the concomitant release of the metalloids As, Sb, and Ge and the trace elements V and W. Similarly, both statistical analyses identified mountainous sectors where the weathering of silicates and carbonates plays a key role in the abundance of HCO₃^-, Ga, and Ba. However, the LISA analysis failed to identify hotspots of carbonate-derived elements such as Ca, Mg, Sr, and U and silicate-derived elements such as Ca, Mg, K, Sr, Rb, Cs, Pb, Ni, and Y. Otherwise, the double-clustering analysis clearly defined high- and low-concentration zones for all these elements in the study region. Unlike the LISA analysis, the double-clustering approach was also successful in determining alluvial areas with high concentrations of Si and Ti and areas where the concentrations of Na, Cl^-, SO₄^2-, NO₃^-, B, Li, Cu, Re, and Se in groundwater were elevated, increasing the groundwater salinity. Overall, this study demonstrated that the double-clustering is an easy-to-apply approach, capable of visualizing disperse zones where specific anthropogenic processes may threaten the groundwater quality.

Hydrochemical and Isotopic Characterization of the Waters of the Manglaralto River Basin (Ecuador) to Contribute to the Management of the Coastal Aquifer

Coastal aquifers are strategic and fundamental in the development of touristic areas. The coastal aquifer within the Manglaralto River Basin in Ecuador is essential, as it is the only source of water supply for a large part of the northern part of the Santa Elena province. It is a semi-arid region where high volumes of water are pumped from the aquifer, causing a significant drawdown of groundwater levels, thus affecting the water quality. This work aims to characterize the characteristics of groundwater in the coastal aquifer using hydrochemistry and stable isotopes to propose a hydrogeological conceptual model. The methodology for determining the chemical and isotopic characteristics of groundwater follows the following scheme: (i) studies of ionic concentrations using the Piper diagram, (ii) assessment of the origin of salinity through the Cl/Br ratio, the presence of seawater intrusion through the Hydrochemical Facies Evolution Diagram HFE-D, (iii) characterization of precipitation events using stable isotopes (18O and 2H), and, (iv) development of a hydrogeological conceptual model of the study area. The results indicate that in the basin there are mixing processes of the existing water in the aquifer with recharge water, direct cation exchange processes in the freshening process during recharge, and evaporation in the unsaturated zone. A conceptual model of the flow system in the basin is built, based on the mentioned processes. The main conclusions are: seawater intrusion is present in the areas of the wells located closest to the coast, urban activity through septic tanks is affecting the quality of the aquifer, and rainfall is highly relevant in the different hydrochemical and isotopic processes that operate in the basin.

Assessment of the Main Geochemical Processes Affecting Surface Water and Groundwater in a Low-Lying Coastal Area: Implications for Water Management

Hydrogeochemical analyses were carried out to identify geochemical processes occurring in the low-lying coastal aquifer of Ravenna, North Adriatic Sea (Italy). The area is characterized by a complex coexistence of several environments: coastal dunes, paleodunes, pine forests, freshwater wetlands, rivers, brackish lagoons, gravel pit lakes, reclaimed lands, agricultural fields and industrial areas. Water quality is of primary importance for the sustainability of these, areas and a full understanding of geochemical processing is fundamental for their management. A total of 104 water samples was collected from groundwater wells and surface water bodies, and analyzed for the major and trace elements (TEs). Field measurements of chemical-physical parameters were carried out by a multiparameter device XS PCD650; major elements were analyzed following the Italian National Environmental Agency standards (APAT-IRSA 2003), while TEs were analyzed by ICP-AES/ICP-MS. The major findings include: organic matter degradation in salinized and anoxic conditions; TEs concentrations related to water–sediment interactions, i.e., adsorption, ion exchange, redox reactions, mineral dissolution and precipitation; anthropogenic contamination from pesticides and fertilizers use; pollution from industrial district; TEs enrichments and depletion due to groundwater salinization and water management practices; comparison of TEs concentrations with respect to national and international thresholds. The findings can provide water managers and local authorities with a comprehensive framework of the coastal water hydrochemistry, allowing a better understanding of the effects of current management practices and the design of mitigation measures to reduce water resource deterioration in the studied coastal area.