Saltwater intrusion in coastal regions of North America

Saltwater has intruded into many of the coastal aquifers of the United States, Mexico, and Canada, but the extent of saltwater intrusion varies widely among localities and hydrogeologic settings. In many instances, the area contaminated by saltwater is limited to small parts of an aquifer and to specific wells and has had little or no effect on overall groundwater supplies; in other instances, saltwater contamination is of regional extent and has resulted in the closure of many groundwater supply wells. The variability of hydrogeologic settings, three-dimensional distribution of saline water, and history of groundwater withdrawals and freshwater drainage has resulted in a variety of modes of saltwater intrusion into coastal aquifers. These include lateral intrusion from the ocean; upward intrusion from deeper, more saline zones of a groundwater system; and downward intrusion from coastal waters. Saltwater contamination also has occurred along open boreholes and within abandoned, improperly constructed, or corroded wells that provide pathways for vertical migration across interconnected aquifers. Communities within the coastal regions of North America are taking actions to manage and prevent saltwater intrusion to ensure a sustainable source of groundwater for the future. These actions can be grouped broadly into scientific monitoring and assessment, engineering techniques, and regulatory approaches.     

The development and control of the seawater intrusion in the eastern coastal of Laizhou Bay, China

Seawater intrusion was observed in China’s coastal aquifers as early as 1960s. At present, it covers an area of more than 2,000 km² in China. In 2002, the seawater intrusion area was increased to 1773.6 km² in Shandong Province, China, and it is continuing to extend in the coastal area of Laizhou Bay. In order to study the characteristic of the seawater intrusion and the pattern of the saltwater/freshwater interface, we set the China’s first three-dimensional seawater intrusion observation network in Longkou City of Shandong Province in 1989. In order to control the development of the seawater intrusion, in 1995, a continuous underground concrete wall, by the method of high-pressure jet grouting, was constructed in the downstream plain area of the Huangshuihe river in Longkou City. The average depth of the 5,842 m long concrete wall is 26.7 m, and the maxim depth is 41 m. It forms a coastal underground reservoir with the total storage capacity of about 5,359 × 10⁴ m³. In the past 10 years, the reservoir obtained great economic benefit, social benefit and ecosystem benefit.     

The integrated impacts of natural processes and human activities on groundwater salinization in the coastal aquifers of Beihai,southern China

Salinization in coastal aquifers is usually related to both seawater intrusion and water–rock interaction. The results of chemical and isotopic methods were combined to identify the origin and processes of groundwater salinization in Daguansha area of Beihai, southern China. The concentrations of the major ions that dominate in seawater (Cl^?, Na⁺, Ca²⁺, Mg²⁺ and SO ₄ ^2– ), as well as the isotopic content and ratios (²H, ¹⁸O, ⁸⁷Sr/⁸⁶Sr and ¹³C), suggest that the salinization occurring in the aquifer of the coastal plain is related to seawater and that the prevailing hydrochemical processes are evaporation, mixing, dissolution and ion exchange. For the unconfined aquifer, groundwater salinization has occurred in an area that is significantly influenced by land-based sea farming. The integrated impacts of seawater intrusion from the Beibuwan Gulf and infiltration of seawater from the culture ponds are identified in the shallowest confined aquifer (I) in the middle of the area (site BBW2). Leakage from this polluted confined aquifer causes the salinization of groundwater in the underlying confined aquifer (II). At the coastal monitoring site (BBW3), confined aquifer I and lower confined aquifer II are heavily contaminated by seawater intrusion. The weak connectivity between the upper aquifers, and the seaward movement of freshwater, prevents saltwater from encroaching the deepest confined aquifer (III). A conceptual model is presented. Above all, understanding of the origin and processes of groundwater salinization will provide essential information for the planning and sustainable management of groundwater resources in this region.     

Coastal-zone shallow-aquifer characterization study using geoelectrical and geochemical methods in Zirconium Complex,Atomic Energy,Pazhayakayal, Thoothukudi,India

Groundwater is a very important component of water resources in coastal aquifers in Thoothukudi. It has been established that the groundwaters in the coastal zone of the Zirconium Complex, Pazhayakayal, Thoothukudi district, Tamilnadu, India, are subjected to wave and tidal impact. The groundwater quality was studied by hydrogeological methods, 2D electrical resistivity imaging (ERI) techniques (six profiles), 11 Wenner vertical electrical sounding (VES), and well log analysis. Also, nine geochemical water samples were taken from the study area. The 2D ERI and VES surveys were carried out using WGMD-4 Ltd., Chennai, resistivity meter, multicore cable, and multielectrodes with Wenner array. The collected resistivity data were interpreted using the Res2DINV software. The research shows that the groundwaters are the result of the paleoriver flow along the Tamirabarani Channel in the western area and of the seawater intrusion in the eastern area. The fresh water is characterized by resistivity of about 10–100 Ohm?m in the study area. The resistivity of 10–50 Ohm?m indicates that the subsurface section is made up of sand, clay, and caliche. Resistivity values of more than 200 Ohm?m are specific to sand dunes. The very low resistivity (<5 Ohm?m) layer might be due to the seawater intrusion in the study area. Six water samples from the well drilled in the coastal area were analyzed, which made it possible to determine the concentrations of major and trace elements in the groundwaters. These data were used to establish the seawater intrusion and coastal environment characteristics in the study area.     

Use of vertical electrical soundings to delineate seawater intrusion in a coastal area of Byunsan,Korea

To delineate spatial extent of seawater intrusion in a small experimental watershed in the coastal area of Byunsan, Korea, electrical resistivity surveys with some evaluation core drillings and chemical analysis of groundwaters were conducted. The vertical electrical sounding (VES) method was applied, which is useful to identify variations in electrical characteristics of layered aquifers. The drilling logs identified a three-layered subsurface including reclamation soil, weathered layer and relatively fresh sedimentary bedrock. The upper two layers are the main water-bearing units in this area. A total of 30 electrical sounding curves corresponded mostly to the H type and they were further divided into three classes: highly conductive, intermediate, and low conductive, according to the observed resistivity values of the most conductive weathered layer. In addition, groundwater samples from 15 shallow monitoring wells were analyzed and thus grouped into two types based on HCO₃/Cl and Ca/Na molar ratios with TDS levels, which differentiated groundwaters affected by seawater intrusion from those not or less affected. According to relationships between the three classes of the sounding curves and groundwater chemistry, locations of the monitoring wells with low HCO₃/Cl and Ca/Na ionic ratios coincided with the area showing the highly conductive type curve, while those with the high ratios corresponded to the area showing low conductive curve type. Both the low electrical resistivity and the low ionic ratios indicated effects of seawater intrusion. From this study, it was demonstrated that the VES would be useful to delineate seawater intrusion in coastal areas.     

Modélisation physique de l'intrusion d'eau de mer dans un aquifère karstique : cas de l'Almyros d'Héraklion (Crète)

Coastal aquifers are subjected to natural seawater intrusion. Laboratory experiments were made to model the hydraulic conditions of saltwater inflow into a porous karstic conduit through which the freshwater flows. The conceptual model is based on the functioning scheme of the coastal karst system Almyros of Heraklion, Crete. The experiments show that seawater intrusion in a karst conduit can be diffuse, does not always lead to density separation of freshwater and seawater, and is not necessarily related to a conduit-network directly connected to the sea. To cite this article: B. Arfib, J. Ganoulis, C. R. Geoscience 336 (2004).     

Use of chemical tracing to study acquisition modalities of the mineralization and behaviour of unconfined groundwaters under a semi-arid climate: the case of the Souss plain (Morocco)

The chemical content of the Souss unconfined groundwater displays spatial variations in conductivity (between 400 and 6,000 µS cm^-1). The chemical tracers (Cl^-, SO₄^2-, Sr²⁺, Br^-), which characterize the different components of the groundwater, allowed the determination of the origin of water salinity. Cl^- and SO₄^2-, reaching respectively 2,000 and 1,650 mg L^-1, display localized salinity anomalies. Br^-/Cl^- ratio distinguishes marine-influenced impoverished zones versus the oceanic domain. Thus, salinity anomalies can be attributed: (1) downstream, to a currently existing salt-encroachment (with added waste water) and sedimentary palaeosalinity, (2) in the middle-Souss, to High Atlas evaporites and to irrigation water recycling. Sr²⁺/Ca²⁺ ratio (evaporites if >1‰), confirms the evaporitic origin of the anomalies along the right bank of oued Souss. Furthermore, it facilitates the distinction between the different aquifer contributions (Cretaceous, Jurassic and Triassic), and it highlights leakage from deep Turonian limestones in the groundwater recharge system. To the south, recharge is from the Anti Atlas (evaporite-free) waters. Oxygen-18 measurements confirm the groundwater recharge from the High and Anti Atlas as piezometric maps and chemical tracers suggested, plus from leakage from the Turonian and the marine aquifers.     

Rapid and Intense Phosphate Desorption Kinetics When Saltwater Intrudes into Carbonate Rock

It is important to understand how phosphate sorption dynamics of coastal carbonate aquifers are affected by seawater intrusion, because many coastal aquifers are composed of carbonate rocks and subject to an increase in saltwater intrusion during relative sea-level rise. Twelve carbonate rock and unconsolidated sediment specimens were acquired from a test corehole spanning the full thickness of the Biscayne aquifer in southeastern Florida. All 12 samples exhibit low phosphorus content but variable contents of iron. Column leaching experiments were conducted with two carbonate aquifer samples, alternating between freshwater and saltwater flow. With the first influx of saltwater, phosphate concentration in leachate increased rapidly from a freshwater value of approximately 0.2 μM to peaks of between 0.8 and 1.6 μM. The phosphate concentration began to diminish as saltwater continued to flow, but sustained desorption continued for over 2 h. Overall, seawater drove sorption behavior much more than chemical composition for the aquifer rocks and sediment from the seven rock samples for which we did isotherm sorption experiments. Our results indicate that an immediate and intense pulse of phosphate desorption from carbonate rock and sediment with low phosphorus content occurs in response to an influx of seawater and that the duration of desorption will vary by layer within a single aquifer.     

Investigation of seawater intrusion in the Dibdibba Aquifer using 2D resistivity imaging in the area between Al-Zubair and Umm Qasr,southern Iraq

Electrical resistivity surveying for delineating seawater intrusion was performed in the Dibdibba aquifer in the area between the cities of Al-Zubair–Safwan and Al-Zubair–Umm Qasr in the vicinity of Khor AL-Zubair Channel, Basrah governorate, southern Iraq. Fourteen 2D resistivity profiles with a total length of 14 km were collected in the study area. The resistivity sections were compared with lithological data extracted from 11 boreholes. Thirty-nine groundwater samples were collected within the area and analyzed for chemical constituents; internal hydrogeological reports and unpublished studies were also evaluated. Results reveal the existence of three major resistivity layers, ranging from 0.1 to 130 Ωm at various depths and locations. The first layer has very low electrical resistivity (0.1–5 Ωm) representing a layer saturated with saltwater intruded from Khor AL-Zubair Channel. The second layer shows resistivity in the range of 5–130 Ωm, attributed to a transition zone and an unaffected zone saturated with brackish groundwater. The last resistivity layer (<?3 Ωm) represents coarse-grain sediments saturated with saline groundwater. Furthermore, a hard clay bed (Jojab) appears with a resistivity of 3–7 Ωm in all 2D imaging lines within a depth of 20–28 m. Electrical conductivity (EC) measurements from seven wells collected in 2014 and 2016 show a positive EC difference increasing landward with an average increase of 1927 µS/cm. In addition, six chemical relationships (Na/Cl, [Ca?+?Mg]/[HCO₃?+?SO₄], SO₄/HCO₃, SO₄/Cl, Mg/Ca and Cl/[HCO₃?+?CO₃]) are used to detect the source of salinity in groundwater. This study proves that extensive use of high-resolution 2D imaging sections, alongside lithological and hydrogeological data, can serve as a useful tool to delineate the boundaries between aquifers, identify hydraulic boundaries between groundwater with different salinities and allocate hard clay layers between the upper and lower Dibdibba aquifer. In general, the combination of 2D imaging and hydrochemistry enables conceptualization of the hydrogeological situation in the subsurface and characterization of the salinity source, here seawater intrusion, in the study area. There have been no studies published so far on the characteristics of saltwater intrusion in the study area, and this study is considered to be important for monitoring and studying the intrusion and regression of seawater.     

A modeling study of seawater intrusion in Alabama Gulf Coast,USA

A numerical model of variable-density groundwater flow and miscible salt transport is developed to investigate the extent of seawater intrusion in the Gulf coast aquifers of Alabama, USA. The SEAWAT code is used to solve the density-dependent groundwater flow and solute transport governing equations. The numerical model is calibrated against the observed hydraulic heads measured in 1996 by adjusting the zonation and values of hydraulic conductivity and recharge rate. Using the calibrated model and assuming all the hydrogeologic conditions remain the same as those in 1996, a predictive 40-year simulation run indicates that further seawater intrusion into the coastal aquifers can occur in the study area. Moreover, the predicted intrusion may be more significant in the deeper aquifer than the shallower ones. As the population continues to grow and the demand for groundwater pumping intensifies beyond the 1996 level, it can be expected that the actual extent of seawater intrusion in the future would be more severe than the model prediction. Better strategies for groundwater development and management will be necessary to protect the freshwater aquifers from contamination by seawater intrusion.

Modeling the dynamics of the freshwater-saltwater interface in response to construction activities at a coastal site

A numerical model was developed to evaluate the response of groundwater flow and the fresh-saltwater interface in relation to the construction of a particle accelerator at the coastal plain of Tokaimura, Japan. Undisturbed conditions were initially simulated and validated against field observations as a prerequisite for the analysis of predictive scenarios. Groundwater heads and the shape of the saltwater interface were appropriately described by the model, although it tended to underestimate salinity concentrations. Saltwater penetrated up to 250 m inland during predevelopment conditions, reaching more than 400 m at the dewatering phase. Flushing of entrapped saline groundwater might occur in addition to seawater intrusion. In depth, multiple saltwater fronts develop in response to the hydraulic properties of the sediments. Groundwater discharges offshore through the sandy aquifers, but salinity fronts prevail in the relatively impermeable layers. Routes for freshwater outflow turned into pathways of seawater intrusion during the pumping phase. The equilibrium would be reestablished within 2 years from the end of the stress, with no evidence of a permanent deterioration of neighbor residential wells. Nonetheless, after construction the accelerator forms a barrier that leads to a sharp rise in piezometric levels and creates a new and long-term disequilibrium in the saltwater wedge. Despite further work is still necessary to test many of the ideas proposed, the present study makes a new contribution to enhance the understanding of the processes occurring in coastal aquifers subjected to anthropogenic influence.     

Evaluation of the fast orthogonal search method for forecasting chloride levels in the Deltona groundwater supply (Florida,USA)

Despite the broad impact and importance of saltwater intrusion in coastal aquifers, little research has been directed towards forecasting saltwater intrusion in areas where the source of saltwater is uncertain. Saline contamination in inland groundwater supplies is a concern for numerous communities in the southern US including the city of Deltona, Florida. Furthermore, conventional numerical tools for forecasting saltwater contamination are heavily dependent on reliable characterization of the physical characteristics of underlying aquifers, information that is often absent or challenging to obtain. To overcome these limitations, a reliable alternative data-driven model for forecasting salinity in a groundwater supply was developed for Deltona using the fast orthogonal search (FOS) method. FOS was applied on monthly water-demand data and corresponding chloride concentrations at water supply wells. Groundwater salinity measurements from Deltona water supply wells were applied to evaluate the forecasting capability and accuracy of the FOS model. Accurate and reliable groundwater salinity forecasting is necessary to support effective and sustainable coastal-water resource planning and management. The available (27) water supply wells for Deltona were randomly split into three test groups for the purposes of FOS model development and performance assessment. Based on four performance indices (RMSE, RSR, NSEC, and R), the FOS model proved to be a reliable and robust forecaster of groundwater salinity. FOS is relatively inexpensive to apply, is not based on rigorous physical characterization of the water supply aquifer, and yields reliable estimates of groundwater salinity in active water supply wells.     

Geophysical characterization of the complex dynamics of groundwater and seawater exchange in a highly stressed aquifer system linked to a coastal lagoon (SE Spain)

Anthropogenic pressure influences the two-way interactions between shallow aquifers and coastal lagoons. Aquifer overexploitation may lead to seawater intrusion, and aquifer recharge from rainfall plus irrigation may, in turn, increase the groundwater discharge into the lagoon. We analyse the evolution, since the 1950s up to the present, of the interactions between the Campo de Cartagena Quaternary aquifer and the Mar Menor coastal lagoon (SE Spain). This is a very heterogeneous and anisotropic detrital aquifer, where aquifer–lagoon interface has a very irregular geometry. Using electrical resistivity tomography, we clearly identified the freshwater–saltwater transition zone and detected areas affected by seawater intrusion. Severity of the intrusion was spatially variable and significantly related to the density of irrigation wells in 1950s–1960s, suggesting the role of groundwater overexploitation. We distinguish two different mechanisms by which water from the sea invades the land: (a) horizontal advance of the interface due to a wide exploitation area and (b) vertical rise (upconing) caused by local intensive pumping. In general, shallow parts of the geophysical profiles show higher electrical resistivity associated with freshwater mainly coming from irrigation return flows, with water resources mostly from deep confined aquifers and imported from Tagus river, 400 km north. This indicates a likely reversal of the former seawater intrusion process.     

Changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the Holocene: evidence from stable carbon isotopes

Stable carbon isotope ratios in the organic fraction of surface sediments from the Laptev Sea shelf were analyzed in order to study the modern distribution pattern of terrestrial organic matter. The '¹³C_org signature of the surface sediments range from -26.6‰ near the coastal margin to -22.8‰ in the north towards the outer shelf. Characterizing the possible sources of organic matter by their '¹³C_org signature reveals that the terrestrial influence reaches further north in the eastern than in the western Laptev Sea. Downcore records of the '¹³C_org, measured on three AMS ¹⁴C-dated cores from water depths between 46 and 77 m, specify the spatial and temporal changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the past 12.7 ka. The major depositional changes of terrestrial organic matter occurred between 11 and 7 ka and comprised the main phase of the southward retreat of the coastline and of the river depocenters due to the postglacial sea level rise.     

The influence of marine inflows on the chemical composition of groundwater in small islands: the example of the Cyclades (Greece)

 Marine contamination of groundwater may be caused by seawater intrusion and by salt spray. The role of both processes was studied in the Cyclades archipelago on four small islands (45–195 km²) whose aquifers consist essentially of fractured, weathered metamorphic rocks. Annual rainfall ranges from 400 to 650 mm and precipitation has high total dissolved solids contents of 45–223 mg l^–1. The chemical characteristics of the groundwater, whose salinity is from 0.4 to 22 g l^–1, are strongly influenced by seawater intrusion. However, the effect of atmospheric input is shown in certain water sampling locations on high ground elevation where the dissolved chloride contents may attain 200 mg l^–1. Received: 14 November 1995 · Accepted: 9 September 1996     

海水入侵研究现状与展望

文中讨论了海水入侵的研究进展：海岸带海水入侵界面或过渡带的模拟，海岛地下淡水透镜体的动力学研究，井孔在界面或过渡带上部抽水所引起的升锥问题以及岸边地下水位动态研究及海平面变化对海水入侵的影响。海水入侵研究经历了从理想假定到合理概化，从室内实验模型、理想模型到数值模型这一过程。数值方法已成为模拟和求解海水入侵问题的最有力工具，海水入侵的过渡带模型成为主要的研究方向。分析了研究中存在的困难，展望了海水入侵研究的未来趋势：边界条件确定，水文地质参数选取，含水层结构估计，海水入侵输运与化学作用的耦合以及海水入侵预报。     

Three-dimensional hydrostratigraphical modelling to support evaluation of recharge and saltwater intrusion in a coastal groundwater system in Vietnam

Saltwater intrusion is generally related to seawater-level rise or induced intrusion due to excessive groundwater extraction in coastal aquifers. However, the hydrogeological heterogeneity of the subsurface plays an important role in (non-)intrusion as well. Local hydrogeological conditions for recharge and saltwater intrusion are studied in a coastal groundwater system in Vietnam where geological formations exhibit highly heterogeneous lithologies. A three-dimensional (3D) hydrostratigraphical solid model of the study area is constructed by way of a recursive classification procedure. The procedure includes a cluster analysis which uses as parameters geological formation, lithological composition, distribution depth and thickness of each lithologically distinctive drilling interval of 47 boreholes, to distinguish and map well-log intervals of similar lithological properties in different geological formations. A 3D hydrostratigraphical fence diagram is then generated from the constructed solid model and is used as a tool to evaluate recharge paths and saltwater intrusion to the groundwater system. Groundwater level and chemistry, and geophysical direct current (DC) resistivity measurements, are used to support the hydrostratigraphical model. Results of this research contribute to the explanation of why the aquifer system of the study area is almost uninfluenced by saltwater intrusion, which is otherwise relatively common in coastal aquifers of Vietnam.     

Delineation of saltwater intrusion into the freshwater aquifer of Lekki Peninsula,Lagos, Nigeria

Recently, the deterioration of water quality in the coastal zones of Lekki Peninsula area of Lagos due to saltwater infiltration into the freshwater aquifer has become a major concern. With the aim of providing valuable information on the hydrogeologic system of the aquifers, the subsurface lithology and delineating the groundwater salinity, vertical electrical resistivity (VES) sounding survey was carried out utilizing surface Schlumberger electrode arrays, and electrode spacing varying between 1 and 150 m. The DC resistivity surveys revealed significant variations in subsurface resistivity. Also, the VES resistivity curves showed a dominant trend of decreasing resistivity with depth (thus increasing salinity). In general, the presence of four distinct resistivity zones were delineated viz.: the unconsolidated dry sand (A) having resistivity values ranging between 125 and 1,028 Ωm represent the first layer; the fresh water-saturated soil (zone B) having resistivity values which correspond to 32–256 Ωm is the second layer; the third layer (zone C) is interpreted as the mixing (transition) zone of fresh with brackish groundwater. The resistivity of this layer ranges from 4 to 32 Ωm; while layer four (zone D) is characterized with resistivities values generally below 4 Ωm reflecting an aquifer possibly containing brine. The rock matrix, salinity and water saturation are the major factors controlling the resistivity of the formation. Moreover, this investigation shows that saline water intrusion into the aquifers can be accurately mapped using surface DC resistivity method.     

Groundwater travel times for unconfined island aquifers bounded by freshwater or seawater

Analytical solutions for calculating groundwater travel times within unconfined island aquifers are given for cases of both a freshwater-bounded island (island located in a fresh water lake) and an oceanic island (freshwater contained above intrusive saltwater). The solutions apply for homogenous aquifers recharged by surface infiltration and discharged by a down-gradient, fixed-head boundary, under steady-state conditions. The solutions are given for two simple island geometries: circular islands and strip islands. A technique is also provided for comparing travel times in inland islands and oceanic islands. Travel times in oceanic islands are found to be shorter than travel times in inland islands, and travel times in circular islands are found to be longer than travel times in strip islands.     

安徽伏川蛇绿岩套的Nd-Sr-O同位素研究

安徽歙县伏川的蛇绿岩套形成于中-晚元古宙,其Nd、Sr和O同位素组成是:ε_Nd(T)=+0.7-+3.8,ε_Sr(T)=+30.7-+53.9,δ¹⁸O=3.2-11.0‰。据地质学和同位素地球化学特征,该岩套位于杨子板块南缘、江南古岛弧的弧后小洋盆地轴部。ε_Nd(T)值的变化是由于蛇绿岩形成过程中受到下伏不成熟硅铝质基底地壳的混染引起的;ε_Sr(T)和δ¹⁸O的变化,是在蛇绿岩形成时或形成后不久遭受海水热液蚀变的结果。