Due to climate change and ever-increasing groundwater exploitation under the background of population growth and economic development since the 1990s, saltwater intrusion (SWI) into coastal aquifers has been recognized as a significant geo-environmental issue in the Mekong Delta (MKD) in southern Vietnam. Previous research indicated that groundwater over-exploitation mainly causes SWI into deep aquifer and seawater is the dominant source of SWI into deep aquifer, however, whether seawater or saline river water is the dominant source of SWI into shallow aquifer remains unknown. In this study, a 3D groundwater flow and salt transport SEAWAT model was developed and calibrated/validated to simulate the processes of shallow groundwater and surface water interactions and saltwater and freshwater interactions in the Mekong River Estuary within the MKD during 2012–2021. Simulation results indicated that: (1) the saltwater/freshwater wedge within the qh aquifer migrates 4.3 km landward and saltwater (TDS 20–35 kg/m3) storage increases 101.5 % from 2012 to 2021; (2) the dominant source of SWI is seawater from annual perspective; (3) the dominant source of SWI is seawater from November to August (10 months) while is the Bassac River from September to October (2 months) from monthly perspective; and (4) contribution ratios of ocean, the Mekong River, and the Bassac River to SWI during one year were 80.29 %, 3.42 %, and 16.29 %, respectively. The outcome of this study can provide a useful reference for implementation of efficient and effective adaptation strategies of coastal water resources management and eco-environment protection and restoration against SWI in a changing climate in the MKD.

Excessive intake of iodine will do harm to human health. In recent years, high iodine groundwater has become a global concern after high arsenic and high fluorine groundwater. A deep understanding of the environmental factors affecting iodine accumulation in groundwater and the mechanism of migration and transformation is the scientific prerequisite for effective prevention and control of iodine pollution in groundwater. The paper comprehensively investigated the relevant literature on iodine pollution of groundwater and summarized the present spatial distribution and hydrochemical characteristics of iodine-enriched groundwater. Environmental factors and hydrogeological conditions affecting iodine enrichment in aquifers are systematically summarized. An in-depth analysis of the hydrologic geochemistry, physical chemistry, biogeochemistry and human impacts of iodine transport and transformation in the surface environment was conducted, the results and conclusions in the field of high iodine groundwater research are summarized comprehensively and systematically. Stable isotope can be used as a powerful tool to track the sources of hydrochemical components, biogeochemistry processes, recharge sources and flow paths of groundwater in hydrogeological systems, to provide effective research methods and means for the study of high iodine groundwater system, and deepen the understanding of the formation mechanism of high iodine groundwater, the application of isotopic technique in high iodine groundwater is also systematically summarized, which enriches the method and theory of high iodine groundwater research. This paper provides more scientific basis for the prevention and control of groundwater iodine pollution and the management of groundwater resources in water-scarce areas.

In this study, a 3D variable-density groundwater flow and salt transport model was developed and calibrated to simulate spatial and temporal variation of groundwater salinity in Binhai New Area located at coastal Tianjin Municipality for tracking saltwater/freshwater interface depth, analyzing saltwater descending velocity, and determining downward saltwater intrusion ceasing time under the background of rigorous groundwater pumping regulations (groundwater pumping rate reducing to zero in 2023). Results indicated that: (1) saltwater/freshwater interface depths in the northern area would increase from 130–170 m to 150–190 m from 2016 to 2031, in the central area would increase from 180–230 m to 210–260 m from 2016 to 2030, and in the southern area would increase from 280–300 m to 300–320 m from 2016 to 2030 and then stay unchanged, respectively; (2) saltwater descending velocities in the northern, central, and southern area decrease linearly and will reach zero in the years of 2031, 2030, and 2030, respectively; and (3) enlargement of the extent of downward saltwater intrusion will cease since 2031 in the northern area or 2030 in the central and southern area. To the best knowledge of the authors, this research is the first effort to unravel the fact of vertical saltwater intrusion (saltwater descending) occurred in coastal Tianjin, which demonstrates an urgent need for rigorous pumping regulations of reducing groundwater exploitation.

Saltwater intrusion into coastal aquifers menace multiple coastal areas globally, degrading groundwater quality, which poses an important threat to freshwater supply for agricultural, industrial and domestic utilization. Groundwater over-exploitation used to be commonly recognized as the principle factor causing saltwater intrusion, while sea-level rise, intensified storm surges and precipitation change have grown to become important drivers of factors inducing saltwater intrusion as well. In the context of exacerbated human activities such as groundwater over-exploitation due to the ever-increasing water demand because of population growth and economic and social development, as well as sea-level rise and increased frequency and intensity of extreme weather events and warmer temperatures and changing precipitation patterns and regimes resulting from climate change, the phenomenon of saltwater intrusion worldwide has been seriously aggravated recently. A deeper understanding of the theories and multiple pathways of saltwater intrusion, the commonly-used methods to investigate the extent of saltwater intrusion, as well as numerical approaches to assess the impacts of anthropogenic activities and climate change on saltwater intrusion in future are of great importance to mitigate its negative effects.

In recent decades, changes in temperature, wind, and rainfall patterns of Southeast Asia induced by climate warming in the Tibetan Plateau result in many environmental changes that have serious impacts on the lower reach of the Mekong River basin, a region already battling severe water-related environmental problems such as pollution, saltwater intrusion, and intensified flooding. In the densely populated Mekong Delta located at the mouth of the Mekong River basin in southern Vietnam, the hydrogeological systems have been transformed from an almost undisturbed to a human-impacted state and saltwater intrusion into surface water and groundwater systems has grown to be a detrimental issue recently, seriously threatening freshwater supply and degrading the eco-environment. In this article, the impacts of human activities and climate change (e.g., groundwater over-exploitation, relative sea-level rise, storm surge, changing precipitation and temperature regimes, uncontrolled drainage canals, operation of hydropower dams, and rapid development of aquaculture) on saltwater intrusion into groundwater systems in the Mekong Delta are briefly reviewed. Based on current status of research findings regarding saltwater intrusion and the subsequent groundwater quality degradation under the impacts of human activities and climate change, major knowledge gaps and challenges are identified and discussed, including thickness and permeability of the silt and clay aquitard, present-day highly heterogeneous 3D distribution of saline groundwater zones, dynamic variation of saltwater/freshwater transition zone, and the most effective and economical control measure. To bridge these gaps, future work should: 1) apply environmental isotope techniques in combination with borehole tests to gain detailed hydrogeological information regarding spatial variation of permeability and thickness of the silt and clay aquitard; 2) intensify regular groundwater monitoring and collect as much groundwater samples from multiple hydro-stratigraphic units at different depths as possible to visualize the present-day highly heterogeneous 3D distribution of saline groundwater; 3) develop a series of variable-density coupled groundwater flow and salt transport models representing various scenarios of human activities and climate change for predicting future extent of saltwater intrusion; and 4) identify the dominant factor causing saltwater intrusion and determine the most effective and economical engineering technique to address saltwater intrusion problems in the Mekong Delta.

Sinkhole development has been recognized as a major geohazard, as sinkholes pose great threats to infrastructure, such as buildings, roads, bridges, and pipelines, resulting in huge financial losses to society. Previous studies indicated that the spatial density of sinkholes increases linearly with the downward groundwater leakage rate (DGLR) (inter-aquifer flow rate from an unconfined to a confined aquifer through the aquitard between them) and that the spatial variation of annual-average DGLR is a useful indicator of the relative probability of sinkhole development. In this study, a groundwater flow model using the MODFLOW computer code was developed and calibrated to simulate the spatial variation of annual-average DGLR to evaluate the relative probability of sinkhole development at an under-construction expressway and its vicinity. The results indicated that the expressway construction site has a relatively high probability of sinkhole development in the designed range of the pavement structures, and it is concluded that engineering action should be taken in advance to minimize potential sinkhole hazards.

Saltwater intrusion into the surficial aquifer in coastal east-central Florida (USA) due to the effects of storm surges from tropical cyclones and continuous sea-level rise is a detrimental issue resulting in groundwater quality deterioration and bio-diverse ecosystem degradation. In this study, groundwater flow and salinity transport models using SEAWAT are developed for quantifying the “superposed” effects of storm surge (SS) from a Category 3 hurricane (Hurricane Jeanne which hit the Florida Atlantic coast on September 24–27, 2004) and continuous sea-level rise (SLR) on saltwater intrusion (SWI) into the surficial aquifer in coastal east-central Florida from the year of 2004 to 2024, and the simulated time-variant extent of SS- and SLR-induced landward migration of saline groundwater is compared for determination of whether the effects of SS from a Category 3 hurricane or continuous SLR on SWI are more significant. Results indicate that (1) the effects of SS are more significant than the effects of SLR within the first 12, 10, or 9 years after its occurrence date if sea level rises in low, mid, or high rate; (2) the effects of SS are more significant than the effects of SLR if SS occurs at least once within its return period (8–12 years); and (3) the maximum effects of SS are “equivalent” to the effects of the 95-year SLR, 45-year SLR, or 28-year SLR if sea level rises to 0.3 m in low, mid, or high rate. The outcome of this study warns public to pay more attention to the detrimental “superposed” effects of SS and SLR on SWI into the surficial aquifer in coastal east-central Florida.

Climate change such as altered frequency and intensity of storm surge from tropical cyclones can cause saltwater intrusion into coastal aquifers. In this study, a reference SEAWAT model and a diagnostic SEAWAT model are developed to simulate the temporal variation of surficial aquifer total dissolved solids (TDS) concentrations after the occurrence of a storm surge for exploration of the effects of storm surge on the extent of saltwater intrusion into the surficial aquifer in coastal east-central Florida (USA). It is indicated from the simulation results that: (1) rapid infiltration and diffusion of overtopping saltwater resulting from storm surge could cause a significant and rapid increase of TDS concentrations in the surficial aquifer right after the occurrence of storm surge; (2) rapid infiltration of freshwater from rainfall could reduce surficial aquifer TDS concentrations beginning from the second year after the occurrence of storm surge in that the infiltrated rainwater could generate an effective hydraulic barrier to impede further inland migration of saltwater and provide a downgradient freshwater discharge for saltwater dilution and flushing counteracting the effects of storm surge on the extent of saltwater intrusion; and (3) infiltrated rainwater might take approximately eight years to dilute and flush the overwhelming majority of infiltrated saltwater back out to the surrounding waterbodies, i.e., the coastal lagoons and the Atlantic Ocean.

基于数值模拟手段,论证水幕方法扰动底层盐水楔实现抑制成潮入侵的可行性.结果表明,在磨刀门河道上游深槽设置水平喷水干扰盐水楔,可以有效减弱底层盐水浓度,在底部高盐水团附近喷水的抑成效果优于其他断面,并且喷水流量存在最优值,当小于该值时,成潮上溯距离随流量的减小逐渐增大,当大于该值时,成潮上溯距离随流量的增大逐渐增大.进一步研究发现磨刀门河口地区小潮期进行盐水楔扰动的效果最好,低盐度等值线(≤0.5‰)向外海退缩程度显著,有利于河口区取水.

Cover-collapse and cover-suffosion sinkholes are widely distributed in central Florida (USA) karst terrains and have been recognized as the primary geo-hazard threatening human lives and destroying infrastructure. Previous studies indicated that the development of cover-collapse and cover-suffosion sinkholes in central Florida might be related to hydrologic/hydrogeologic conditions such as rainfall, groundwater downward leakage and groundwater hydraulic head differences (groundwater level differences between the water tables in unconfined aquifer and the potentiometric levels in confined aquifer). Here, a case study in central Florida urban areas is conducted to quantify the effects of rainfall, groundwater downward leakage and groundwater head differences on the development of cover-collapse and cover-suffosion sinkholes in central Florida with a focus on the timing of their occurrences. Results indicate that heavy rainfall/storm(s) and rapid increase of head differences within a relatively short period of time are major factors affecting the timing of sinkhole occurrences, and the spatial variation of groundwater downward leakage rate can be used to generate sinkhole susceptibility zonation maps for serving as a useful indicator of the likelihood of sinkhole development at certain areas. Results caution that the groundwater pumping and mining dewatering rate should be setup properly and the starting time of groundwater pumping and/or mining dewatering should be selected carefully in central Florida, i.e., the activities should be put into abeyance after a heavy rainfall/storm(s) to reduce the probability of sinkhole occurrences.

Saltwater intrusion (SWI) into root zone in low-lying coastal areas can affect the survival and spatial distribution of various vegetation species by altering plant communities and the wildlife habitats they support. In this study, a baseline model was developed based on FEMWATER to simulate the monthly variation of root zone salinity of a geo-typical area located at the Cape Canaveral Barrier Island Complex (CCBIC) of coastal east-central Florida (USA) in 2010. Based on the developed and calibrated baseline model, three diagnostic FEMWATER models were developed to predict the extent of SWI into root zone by modifying the boundary values representing the rising sea level based on various sea-level rise (SLR) scenarios projected for 2080. The simulation results indicated that the extent of SWI would be insignificant if SLR is either low (23.4cm) or intermediate (59.0cm), but would be significant if SLR is high (119.5cm) in that infiltration/diffusion of overtopping seawater in coastal low-lying areas can greatly increase root zone salinity level, since the sand dunes may fail to prevent the landward migration of seawater because the waves of the rising sea level can reach and pass over the crest under high (119.5cm) SLR scenario.

Sinkholes are one of the most frequent hydrogeological hazards in the Central Florida region. The risks related to sinkholes pose a significant threat to humans, infrastructure, and natural resources. It is important to identify the sinkhole-prone areas in order to mitigate or prevent damage and losses caused by sinkholes. The main objective of this study is to develop sinkhole hazard mapping for Central Florida region with a GIS technique. A sinkhole hazard map was generated by using Frequency Ratio (FR) model that analyzes the relationship between the sinkhole occurrence and sinkhole-affecting factors. Various hydrogeological factors that potentially contribute to the sinkhole formation in the region were identified and used for the analysis. The Sinkhole Hazard Index (SHI) values for the entire study area were determined using the developed hazard model, and then used to produce the sinkhole hazard map of the Central Florida region. The developed sinkhole hazard map shows a strong correlation with the reported sinkhole.

Sinkholes occur when surface soils gradually subside or suddenly collapse into subsurface cavities and voids due to raveling and erosion of surficial soils caused by dissolution and washing-off of underlying soluble carbonate bedrock. Sinkhole occurrence is related to local-scale hydrogeologic conditions (groundwater recharge rate and hydraulic head difference between water table and potentiometric level). Historical data have shown that sinkholes are more likely to occur in the beginning of wet season and the frequency of occurrence varies seasonally. In this study, the East-Central Florida region, which is vulnerable to sinkhole hazard, is selected as the study area, and the relationships between temporal and spatial distributions of observed sinkholes and hydrogeologic conditions are quantitatively investigated. The analysis results indicate that the seasonality of sinkhole occurrence is due to the seasonal variation of rainfall and groundwater level, and sinkholes are most likely to occur when the local-scale head difference stays constant at a peak value after a sharp increase over a short period of time. In space, sinkhole density increases linearly with increases in groundwater recharge rate and hydraulic head difference.

A three-dimensional variable-density groundwater flow and salinity transport model is implemented using the SEAWAT code to quantify the spatial variation of water-table depth and salinity of the surficial aquifer in Merritt Island and Cape Canaveral Island in east-central Florida (USA) under steady-state 2010 hydrologic and hydrogeologic conditions. The developed model is referred to as the ‘reference’ model and calibrated against field-measured groundwater levels and a map of land use and land cover. Then, five prediction/projection models are developed based on modification of the boundary conditions of the calibrated ‘reference’ model to quantify climate change impacts under various scenarios of sea-level rise and precipitation change projected to 2050. Model results indicate that west Merritt Island will encounter lowland inundation and saltwater intrusion due to its low elevation and flat topography, while climate change impacts on Cape Canaveral Island and east Merritt Island are not significant. The SEAWAT models developed for this study are useful and effective tools for water resources management, land use planning, and climate-change adaptation decision-making in these and other low-lying coastal alluvial plains and barrier island systems.