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The Salt Water Intrusion Meetings, or SWIMs, are a series of meetings that focus on seawater intrusion in coastal aquifers and other salinisation processes. 2018 marks the 50th year of the SWIM and the 25th biennial meeting. The SWIM proceedings record half a century of research progress on site characterisation, geophysical and geochemical techniques, variable-density flow, modelling, and water management. The SWIM is positioning itself to remain a viable platform for discussing the coastal aquifer management challenges of the next 50 years.

Conductivity (EC) and total dissolved solids (TDS) are water quality parameters, which are used to describe salinity level. These two parameters are correlated and usually expressed by a simple equation: TDS = k EC (in 25 °C). The process of obtaining TDS from water sample is more complex than that of EC. Meanwhile, TDS analysis is very important because it can illustrate groundwater quality, particularly in understanding the effect of seawater intrusion better than EC analysis. These conditions make research in revealing TDS/EC ratios interesting to do. By finding the ratio value, TDS concentration can be measured easily from EC value. However, the ratio cannot be defined easily. Previous research results have found that the correlation between TDS and EC are not always linear. The ratio is not only strongly influenced by salinity contents, but also by materials contents. Furthermore, the analysis of TDS concentration from EC value can be used to give an overview of water quality. For more precision, TDS concentrations need to be analyzed using the gravimetric method in the laboratory.

Tidal freshwater marshes are threatened by seawater intrusion globally due to freshwater discharge reduction and sea‐level rise. However, terrestrial nitrate (NO3−) transport responding to seawater intrusion remains poorly understood in tidal marshes. After validation against laboratory experiments, numerical simulations were conducted to analyze seawater intrusion effects on terrestrial NO3− transport and transformation in tidal marsh aquifers. Results reveal that seawater intrusion noticeably affects NO3− transport from the marsh aquifer to the tidal creek. Seawater intrusion results in an upper saline plume and a saltwater wedge within the aquifer, which markedly narrows the discharge outlet width of the NO3− plume and intensifies the peak NO3− flux across the creek bank. Consequently, both the NO3− removal efficiency and total nitrogen gas load to the creek decrease substantially after seawater intrusion. This is because the reduction of the transit time and the mixing zone width of the NO3− plume after seawater intrusion weakens denitrification. Sensitivity analyses indicate that the difference of the NO3− removal efficiency before and after seawater intrusion depends on soil properties. A larger unsaturated flow effect, saturated hydraulic conductivity or effective porosity leads to a greater difference of the NO3− removal efficiency before and after seawater intrusion. The predicted decrease of the NO3− removal efficiency after seawater intrusion is consistent with existing field data. Plain Language Summary Tidal marshes, including salt marshes and freshwater marshes, are the last barrier to discharging land‐sourced solutes to the ocean. As a result of sea‐level rise and anthropogenic activities, seawater intrusion is a widespread threat to tidal freshwater marshes. Conversely, salt marshes can be freshened by inland freshwater during the flood season. Based on a combination of laboratory experiments and computer simulations, the present study explores how terrestrial nitrate (NO3−) transport responds to seawater intrusion in tidal marsh aquifers. The NO3− removal efficiency and total nitrogen gas load to the tidal creek are calculated to quantify seawater intrusion effects on terrestrial NO3− transport. Both the NO3− removal efficiency and total nitrogen gas load to the tidal creek decrease substantially after seawater intrusion. This study has wide applications while designing strategies for coastal water and marsh management. Key Points Seawater intrusion plays an important role in terrestrial nitrate transport Nitrate removal efficiency and total nitrogen gas load to the tidal creek decrease substantially after seawater intrusion The difference of the nitrate removal efficiency before and after seawater intrusion depends on soil properties

Vietnam’s coastal aquifers face the challenge of saltwater intrusion due to factors like climate change, sea level rise, and over-exploitation. This issue is particularly prominent in the Central Coastal Region, where the plains are narrow and water resources are limited. In order to ensure sustainable management of groundwater, it is crucial to assess the vulnerability of Quaternary aquifers to saltwater intrusion. For evaluation of salt, fresh water distribution in the aquifers, we applied geophysical methods, along with water sampling and analysis from boreholes and wells. Additionally, we employed Geographic Information System (GIS) to delineate vulnerability zones to seawater intrusion. The GALDIT method, incorporating the Analytical Hierarchical Process (AHP), was used to assign weights to key indicators. The results indicate that saltwater intrusion has changed in recent years. In the North-central coastal plains, over half of the area (56.8% or 4061.8 km 2 ) exhibits high vulnerability, while the South-central coastal plains show high vulnerability in approximately one-fifth of the area (18.8% or 1699 km 2 ). Groundwater occurrence and the impact of existing seawater intrusion play pivotal roles in determining coastal aquifer vulnerability to saltwater intrusion. This underscores the significance of narrow, steep plains, and thin aquifers as key influential factors. To mitigate the risk of saltwater intrusion in production wells, it is crucial to consider regulating groundwater extraction in high vulnerability areas. This would limit capacity and minimize the possibility of saltwater intrusion. Effective management and exploitation of groundwater resources are essential for the sustainable development of Vietnam’s coastal regions.

Seawater intrusion (SI) poses a substantial threat to water security in coastal regions, where numerical models play a pivotal role in supporting groundwater management and protection. However, the inherent heterogeneity of coastal aquifers introduces significant uncertainties into SI predictions, potentially diminishing their effectiveness in management decisions. Data assimilation (DA) offers a solution by integrating various types of observational data with the model to characterize heterogeneous coastal aquifers. Traditional DA techniques, like ensemble smoother using the Kalman formula (ESK) and Markov chain Monte Carlo, face challenges when confronted with the non‐linearity, non‐Gaussianity, and high‐dimensionality issues commonly encountered in aquifer characterization. In this study, we introduce a novel DA approach rooted in deep learning (DL), referred to as ESDL, aimed at effectively characterizing coastal aquifers with varying levels of heterogeneity. We systematically investigate a range of factors that impact the performance of ESDL, including the number and types of observations, the degree of aquifer heterogeneity, the structure and training options of the DL models. Our findings reveal that ESDL excels in characterizing heterogeneous aquifers under non‐linear and non‐Gaussian conditions. Comparison between ESDL and ESK under different experimentation settings underscores the robustness of ESDL. Conversely, in certain scenarios, ESK displays noticeable biases in the characterization results, especially when measurement data from non‐linear and discontinuous processes are used. To optimize the efficacy of ESDL, attention must be given to the design of the DL model and the selection of observational data, which are crucial to ensure the universal applicability of this DA method. Key Points Non‐linearity and non‐Gaussianity in coastal aquifer characterization problems pose challenges to traditional data assimilation (DA) methods We propose to address these issues with a deep learning‐based DA method called ESDL Various factors influencing the performance of ESDL are systematically investigated

Releasing freshwater from upstream reservoirs is a reasonable strategy to mitigate saltwater intrusion, however, its effectiveness may vary depending on weather events. Previous studies have primarily examined the effects on saltwater intrusion of seasonal regulation in reservoir discharge, with a limited attention given to the synoptic scale. This study applied the ECOM‐si to quantitatively analyze the impacts of the October 2022 emergency freshwater release on saltwater intrusion, also elucidating the mechanisms by which cold fronts (defined as northerly winds with speeds exceeding 10 m/s) impaired the effects of that release. The release reduced landward advective salt flux, shifting the 0.45 psu isohaline 17 km downstream during the neap tide period. On October 21, the salinity at the Qingcaosha Reservoir (QCSR) intake point fell to 0.45 psu, creating a 12.75‐hr window for freshwater intake. Cold fronts greatly diminished the effectiveness of freshwater release, shortening the water intake period by 24.14 hr. During the cold front period, northerly winds induced landward Ekman transport, creating a horizontal recirculation pattern with inflow through the North Channel (NC) and outflow through the South Channel (SC). The net landward water flux per unit width in the NC reached −1 m2/s. During the first cold front, steady shear salt flux contributed most significantly, with a magnitude of −70 ton/s, while advective salt flux dominated during the second cold front, reaching −239 ton/s. Without the cold fronts, the potential water intake time could have increased to 36.89 hr.

Surface electrical resistivity tomography (ERT) is a widely used tool to study seawater intrusion (SWI). It is noninvasive and offers a high spatial coverage at a low cost, but its imaging capabilities are strongly affected by decreasing resolution with depth. We conjecture that the use of CHERT (cross-hole ERT) can partly overcome these resolution limitations since the electrodes are placed at depth, which implies that the model resolution does not decrease at the depths of interest. The objective of this study is to test the CHERT for imaging the SWI and monitoring its dynamics at the Argentona site, a well-instrumented field site of a coastal alluvial aquifer located 40 km NE of Barcelona. To do so, we installed permanent electrodes around boreholes attached to the PVC pipes to perform time-lapse monitoring of the SWI on a transect perpendicular to the coastline. After 2 years of monitoring, we observe variability of SWI at different timescales: (1) natural seasonal variations and aquifer salinization that we attribute to long-term drought and (2) short-term fluctuations due to sea storms or flooding in the nearby stream during heavy rain events. The spatial imaging of bulk electrical conductivity allows us to explain non-monotonic salinity profiles in open boreholes (step-wise profiles really reflect the presence of freshwater at depth). By comparing CHERT results with traditional in situ measurements such as electrical conductivity of water samples and bulk electrical conductivity from induction logs, we conclude that CHERT is a reliable and cost-effective imaging tool for monitoring SWI dynamics.

Saltwater intrusion (SWI) is a well‐studied phenomenon that threatens the freshwater supplies of coastal communities around the world. The development and advancement of numerical models has led to improved assessment of the risk of salinization. However, these studies often fail to include the impact of surface waters as potential sources of aquifer salinity and how they may impact SWI. Based on field‐collected data, we developed a regional, variable‐density groundwater model using SEAWAT for east Dover, Delaware. In this location, major users of groundwater from the surficial aquifer are the City of Dover and irrigation for agriculture. Our model includes salinized marshland and tidal streams, along with irrigation and municipal pumping wells. Model scenarios were run for 100 years and included changes in pumping rates and sea‐level rise (SLR). We examined how these drivers of SWI affect the extent and location of salinization in the surficial aquifer by evaluating differences in chloride concentration near surface waters and the subsurface freshwater‐saltwater interface. We found the presence of the marsh inverts the typical freshwater‐saltwater wedge interface and that the edge of the interface did not migrate farther inland. Additionally, we found that tidal streams are the dominant pathways of SWI at our site with salinization from streams being exacerbated by SLR. Our results also show that spatial distribution of pumping affects both the magnitude and extent of salinization, with an increase in concentrated pumping leading to more intensive salinization than a more widely distributed increase of the same total pumping volume. Key Points Presence of a saltmarsh inverts the freshwater‐saltwater interface in our study location Tidal streams contribute substantially to salinization of inland groundwater Concentrated pumping led to more intensive salinization than widespread pumping

Uncertainties in the rate and magnitude of sea-level rise (SLR) complicate decision making on coastal adaptation. Large uncertainty arises from potential ice mass-loss from Antarctica that could rapidly increase SLR in the second half of this century. The implications of SLR may be existential for a low-lying country like the Netherlands and warrant exploration of high-impact low-likelihood scenarios. To deal with uncertain SLR, the Netherlands has adopted an adaptive pathways plan. This paper analyzes the implications of storylines leading to extreme SLR for the current adaptive plan in the Netherlands, focusing on flood risk, fresh water resources, and coastline management. It further discusses implications for coastal adaptation in low-lying coastal zones considering timescales of adaptation including the decisions lifetime and lead-in time for preparation and implementation. We find that as sea levels rise faster and higher, sand nourishment volumes to maintain the Dutch coast may need to be up to 20 times larger than to date in 2100, storm surge barriers will need to close at increasing frequency until closed permanently, and intensified saltwater intrusion will reduce freshwater availability while the demand is rising. The expected lifetime of investments will reduce drastically. Consequently, step-wise adaptation needs to occur at an increasing frequency or with larger increments while there is still large SLR uncertainty with the risk of under- or overinvesting. Anticipating deeply uncertain, high SLR scenarios helps to enable timely adaptation and to appreciate the value of emission reduction and monitoring of the Antarctica contribution to SLR.

Subsurface physical barriers have been effectively used to mitigate seawater intrusion (SWI). Traditionally, the primary emphasis in both numerical studies and practical implementations has been on vertical barriers. The current research aims to explore the dynamics of SWI under various cutoff-wall inclination angles and depths, as well as aquifer heterogeneity using both experimental and numerical simulations. The impact of aquifer characteristics was assessed by utilizing a low hydraulic conductivity (K) aquifer (case L), a high hydraulic conductivity aquifer (case H), and two stratified aquifers. The stratified aquifers were created by grouping different hydraulic conductivity layers into two cases: high K above low K (case H/L) and low K above high K (case L/H). The model simulations covered seven different cutoff-wall inclination angles: 45.0°, 63.4°, 76.0°, 90.0°, 104.0°, 116.6°, and 135.0°. The maximum repulsion ratio of SWI wedge length was observed at an inclination angle of 76.0° for cutoff-wall depth ratios up to 0.623. However, as the depth ratio increased to 0.811, the maximum repulsion ratio shifted to an angle of 63.4° for all aquifers studied. At an inclined cutoff depth ratio of 0.811, the cutoff-wall inclination angle of 45.0° had the most significant impact on the saltwater wedge area. This results in SWI area reductions of 74.9%, 79.8%, 74.7%, and 62.6% for case L, case H, case H/L, and case L/H, respectively. This study provides practical insights into the prevention of SWI. Nevertheless, a thorough cost–benefit analysis is necessary to assess the feasibility of constructing inclined cutoff-walls.