Journal of Groundwater Science and Engineering

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Predicting and managing seawater intrusion using a hybrid machine learning–reinforcement learning framework: The Mnasra coastal aquifer, Northwestern Morocco

Hanaa Aguedai, Oussama Gliti, Abdelaziz Mridekh, Mohammed Igouzal

, Available online , doi: 10.26599/JGSE.2026.9280100

Abstract(22) FullText HTML (9) PDF(0)

Abstract:
Coastal aquifers are essential freshwater sources for irrigated agriculture in semi-arid regions, but they are increasingly threatened by seawater intrusion driven by intensive pumping and climate-related stressors. This study develops a preliminary hybrid machine learning–reinforcement learning (ML–RL) framework to explore seawater-intrusion management scenarios in the Mnasra coastal aquifer, northwestern Morocco. The Seawater Intrusion Index (SWI), defined as the seawater fraction, was analytically derived from chloride concentration. Ridge Regression was used as a static surrogate estimator of this chloride-derived SWI using 13 hydrochemical variables measured in 36 wells from a single hydrochemical sampling campaign. Because chloride is directly involved in the analytical definition of SWI, predictive performance was interpreted cautiously. Model robustness was assessed using chloride-ablation analysis, leave-one-out cross-validation, repeated 5-fold cross-validation, and feature-contribution diagnostics. The full model including Cl⁻ reproduced the chloride-derived SWI with high cross-validated performance, whereas the chloride-excluded model provided a more conservative assessment of the broader hydrochemical signature. In the second stage, a Deep Q-Network (DQN) agent explored well-specific pumping-reduction actions (0%, 19%, 30%, and 40%) within a simplified synthetic surrogate environment initialized from the observed hydrochemical state. The DQN component was designed to reduce SWI toward the agricultural protection threshold of 0.01 while preserving usable groundwater supply. Because the dataset contains no observed temporal pumping-response trajectories, the RL outputs are interpreted as synthetic scenario-screening results rather than field-validated dynamic predictions. The management scenarios indicate partial mitigation of salinization: SWI was reduced in moderate- and high-intrusion wells while a substantial fraction of water supply was preserved, but several wells remained above the 0.01 threshold after management. The proposed framework should therefore be viewed as a proof-of-concept decision-support approach that complements, rather than replaces, geological characterization and process-based density-dependent groundwater models such as SEAWAT or FEFLOW.

Release mechanisms of heavy metals during compaction of clayey sediments: Studies using simulation experiments

Juan Chen, Shi-xiong Wang, Li-qiang Tang, Rui Liu, Xin-yao Liu, Teng Ma

, Available online , doi: 10.26599/JGSE.2026.9280098

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Abstract:
Compaction-induced release of porewater containing elevated concentrations of heavy metals from clayey aquitards poses a potential risk to groundwater quality in adjacent aquifers. However, the enrichment mechanisms of heavy metals in clay porewater and their subsequent environmental impacts on groundwater systems remain inadequately elucidated. In this study, two sets of simulation experiments were conducted on clayey sediments collected from the Chen Lake wetland using a novel integrated compaction simulator, with continuous monitoring of sediment, porewater, and gas compositions. Under the natural compaction condition (0.02 MPa/h), porewater was continuously released with pressurization, accompanied by a redox transition from oxidative to reductive environments. The release of sediment As, Ba, Mn, and Sr into porewater was associated with the reductive dissolution of Fe(III) oxides, whereas Cr, Rb, and Zn were primarily released through OM mineralization. Furthermore, portions of Rb and Sr were bound to carbonates, with their concentrations in porewater increasing due to inorganic carbon dissolution. In the later compaction stages, reduced moisture content inhibited further heavy metal release. Under the artificial compaction condition (0.1 MPa/24h), heavy metal mobilization was predominantly driven by pressure-induced dissolution under enhanced water-rock interactions. Following each pressurization, sediment As, Ba, Mn, and Sr were released alongside Fe mineral dissolution. Although the concentrations of V, Ni, Co, Cu, Cd, and Pb in porewater remained relatively low, they exhibited marked increases in response to each pressurization during the later stages of the experiment. Quantitative assessments for the Chen Lake wetland indicate that natural compaction of clayey sediments (–20 m thick) elevated groundwater As, Ba, Co, Mn, Rb, Sr, and V concentrations in the underlying aquifer (–70 m thick) by 5.81 μg/L, 57.81 μg/L, 0.23 μg/L, 561.46 μg/L, 0.57 μg/L, 90.03 μg/L, and 0.04 μg/L, respectively. Under artificial compaction, the contributions of clayey sediments to groundwater As, Ba, Co, Mn, Rb, and Sr concentrations were estimated to range from 0.07 μg/L to 1.81 μg/L, 1.24 μg/L to 17.1 μg/L, 0.006 μg/L to 0.074 μg/L, 25.89 μg/L to 261.8 μg/L, 0.01 μg/L to 0.24 μg/L, and 2.45 μg/L to 37.51 μg/L, respectively, underscoring the significant influence of clayey sediment compaction on heavy metal enrichment in groundwater systems.

Distribution and controlling mechanisms of rare earth elements in geothermal waters of Xiamen, Southeastern China

Shuai-chao Wei, Wei Zhang, Ruo-xi Yuan, Feng Liu, Long Li, Xiao-xue Yan

, Available online , doi: 10.26599/JGSE.2026.9280104

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Abstract:
Rare Earth Elements (REEs) are effective tracers of fluid–rock interaction and fluid mixing in geothermal systems, but their behavior in coastal geothermal waters remains poorly constrained owing to the combined effects of seawater intrusion, high ionic strength, and complex ligand competition. In this study, five geothermal water samples from the Xiamen coastal geothermal system, southeastern China, were characterized by hydrochemical analysis, Post-Archean Australian Shale (PAAS)-normalized REE patterns, anomaly indices, and PHREEQC speciation modeling to constrain the distribution and controls of dissolved REEs. Total dissolved REE concentrations (∑REE) range from 0.044 μg/L to 1.651 μg/L, with higher values in coastal waters than in the inland geothermal end-member. All samples show the fractionation pattern light rare earth elements (LREEs) > Middle Rare Earth Elements (MREEs) > Heavy Rare Earth Elements (HREEs), whereas PAAS-normalized patterns indicate relative HREE enrichment and generally negative Ce anomalies. Eu anomalies vary systematically from positive in the inland geothermal water to weak or absent in coastal waters. Regional comparison suggests that deep water–rock interaction controls the overall REE source and abundance levels, whereas seawater mixing primarily modifies REE fractionation, anomaly signatures, and aqueous speciation. Speciation modeling further shows that REEs in coastal geothermal waters are mainly associated with fluoride-, sulfate-, and carbonate-bearing complexes, whereas carbonate complexes dominate in the inland sample. Overall, REE behavior in the Xiamen system is governed by the coupled effects of water–rock interaction and seawater mixing, providing new insight into REE migration and fractionation in fault-controlled coastal geothermal environments.

Heat transfer performance and dynamic effects of middle-shallow U-tube ground heat exchangers under geological stratification

Chang-zhe Wang, Feng Liu, Li-juan Yuan, Hua-jun Wang

, Available online , doi: 10.26599/JGSE.2026.9280101

Abstract(2198) FullText HTML (707) PDF(6)

Abstract:
Previous research has lacked sufficient attention to the heat exchange performance of middle-shallow U-tube ground heat exchangers (GHEs), particularly regarding the impact of vertical lithology heterogeneity. In this study, a heat transfer model of GHEs coupling vertical lithological variations and ground temperature distribution is established, based on field test data of middle-shallow boreholes in Langfang, Hebei Province. The heat transfer characteristics of GHEs within the depth of 200–300 m and their influencing factors are analyzed. Results show that the flow velocity and wall thickness strongly affect the heat transfer of middle-shallow GHEs. Increasing the flow rate helps to enhance heat transfer, but is not conducive to improving energy efficiency of the system due to higher power consumption of circulating pumps. There is an optimal flow rate range of 2–3 m³/h for PE-RT GHEs. Furthermore, reducing the wall thickness from 8 mm to 3 mm can significantly improve the heat transfer per unit depth by 10–12% for PE-RT GHEs. The thermal influence distance (TID) of GHEs exhibits significant lithological differences and seasonal variations along the depth. As the depth increases, the TID in winter and summer exhibits increasing and decreasing trends, respectively. Especially, the TID of sandy layers due to a high thermal conductivity is greater than that of clay layers under the same conditions. For 250–300 m deep GHEs, the maximum TID reaches 5.8 m in winter and 7.3 m in summer, respectively, after running for five years. The heat transfer performance of middle-shallow GHEs has an attenuation risk of up to 33–35% during long-term operation, which can be alleviated using an intermittent operation strategy. The present findings can offer a useful reference for the design and optimization of middle-shallow GHEs in similar geological conditions.