Anthropogenic and marine forcings on coastal groundwater quality degradation: Tracing, hydrogeochemical signatures, seawater intrusion trends, and cumulative health threats

Groundwater is an important resource, but human activities in coastal areas have altered its chemical composition, damaged its utility, and hindered sustainable development. This study systematically investigates the chemical characteristics and related health hazards of Longkou groundwater using methods such as K-means clustering, principal component analysis (PCA), positive matrix factorization (PMF), and Monte Carlo simulation (MCs). Analytical results reveal that Ca²⁺ and Na⁺ constitute the predominant cations, while NO₃^- emerges as the dominant anion due to intensive agricultural activities. Silicate and carbonate weathering, and gypsum dissolution represent the primary geogenic processes controlling regional hydrochemical signatures. Concurrently, seawater intrusion, reverse cation exchange, and anthropogenic inputs significantly influence groundwater chemistry. Stable isotope analysis confirms atmospheric precipitation as the principal recharge source, with evident evaporative enrichment. Spatial analysis demonstrates increasing Na⁺ and Cl^- proportions toward coastal areas, with hydrochemical facies transitioning from Cl and Cl·HCO₃ types to Cl·NO₃ and NO₃·Cl types. While seawater intrusion remains severe with expanding influence zones, its intensity shows an annual decreasing trend. Source apportionment using PCA-PMF reveals three major contributors: anthropogenic activities, accounting for 40.56%; seawater intrusion, accounting for 25.18%; and natural processes, accounting for 34.25%. Water quality assessment reveals that most groundwater in the study area exceeds drinking water standards, with seawater intrusion causing severe salt damage to crops and land. MCs show that over 50% of samples exhibit HI values > 1 under NO₃^- exposure, and the pediatric population is more sensitive. These findings underpin the sustainable management of groundwater in economically important coastal regions.