Water quality assessment in multi-salinity groundwater of muddy coastal zones using a function-adaptive weight water quality index.

Muddy coastal aquifer subject to seawater (saltwater) intrusion suffer from severe freshwater scarcity and fragile groundwater environments. Traditional groundwater water quality evaluation frameworks relying merely on drinking water standards often categorize high-salinity groundwater (total dissolved solids, TDS > 3 g/L) as low-quality resources, while neglecting their exploitable value for diversified functional applications. To fill this research gap, this study proposes a salinity-based Function-Adaptive Weight Water Quality Index (FAWQI) for targeted, multi-functional groundwater quality assessment. Based on salinity thresholds, groundwater bodies are categorized into three functional types: low-salinity water (3-10 g/L) applicable to livestock breeding, medium-salinity water (10-35 g/L) fit for aquaculture, and high-salinity water (>35 g/L) that can serve as liquid mineral raw materials. Hydrochemical evaluation results indicate that freshwater zones are generally of fair quality, with 27% of samples rated "good" and 43% rated "fair." Brackish groundwater quality is largely restricted by saltwater intrusion and elevated nitrate, which severely restricts its applicability for farmland irrigation. Approximately 52% of low-salinity groundwater samples is acceptable for livestock use, and medium-salinity water consistently meets the standard thresholds for marine aquaculture. Though hypersaline brines are unsuitable for conventional purposes, more than 76% of brine samples are enriched in K+, Na+, Mg2+ Br- and I-, demonstrating great exploitation potential as industrial mineral resources. The proposed FAWQI framework transforms conventional groundwater assessment dominated by single drinking-water-oriented standards into a function-driven classification framework. This novel evaluation method provides robust scientific support for zoned groundwater management and sustainable exploitation of water and mineral resources in saline coastal areas with severe water scarcity.