Groundwater Hydrochemical Evolution and Contaminant Transport Within Lacustrine Estuarine Aquifers: Implications for Potable Water Security and Terminal Lake Ecological Preservation

Lacustrine estuarine aquifers play an essential role in regulating solute transport to terminal lakes and maintaining downstream water supplies in endorheic basins. This research takes the Buha river watershed, the primary tributary to Lake Qinghai, as a case study to explore hydrochemical signatures, evolutionary characteristics, and water quality suitability of lacustrine estuarine groundwater using isotopic and modified hydrochemical tools. The results reveal that 37.50% of groundwater samples exceeded the WHO guideline value for Ca 2+ (75 mg/L) and 43.75% surpassed the geogenic nitrate threshold of 10 mg/L, with the HCO 3 –Ca facies dominating the estuarine aquifer. Although overall water suitability was classified as excellent to good, this macroscopic assessment masked localized toxicological threats, particularly noncarcinogenic risks to infants from nitrate exposure. These exposure hazards are primarily linked to the rapid downward percolation of synthetic crop fertilizers and domestic wastewater. The discrepancy between overall water suitability and localized health risks is driven by the interaction of regional land use and hydrogeological architecture. In the upper reaches, concentrated agricultural and residential activities overlie highly permeable coarse alluvial deposits, causing localized contamination. Conversely, finer sediments in the lower reach promote natural lithological attenuation, which prevents downstream nitrate accumulation and protects the terminal Lake Qinghai ecosystem. These findings provide critical insights into groundwater quality dynamics in endorheic basins, highlighting the necessity of implementing targeted groundwater protection to safeguard domestic drinking water safety in vulnerable lacustrine estuarine plains.