Assessing major factors affecting shallow groundwater geochemical evolution in a highly urbanized coastal area of Shenzhen City, China

Groundwater is the most important alternative drinking water source in the coastal urban area of Shenzhen City, China. Understanding the main geochemical factors affecting groundwater chemistry is vital for sustainable water resource management in the long term. Seawater intrusion has been a serious problem, but the situation seems to have recently improved with the establishment of a water supply system and land reclamation. However, total dissolved solids (TDS) concentrations in the groundwater have increased. To determine major factors controlling groundwater geochemical evolution, studies were carried out in the area in Shenzhen most vulnerable to seawater intrusion. Along the groundwater flow path from the bedrock outcrop area to the coastal plain, groundwater evolution processes were studied using multiple hydrogeochemical analyses. Piper plots and hydrochemical facies evolution diagrams show the aquifer vulnerable to seawater intrusion is currently dominated by fresh groundwater of Ca-HCO3 or Ca-Na-HCO3-Cl type; only a few locations close to Shenzhen Bay appear to be Na-Cl type, indicating the threat of seawater intrusion. Anthropogenic pollution is mainly characterized by elevated NO3- and NH4+ in the form of point sources. Correlation coefficient analysis to evaluate water-rock interactions during granite weathering indicates this process can contribute significant amounts of Ca2+, Mg2+, Na+, K+, and HCO3- to groundwater. Evaluation of the aggressiveness index (AI), Langelier saturation index (LSI), and Ryznar stability index (RSI) suggest all groundwaters are corrosive and the dissolution of concrete material foundations in urban areas contributes significant amounts of Ca2+ and SO42- to groundwater. Principal components analysis (PCA) demonstrates water-rock interactions and concrete material dissolution are more important factors than seawater intrusion in terms of affecting shallow groundwater chemistry. This study demonstrates that hydrogeochemical tools are effective in assessing groundwater evolution under the influence of multiple factors in a highly urbanized coastal area.