Assessing the impacts of motorway construction on groundwater in karst mountain areas, Southwest China

Motorways in karst mountainous regions play a key role in promoting economic development and enhancing local living standards. However, the construction of motorways in these sensitive ecosystems poses substantial risks to groundwater resources. In this study, the effects of a motorway project under construction in Southwest China on karst groundwater systems. Hydrochemistry and flow rate monitoring and isotopic analysis were conducted in two groundwater watersheds serving as drinking water sources. Throughout the construction period, the hydrochemical characteristics and groundwater flow rates remained stable, and water quality met the drinking water standards of China in both watersheds. However, tunnel excavation led to increased levels of potassium (K) and sodium (Na) in tunnel drainage owing to cutting fluid used during tunnel construction. Isotopic signatures of δ 13 C-HCO 3 − confirmed that carbonate rock weathering is the primary source of HCO 3 - in groundwater, whereas δ 15 N-NO 3 − and δ 18 O-NO 3 − tracers identified manure and domestic sewage used in agricultural activities in recharge areas as the sources of nitrate. The study further revealed that the location of springs relative to the motorway and the degree of karstification significantly of the construction area affect the impact on groundwater. The construction motorway located downstream of the spring outlet caused no notable spring damage, whereas the motorway construction located upstream of the spring outlet, in conjunction with cave landfilling operations in highly karstification regions, induced significant hydrogeological disturbances, resulting in shifts in hydrogeochemical properties and temporary spring depletion until a new equilibrium was established. These findings provide crucial insights for mitigating impacts on groundwater resource through route planning and construction practices in karst regions. • The hydrochemical characteristics remained stable during highway construction • Tunnel Construction activities resulted in K and Na contamination. • The motorway under construction lies below a karst spring outlet, with minimal impact on water quality.