JINAN -- China's first subway line passing through a karst spring area began operation on Saturday in Jinan city of East China's Shandong province, marking an engineering breakthrough that balances urban transit demands with the protection of natural water resources.

The subway line 4 runs beneath one of Jinan's busiest transport corridors, significantly reducing travel time and easing road congestion.

Yet concerns over whether subway infrastructure could coexist with the city's sensitive spring environment delayed construction for years. Many feared that tunneling might disrupt groundwater flows from the southern mountains, which sustain the springs to the north.

Jinan is called the "City of Springs" for its roughly 1,000 natural artesian springs. However, its karst geology also makes it one of the most fragile environments for underground construction.

To address the complex geology, survey teams identified spring-sensitive zones and adjusted the route to avoid aquifers, increased tunnel depth and minimized impacts on water systems.

The construction team carried out comprehensive geological scans, customized shield tunneling machines, and adopted earth pressure balance technology to prevent ground settlement. At key stations, U-shaped groundwater diversion channels were installed to maintain natural water flow.

Monitoring data shows that the water level of iconic Baotu Spring recently reached 30.32 meters, the highest recorded since 1966.

"This demonstrates that our spring protection measures and innovative engineering approaches are scientifically effective," said Huang Yongliang, a senior engineer for spring protection from Jinan Rail Transit Group Co., Ltd.