GROUND-WATER DISCHARGE TO THE WASHINGTON METRO RED LINE, MONTGOMERY COUNTY, MD

Excessive water intrusion has been observed along stretches of the underground subway tunnels constructed in fractured rock in the Washington Metropolitan Area Transit Authority rail system that services Washington, D.C. and parts of neighboring Maryland and Virginia. The tunnels along the Red Line are suffering the most leakage and were constructed in the late 1970’s and early 1980’s, when no water-sealing technology existed to effectively mitigate water intrusion. WMATA is currently investigating technologies for reducing or eliminating water inflow into stretches of the Red Line tunnel. To evaluate the costs of design, installation, and operation and maintenance of pumping and other alternatives, the hydrogeologic and geochemical controls on ground-water flow in the fractured rock are being investigated.

The subway tunnel along the Red Line between Bethesda Station and the portal where the subway goes above ground includes the Medical Center Station and Crossover, the site of the greatest volume of leakage. This part of the tunnel was constructed through gneissic rock and igneous intrusions, approximately 45 meters below land surface and approximately 25 meters below the water table. Fractures in the gneissic rock have a general north-south orientation and are poorly connected, whereas, fractures in the igneous intrusion are more numerous than in the gneissic rock and have a general east-west orientation with a greater degree of interconnectivity than fractures in the gneissic rock.

The A6 pump station collects intruded water (about 380,000 liters/day) from the subway tunnel along the Red Line between Bethesda Station and the portal (about 3,500 meters). Weirs were installed in the tunnel drains below the electrified third rail and were used to proportion leakage to the tunnel by geologic type and tunnel geometry. Leakage into the tunnel from fractured gneiss is about 20 liters/day/meter of tunnel, and from the igneous intrusion including the station and crossover about 148 liters/day/meter. In contrast, tunnel inflows from the igneous intrusion that do not include the station and crossover are about 120 liters/day/meter of tunnel. Areas of excessive leakage into the subway tunnels appear to be correlated with geology and tunnel construction.