Hydrogeologic Setting of the Washington Metro Subway Tunnel Area, Montgomery County, Maryland
WRD PROJECT #: MD143
PROJECT CHIEF: Greene, Earl A.
BEGIN DATE: 01-March-2000
END DATE: 30-September-2001
Customers currently supporting the project:
Washington Metropolitan Area Transit Authority
U.S. Geological Survey
Problem
The Washington Metropolitan-Area Transit Authority (WMATA) is planning to conduct a pilot study to investigate the causes of ground-water intrusion in the Medical Center Station crossover. To supplement the WMATA's structural and hydrogeological data-collection effort, a comprehensive geophysical testing of the rock above the crossover is required. Accurate geological information will help to understand the nature of water leaks, and eventually will help in devising a solid strategy to take proper action for eliminating the problem in a WMATA underground facilities without a final liner.
Objectives
The objectives of this Pilot Study combined with the Medical Center Station Crossover Structural and Hydrogeological Investigations are to identify the specific geological and ground-water characteristics of the ground above the Medical Center Station crossover between Stations A439+70 and A444+60, and identify rock joints, faults, fissures and other ground features that can work as a conduit for conveying ground water to the crossover.
Approach
The USGS proposes to provide personnel to work with WMATA staff in three distinct areas of scientific expertise: geology, geophysics, and hydrology. The USGS proposes to work with WMATA staff to help develop workplans for conducting geologic, hydrologic, and geophysical investigations of the area around the Medical Center crossover tunnel. These workplans are expected to specifically address the question of the types of data that should be gathered and to prioritize the data collection. After the workplans have been reviewed and revised, the USGS will assist WMATA with implementation of the Task Orders sent to WMATA's on-call consultants. The USGS will also assist with periodic review of the progress and results of the data collection.
Because of the expertise required for critical parts of this effort, the USGS proposes to carry out selected tasks associated with the general characterization of the hydrogeologic framework and ground-water-flow system in the middle of the Rock Creek Watershed along the Washington Metro Railway near the National Institutes of Health and the Naval Medical Center. These tasks include:
1. Conduct an inventory of existing data from USGS data bases: Water-Quality Database (QWDATA); Ground-Water Site Inventory Data base (GWSI); State Water-Use Database (Surface WaterUD), existing Surface-Water Database (ADAPS); and literature from previous studies to establish the groundwork of known hydrogeologic data near the study area.
2. Design a watershed ground-water-monitoring network in the crystalline bedrock and saprolite. Several wells (either newly constructed or existing) will be instrumented with continuous recorders to monitor temporal changes in water levels over time, and develop water-table maps in the saprolite and head maps in the crystalline bedrock. Borehole geophysics will be used in newly drilled wells to identify fractures that are transmitting water. Packers will be installed in observation wells to determine vertical head potentials and identify the spatial and temporal variations in hydraulic heads with depth.
3. Set up a stream-gage-monitoring network and make streamflow measurements to understand the connection of the ground-water system to the streams to investigate surface/ground water interaction of Rock Creek and smaller tributaries. Continuous gages and seepage runs will be used to determine potential leakage into the crystalline bedrock. Three seepage runs will be conducted along Rock Creek to determine gaining and losing reaches. These data will indicate whether streamflow is recharging the underlying bedrock aquifer and may be contributing to tunnel leakage. One stream gage will be installed on an unnamed tributary to Rock Creek that crosses over the tunnel. Data from this gage will provide insight into flow volumes available for infiltration into the tunnel system.
4. A report will be prepared that summarizes the data collected and provides a preliminary interpretation of the factors controlling fluid migration into metro tunnels. A draft of the report will be prepared by the end of the contract period. Formal release of this report will follow within 4 months after it has been approved by the USGS for distribution.
5. A complete scientific study plan will be developed for a more comprehensive assessment of the hydrogeology of the bedrock aquifer system affecting the metro tunnels. Although the proposed contract period will focus on basic data collection, the complexity of the geologic terrane is such that basic data are required in order to design a comprehensive study plan to fully characterize the hydrogeology and the factors affecting water flow into metro tunnels.
