Land Subsidence Monitoring Network
Decades of groundwater withdrawals from unconsolidated, confined (artesian) coastal plain aquifers in Maryland have resulted in significant drawdown of groundwater levels. Water levels have declined in some aquifers to more than 170 feet below sea level (Staley and others, in press). Projected increases in withdrawals to supply a growing population will result in additional drawdown. Withdrawing water from a confined aquifer reduces the hydrostatic pressure head in the pumped aquifer and in the adjacent confining layers (clay and silt). Reduction of hydrostatic pressure in the aquifer system resulting from the drawdown increases the load on the sediment which may lead to compaction and land subsidence. In the mid-Atlantic region, land subsidence ranging from 1.5 to 3.7 millimeters per year has occurred in the Franklin and Suffolk area of Virginia (lower Chesapeake Bay region) and is attributed to groundwater withdrawals from the Potomac Group aquifer system in Virginia (Patapsco and Patuxent aquifer systems in Maryland) ( Davis, 1987; Eggleston and Pope, 2013). While not likely to cause major engineering problems, land subsidence related to groundwater withdrawals could exacerbate the problem of tidal flooding in low-lying areas caused by future sea-level rise. Permanent reduction in reservoir capacity by irreversible compaction of sediments may also occur.
Regional network for measuring land subsidence
Starting in 1999, three deep rod benchmarks were installed at major pumping centers in Anne Arundel County, Maryland (Arnold, Broad Creek, and Crofton Meadows well fields). Yearly GPS surveys were undertaken by Maryland State Highway personnel until 2016 when the GPS observations were conducted as a joint effort between the Maryland Geological Survey and the National Geodetic Survey (NOAA).
Starting in 2019, MGS joined a multi-agency effort to isolate vertical land motion (vlm) attributed to human activities (groundwater withdrawals) from long-term geologic signals due to glacial cycles and deep Earth processes. The group is utilizing yearly static GNSS measurements to determine vlm over a 5-year period.
Andreasen, D.C., 2007, Optimization of groundwater withdrawals in Anne Arundel County, Maryland, from the Upper Patapsco, Lower Patapsco, and Patuxent aquifers projected through 2044: Maryland Geological Survey Report of Investigations No. 77, 107 p.
Davis, G.H., 1987, Land subsidence and sea level rise on the Atlantic Coastal Plain of the United States: Environ. Geol. Water Science, vol. 10, no.2, p. 67-80. Eggleston, Jack and Pope, Jason, 2013, Land subsidence and relative sea-level rise in the southern Chesapeake Bay region: U.S. Geological Survey Circular 1392, 30 p.
Staley, A.W., Andreasen, D.C., and Curtin, S.E., in press, Potentiometric surface and water-level difference maps of selected confined aquifers in Southern Maryland and Maryland's Eastern Shore, 1975-2015: Maryland Geological Survey Open-File Report 16-02-02, 30 p.