Anne Arundel County Land Subsidence Network
A GPS survey was completed May 3-13, 2016 to determine the heights of 3d marks at the Arnold, Broad Creek, and Crofton Meadows well fields in Anne Arundel County, Maryland. The GPS data were processed using the National Geodetic Survey’s Online Positioning User Service (OPUS) Projects utility. The ellipsoid heights determined through OPUS Projects were 3.640 +/-0.001 m at Arnold, 6.175 +/-0.001 m at Broad Creek, and 7.101 +/-0.002 m at Crofton Meadows. Historical GPS data (1999 to 2015) for the same marks were re-processed using OPUS Projects. The range in ellipsoid height elevation over the period 1999 to 2016 was 0.047 m at Arnold, 0.028 m at Broad Creek, and 0.040 m at Crofton Meadows. There are no discernible trends in ellipsoid heights over the period of record to indicate land subsidence.
Decades of groundwater withdrawals from unconsolidated, confined (artesian) coastal plain aquifers in Anne Arundel County, Maryland have resulted in significant drawdown of groundwater levels. Water levels have declined in some aquifers by as much as 130 ft from pre-pumping (Andreasen, 2007; Staley and others, in press). Projected increases in withdrawals to supply a growing population will result in additional drawdown (Andreasen, 2007). 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, la nd 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.
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.