Anne Arundel County Land Subsidence Network
A GPS campaign was conducted October 16-20, 2017 to determine the heights of 3d marks at well fields in Anne Arundel County to assess the potential effects of groundwater withdrawals from Maryland’s coastal plain aquifers. The marks are located at major well fields at Arnold (mark ARNO-1), Broad Creek (mark BROA-1), and Crofton Meadows (mark CROF-1). The GPS data were processed using the National Geodetic Survey’s Online Positioning User Service (OPUS) Projects utility. The ellipsoid height determined through OPUS Projects was 3.644 m at ARNO-1, -6.182 m at BROA- 1, and 7.085 m at CROF-1. Height uncertainty for the measurements as reported by OPUS Projects is approximately +/- 0.1 centimeter for all marks. Ellipsoid heights at ARNO-1, BROA-1 and CROF-1 show a slight decline over the period of record, however, the error associated with the heights (reported at +/- 0.1 cm by OPUS Projects for the 2017 values) and fairly significant scatter in the data makes identifying a clear trend difficult.
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.