Establishment of a land subsidence-monitoring network to assess the potential effects of groundwater withdrawals in Southern Maryland
2016, Andreasen, D.C.
Open File Report 16-02-01
A network consisting of four 3D rod survey marks was installed to monitor potential changes in land elevation as a result of groundwater withdrawals in Southern Maryland. Decades of groundwater use in Southern Maryland have caused groundwater levels to decline by as much as ~70 meters (230 feet), reducing interstitial hydrostatic pore pressures and raising the potential for compaction and land subsidence. While the magnitude of land subsidence will likely be small (millimeter per year scale) and therefore not pose major engineering problems, it could exacerbate the problem of tidal flooding in low-lying areas caused by sea-level rise. Three of the survey marks installed − Cove Point (COV-1) in Calvert County, Lexington Park (LEX-1) in St. Mary’s County, and Waldorf (WAL-1) in Charles County − are located in areas of high groundwater withdrawals where relatively deep cones of depression have formed. For comparison, a fourth mark − Rosaryville State Park (ROS-1) in southern Prince George’s County − was installed in an area of relatively low groundwater withdrawals. One set of 5-hour GPS occupations was made in the fall of 2015 and processed using the National Geodetic Survey’s OPUS Projects online utility. Estimated vertical accuracy ranged from 0.002 to 0.003 meters for ellipsoid heights. Heights obtained by this survey provide an initial set of measurements to establish a baseline for future monitoring.