Southern Maryland Land Subsidence Network
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.
A GPS campaign was conducted October 16-20, 2017 to determine the heights of 3d marks. 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 -1.508 m at COV-1, -2.088 m at LEX-1, 33.834 m at ROS-1, and 28.797 m at WAL-1. Height uncertainty for the measurements as reported by OPUS Projects is approximately +/- 0.1 centimeter for all marks with the exception of WAL-1 which had a reported uncertainty of +/- 0.2 centimeters. The greater uncertainty for that mark is possibly attributed to a shorter occupation time. No trends are discernable for the marks in southern Maryland since GPS readings only started in 2016.
Andreasen, D.C., 2016, Establishment of a land subsidence-monitoring network to assess the potential effects of groundwater withdrawals in Southern Maryland: Maryland Geological Survey Open-File Report 16-02-01, 38 p.(pdf, 8.8 MB)