Reports
Land subsidence monitoring to assess potential effects of groundwater withdrawals from coastal plain aquifers in Maryland; Fall, 2020 survey
2021, Ulizio, T.
Open File Report 21-02-01
Key Results
Ellipsoid heights at seven 3D survey marks were determined by GPS for comparison with previous measurements to help assess potential changes resulting from groundwater withdrawals in the coastal plain sediments of Maryland. Initial elevations were also determined by GPS for two additional marks, Money Stump (MSTP) and Peter’s Neck (PTNK), installed in 2019 and 2020, respectively. The GPS campaign was conducted from October 5 to October 9, 2020 at marks Arnold (ARNO), Broad Creek (BROA), and Crofton Meadows (CROF) well fields, Cove Point (COV1), Lexington Park (LEX1), Rosaryville State Park (ROS1), and Waldorf (WAL1). PTNK was monitored from October 7 to October 10, 2020. MSTP was monitored from October 26 to October 29. GPS data were processed using the National Geodetic Survey’s online OPUS Projects utility to determine ellipsoid heights referenced to the International Terrestrial Reference Frame 2014. These data were processed in conjunction with 21 years of GPS data for marks ARNO, BROA, and CROF, and 4 years of GPS data for rod marks COV1, LEX1, ROS1, and WAL1, to assess changes in ellipsoid heights over time. GPS data from the two additional marks, MSTP and PTNK, were processed to establish a baseline ellipsoid height and lateral position for future monitoring. Processed results indicate a trend of decreasing ellipsoid heights ranging from 2.3 to 3.1 millimeters per year over the 21-year period of record (1999 to 2020) and 2.3 to 7 millimeters per year over the 5-year period of record (2016 to 2020). The results also show a clear and consistent northwest shift in mark location, coinciding with the movement of the North American Plate. Land subsidence velocities were interpolated for the State of Maryland using the survey data in conjunction with vertical velocity data from continuously operating reference stations published by the National Geodetic Survey. The interpolated subsidence map shows greater subsidence in unconsolidated sediments of the Maryland Coastal Plain. Assuming the background subsidence rate attributable to glacial isostatic adjustment comprises part of the subsidence rate in the Maryland Coastal Plain region, an additional 1.35 mm/yr. of subsidence on average is occurring that is possibly due to groundwater withdrawal.
