MGS geologists presented talks at the 2016 Maryland Groundwater Symposium, September 28, 2016

Monitoring Potential Land Subsidence Related to Groundwater Withdrawals in Maryland’s Coastal Plain Province

David Andreasen

ABSTRACT

The potential for land subsidence related to groundwater withdrawals in Maryland’s Coastal Plain Province is being monitored by the Maryland Geological Survey through yearly, high-resolution GPS measurements in a small network of survey marks in Anne Arundel County and Southern Maryland (Calvert, Charles, and St. Mary’s Counties). The network of monitoring stations consists of six 3D rod survey marks at major well fields, one mark in an area of relatively low groundwater use, and one mark in the relatively stable Piedmont Province. Decades of groundwater use in the Coastal Plain Province have caused groundwater levels to decline significantly, reducing interstitial hydrostatic pore pressures and raising the potential for compaction and land subsidence. The impact of withdrawals has been especially acute in Southern Maryland where groundwater levels have declined by as much as 230 feet from estimated pre-pumping levels. 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. The period of monitoring in Anne Arundel County spans 20 years, while monitoring in Southern Maryland was just initiated in 2015. To date, there has been no indication of land subsidence in the GPS record.

Potentiometric Surface and Water-Level Difference Maps of Selected Confined Aquifers in Southern Maryland and Maryland’s Eastern Shore, 1975-2015

Andrew Staley

ABSTRACT

The Maryland Geological Survey (MGS) and U.S. Geological Survey (USGS) have maintained a groundwater-level monitoring network since the 1940s to observe changes in groundwater levels through time. Groundwater-level monitoring has been especially critical for Southern Maryland and Maryland’s Eastern Shore where groundwater is the primary source of water supply. It is also the source of freshwater supply used in the operation of the Calvert Cliffs, Chalk Point, Morgantown, and Panda Brandywine power plants. Increased groundwater withdrawals over the last several decades have caused groundwater levels to decline. This talk presents potentiometric-surface maps of the Aquia and Magothy aquifers and the Upper Patapsco, Lower Patapsco, and Patuxent aquifer systems using water levels measured during September 2015. Water-level difference maps are also presented for these aquifers. The water-level differences in the Aquia aquifer are shown using groundwater-level data from 1982 and 2015, while the water-level differences are shown for the Magothy aquifer using data from 1975 and 2015. Water-level difference maps for both the Upper Patapsco and Lower Patapsco aquifer systems are shown using data from 1990 and 2015. The water-level differences in the Patuxent aquifer system are shown using groundwater-level data from 2007 and 2015.

The potentiometric surface maps show water levels ranging from 53 ft above sea level to 164 ft below sea level in the Aquia aquifer, from 86 ft above sea level to 106 ft below sea level in the Magothy aquifer, from 115 ft above sea level to 115 ft below sea level in the Upper Patapsco aquifer system, from 106 ft above sea level to 194 ft below sea level in the Lower Patapsco aquifer system, and from 165 ft above sea level to 171 ft below sea level in the Patuxent aquifer system.. Water levels have declined by as much as 116 feet in the Aquia aquifer since 1982, 99 feet in the Magothy aquifer since 1975, 66 and 83 feet in the Upper Patapsco and Lower Patapsco aquifer systems, respectively, since 1990, and 80 feet in the Patuxent aquifer system since 2007.