Reports
Comparison of brackish-water interface in 1988-90 and 2005-06 in the Aquia and Monmouth aquifers in east-central Anne Arundel County, Maryland, using induction logging and chloride analysis
2006, Andreasen, D.C.
Administrative Report
Abstract
Induction logs were run and chloride concentrations were measured in a set of test wells on the Broadneck, Annapolis Neck, and Mayo Peninsula to determine changes in the position of the brackish-water/freshwater interface in the Aquia and Monmouth aquifers, and to investigate whether brackish-water intrusion has occurred in the Magothy aquifer. The logging and water sampling follows an earlier investigation of brackish-water intrusion conducted by the Maryland Geological Survey between 1988 and 1990.
- Overall, the brackish-water/freshwater interface in the Aquia and Monmouth aquifers on Annapolis Neck and Mayo Peninsula, observed in a set of test wells, is stable. The extent of brackish-water intrusion in the Aquia and Monmouth aquifers is similar to that mapped in 1988-90.
- The induction logs indicate that there has been no intrusion of brackish water at sites where intrusion was previously undetected in the Aquia and Monmouth aquifers. Those sites include Annapolis Roads, Hillsmere, and South River Farms Park.
- There has been no intrusion of brackish water in the Magothy aquifer at Sherwood Forest (Annapolis Neck) and Moorings-on-the-Magothy (Broadneck) test sites
- At sites previously intruded with brackish water in the Aquia and Monmouth aquifers, there has been no appreciable change in the depth and thickness of the brackish-water/freshwater interface. Those sites include Londontown Park, Quiet Waters Park, Bay Ridge, Arundel-on-the-Bay, and Mayo Beach Park.
- The only movement of the brackish-water zone occurred deeper in the section at Arundel-on-the-Bay. At that site, the induction log showed a reduction in resistivity below about 170 feet below sea level in the Monmouth aquifer. The chloride concentration increased from 2,300 milligrams per liter in 1989 to 5,800 milligrams per liter measured in 2006. The increase in chloride concentration may be a delayed response to pumpage from the Aquia aquifer caused by intervening low permeability sediments, possibly cemented sandstone, that may impede the flow of brackish water.
