Reports
An evaluation of the Magothy Aquifer in the Annapolis area, Maryland
1974, Mack, F.K.
Report of Investigations 22
Abstract
This study was made to develop information needed for planning the role of the Magothy aquifer as a source for future water supplies in the Annapolis area in Anne Arundel County, Maryland.
The Magothy aquifer is a layer of coarse gray to white sand from 100 to 200 feet thick cropping out in a belt 2 to 5 miles wide, extending in a northeast-southwest direction, 8 to 12 miles northwest of Annapolis on the west side of Chesapeake Bay. The aquifer dips southeastward beneath younger formations of clay, silt, and sand at approximately 30 feet per mile from an altitude of about 100 feet in upland parts of the outcrop area to about 300 feet below sea level at the southeast end of the Annapolis peninsula. The saline waters of Chesapeake Bay and its estuaries overlie a large part of the area in which the aquifer occurs at depth and also a part of its outcrop area. A confining bed, the Matawan Formation, lies directly above the Magothy aquifer in much of the area.
The Magothy aquifer is a part of a larger hydrologic system (including the streams in its outcrop area, overlying and underlying geologic formations, and Chesapeake Bay and its estuaries), not an isolated source of water. Heavy development of the Magothy aquifer will reduce the amount of water available from the streams and adjacent formations and, conversely, further development of adjacent formations will reduce the availability of water from the Magothy aquifer.
The aquifer is currently yielding water at a rate of 4 mgd (million gallons per day) in the Annapolis area. The cone of depression created by this pumping is relatively shallow and the potentiometric surface for the aquifer, when pumped at this rate, is still at least a few feet above sea level in all but a few areas near larger well fields. Because the potentiometric surface of the Magothy aquifer is still at least a few feet above sea level, the hydraulic gradient is upward in low lying areas such as the estuaries of Chesapeake bay, and there is presently (1973) little danger of salt-water contamination.
Electric-analog model studies indicate that the Magothy aquifer is capable of yielding about 60 mgd in the Annapolis area, if the potentiometric surface is lowered to near its top. The model also indicates that most of the recharge to the aquifer would be received by leakage from adjacent formations. Concern has been expressed regarding the side effects that might be expected as a result of lowering the potentiometric surface, particularly the possibility that salt water from Chesapeake Bay or its estuaries might be induced into the aquifer, thus impairing the quality of its water. The Matawan Formation and younger formations serve as protective barriers, in most of the area, to impede the vertical movement of water between the estuaries and the Magothy aquifer. About 100 years would be required for water from the estuaries to move through the confining beds to the aquifer under the maximum head differentials that could be developed. However, the aquifer is unprotected by overlying layers in the upper parts of the Severn and Magothy Rivers. In those exposed areas, the aquifer would be most susceptible to contamination, which could migrate downdip to other parts of thee aquifer. Probably the amount of water that could leak into the aquifer in this area is relatively small and could be controlled by proper management.
The source of most recharge to the aquifer will be land areas. Therefore, the concentration of salt water that may eventually reach well fields will be substantially diluted by water from the fresh-water sources.
Because many broad assumptions have had to be made in the preparation of the models of the Magothy aquifer, the models would be made more accurate by updating and verification as new data become available.
