Hydrogeology of the Masonville Confined Aquatic Disposal Test Site, Baltimore City, Maryland
The Maryland Port Administration has proposed to install a Confined Aquatic Disposal (CAD) cell at the Masonville Vessel Berth for the containment of dredged materials from the Baltimore Harbor. Construction of the CAD cell requires excavation of predominantly sandy sediment from the Patapsco Formation to a depth of approximately 75 ft below sea level. The base of the CAD cell would extend down to the top of the Arundel Clay. The proposed CAD pilot project at the Masonville Vessel Berth is located in the approximate outcrop area of the Lower Patapsco aquifer. The Lower Patapsco aquifer at this location is likely unconfined and under water-table conditions. The Lower Patapsco aquifer overlies the Arundel Clay, which separates it from the deeper Patuxent aquifer. Locally, the thickness of the Arundel Clay ranges from 20 – 70 feet. Both the Lower Patapsco and Patuxent aquifers are important sources of water supply in northern Anne Arundel County. Anne Arundel County Department of Public Works (the County) withdraws water from the Patuxent aquifer at its Dorsey Road Facility, and from the Lower Patapsco aquifer at seven well sites in the Glen Burnie area. The County well fields are located as near as six miles south of the proposed CAD site. The CAD site lies between two cones of depression in the Patuxent aquifer; one approximately five miles to the northeast surrounding an industrial well field and the other approximately 7 miles to the southwest surrounding the Dorsey Road well field. Currently there are no water-level data to define the head gradients in the outcrop portion of the Lower Patapsco aquifer in which the proposed CAD site will be located. Withdrawals from the Dorsey Road Patuxent aquifer well field and from the Lower Patapsco aquifer well fields in the Glen Burnie area are likely to increase in the future, resulting in steeper hydraulic gradients. Anne Arundel County Department of Public Works has expressed a concern that excavating into the Patapsco Formation may accelerate the movement of contaminants possibly from dredged material, and the contaminants may migrate towards the Lower Patapsco and possibly the Patuxent aquifer well fields.
The objective of the MGS portion of the Confined Aquatic Disposal Pilot Project Hydrogeological Study is to determine whether conditions are favorable for migration of potential contaminants from the proposed CAD pilot project site by measuring head gradients and hydraulic continuity of aquifer layers. This will be established through the installation of monitoring wells that will assess future changes in the flow regime.
One pilot hole will be drilled into the basement rock (approximately 300 ft) at the proposed CAD pilot project site. Two four-inch diameter test wells will be installed at the same location as the pilot hole; one into the Lower Patapsco aquifer (approximately 40-50 ft well) and one into the Patuxent aquifer (approximately 130-150 ft well). One four-inch monitoring well will be installed in the Lower Patapsco aquifer at a remote site (Bay Brooks Park) approximately 1.5 miles to the southwest of the proposed CAD site. Additionally, two 2-inch piezometers (at Garrett Park and Childs Street area) will be installed in the Lower Patapsco aquifer south of the proposed CAD Site to provide greater resolution of the water-table surface. Monitoring wells will be installed by MES or MES subcontractors.
MGS will define the hydrogeologic framework at the CAD site and its relations to the regional framework. MGS will also will determine the head gradient in the Lower Patapsco aquifer in the vicinity of the CAD site and its relation to the regional potentiometric surface, as well as determine the head gradient between the Lower Patapsco aquifer and Patuxent aquifer at the CAD site.
MGS will determine the hydrogeologic units present beneath the CAD site through inspection of drill cuttings, geophysical logs, and correlation to the nearest borehole control. MGS will also prepare structure maps and cross sections.
Head Gradient Determination
MGS will run a 4-hour aquifer test on each test well to estimate aquifer transmissivity and storage coefficient, and to evaluate the effectiveness of the Arundel Clay confining unit. MGS will collect approximately three months of continuous water-level records in all test wells and piesometers.
Four months after receiving all data from the wells, pump tests, and water-quality data, MGS will provide a summary of the hydrogeologic characteristics in the area and their relation to the potential use of the CAD site. These will include, but not be limited to, the following: the head gradient in the Lower Patapsco Aquifer, the regional potentiometric surface of the Lower Patapsco and Patuxent Aquifers, and the effectiveness of the Arundel Clay as a confining unit above the Patuxent Aquifers. Six months after completion of the wells and pump tests, MGS will prepare a final report including all data, structure maps, cross sections and results of the study.