Evaluation of Artesian Pressures in a Multi-Layered Aquifer System

Several pumping tests were performed in Washington D.C. in support of the District of Columbia Water and Sewer Authority’s DC Clean Rivers Project to control combined sewer overflows (CSOs) to surface waters of the District. The test site is located on the west bank of the tidally influenced portion of the Anacostia River approximately 3 miles northeast of its confluence with the Potomac River. The tests were conducted to evaluate the hydraulic properties of aquifers underlying the locations of two proposed large diameter deep shafts. These shafts will provide access to miles of future large diameter tunnels that will provide storage capacity to control the CSOs. During drilling of exploratory borings, two zones with significantly different groundwater pressure conditions were discovered within the confined aquifer beneath the site. While the artesian pressures in the confined aquifer were apparent, the higher pressures at depth within this same unit were not well understood. Pressures in the deeper zone were high enough to cause flowing conditions at the surface with its potentiometric surface more than 10 feet above the ground surface. A pumping well supplemented with observation wells was designed with two discrete screened intervals to evaluate the two zones and two pumping tests were performed. The first pumping test was performed with the pump intake placed between the two screens. Prior to the second test, the well was modified by filling the bottom portion of the well with sand and installing a K-type packer to test only the upper zone. Information from both tests was used to calculate the flow contribution and the hydraulic conductivities of each zone. The information was critical for the design of a groundwater control system for construction of the shafts.