A SEMI-EMPIRICAL ANALYTICAL MODEL FOR DESIGNING AN AQUIFER DEPRESSURIZATION SYSTEM

As part of a construction project in the Washington DC area, depressurization of the uppermost Patuxent Aquifer (PTX) was required. To determine the hydraulic properties of the upper PTX a test well, that could be later used as a dewatering well, and a number of observation wells were installed, and a 7-day, constant-rate pumping test was conducted. The drawdowns observed in the test well and in the observation wells indicated the potential presence of impermeable boundaries or of significant reductions in hydraulic conductivity at a relatively short distance from the site. These results suggested that this portion of the PTX could be an isolated lens of sand within the Potomac Group, a hypothesis supported by regional water level observations. Although an estimate of the hydraulic properties of the PTX was obtained from the early test data, the lack of information on the exact nature of the observed “boundary effects” precluded the development of a numerical groundwater flow model for the design of the dewatering system. Instead, relatively simple semi-empirical analytical models, based on specific drawdowns, and distance-drawdown relationships derived from the test data, were developed to predict drawdowns that would result from pumping at different rates, in varying numbers of dewatering wells and at different distances from these wells. These semi-empirical models were then used to design a dewatering system that will conservatively meet the drawdown targets required during the construction of the project. The basis for these models, their use in the design of the dewatering system, and the resulting dewatering system operation schedule are detailed in this presentation.