Sinusoidal Hydraulic Testing of a Multi-Layer Aquifer at the Waste Isolation Pilot Plant, Carlsbad, NM, USA

The Waste Isolation Pilot Plant, located near Carlsbad, New Mexico, USA, is an operating repository for transuranic nuclear materials. Performance testing and monitoring is routinely conducted at the site to ensure that the facility is operating as designed. Three sinusoidal aquifer tests were performed by pumping at a periodic rate in the Culebra Dolomite Member of the Rustler Formation, a hydrogeologic unit that overlies the repository and a hypothetical pathway for repository releases. Drawdowns in nearby observation wells displayed sinusoidal behavior, with drawdown amplitudes that diminished – and time lags that increased – with distance from the pumping well. An analytic solution was developed to obtain the hydraulic properties of the two dominant units (Lower and Upper) within the vertically heterogeneous Culebra Dolomite using the sinusoidal data. The problem is approached mathematically using a matrix formulation that vertically discretizes the section into multiple layers and employs eigenvalues and eigenvectors to evaluate a Bessel function with complex matrix arguments. The solution for pumping from discrete intervals within an isotropic aquifer is consistent with a partially penetrating solution. While the multi-layer approach confirms previous hydraulic properties in the more conductive Lower Culebra, new information about spatial variability in the less conductive Upper Culebra is provided.