Transient Hydraulic Tomography in the Field: 3-D K Estimation and Validation in a Highly Heterogeneous Unconfined Aquifer

Abstract:

Characterizing subsurface hydraulic properties is essential for predicting flow and transport, and thus, for making informed decisions, such as selection and execution of a groundwater remediation strategy; however, obtaining accurate estimates at the necessary resolution with quantified uncertainty is an ongoing challenge. For over a decade, the development of hydraulic tomography (HT) – i.e., conducting a series of discrete interval hydraulic tests, observing distributed pressure signals, and analyzing the data through inversion of all tests together – has shown promise as a subsurface imaging method. Numerical and laboratory 3-D HT studies have enhanced and validated such methodologies, but there have been far fewer 3-D field characterization studies.

We use 3-D transient hydraulic tomography (3-D THT) to characterize a highly heterogeneous unconfined alluvial aquifer at an active industrial site near Assemini, Italy. With 26 pumping tests conducted from 13 isolated vertical locations, and pressure responses measured at 63 spatial locations through five clusters of continuous multichannel tubing, we recorded over 800 drawdown curves during the field testing. Selected measurements from each curve were inverted in order to obtain an estimate of the distributed hydraulic conductivity field K(x) as well as uniform (“effective”) values of specific storage S_s and specific yield S_y. The estimated K values varied across seven orders of magnitude, suggesting that this is one of the most heterogeneous sites at which HT has ever been conducted. Furthermore, these results are validated using drawdown observations from seven independent tests with pumping performed at multiple locations other than the main pumping well. The validation results are encouraging, especially given the uncertain nature of the problem. Overall, this research demonstrates the ability of 3-D THT to provide high-resolution of structure and local K at a non-research site at the scale of a contaminant source zone.

We would like to especially thank Michael Cardiff, Carsten Leven, Francesco Chidichimo, and Riccardo Borella for their insightful and tireless efforts as part of the field campaign for this research.