Estimating the hydraulic properties of an aquitard from in situ pore pressure measurements

A workflow is described to estimate specific storage (S_s) and hydraulic conductivity (K) from a profile of vibrating wire piezometers embedded into a regional aquitard in Australia. The loading efficiency, compressibility and S_s were estimated from pore pressure response to atmospheric pressure changes, and K was estimated from the earliest part of the measurement record following grouting. Results indicate that S_s and K were, respectively, 8.8 × 10^‑6 to 1.2 × 10^‑5 m^‑1 and 2 × 10^‑12 m s^‑1 for a claystone/siltstone, and 4.3 × 10^‑6 to 9.6 × 10^‑6 m^‑1 and 1 × 10^‑12 to 5 × 10^‑12 m s^‑1 for a thick mudstone. K estimates from the pore pressure response are within one order of magnitude when compared to direct measurement in a laboratory and inverse modelled flux rates determined from natural tracer profiles. Further analysis of the evolution and longevity of the properties of borehole grout (e.g. thermal and chemical effects) may help refine the estimation of formation hydraulic properties using this workflow. However, the convergence of K values illustrates the benefit of multiple lines of evidence to support aquitard characterization. An additional benefit of in situ pore pressure measurement is the generation of long-term data to constrain groundwater flow models, which provides a link between laboratory scale data and the formation scale.