Single-well interference slug tests to assess the vertical hydraulic conductivity of unconsolidated aquifers

Meaningful understanding of flow and solute transport in general requires the knowledge of hydraulic conductivity and its anisotropy. Various field methods allow the measurement of the horizontal component (K-h), but vertical hydraulic conductivity (K-v) is rarely measured, for lack of practical field tests. This paper proposes vertical interference slug tests, an adaptation of inter-well interference slug tests to a single well, for the efficient field measurement of K-v. The test is carried out in a single well between a stress and an observation interval that are vertically isolated with a three-packer assembly. An instantaneous pressure pulse is induced in the stress interval and resulting drawdowns are recorded in both the stress and the observation intervals. In a proof-of-concept field study, 12 vertical interference tests were carried out sequentially along a fully-screened well across a moderately heterogeneous and highly anisotropic aquifer made up of littoral silts and sands. A direct-push method was used to install the well, which was completed without sand-pack to allow the natural collapse of sediments in the thin annular space around the screen. Direct-push wells allow the measurement of in situ hydraulic properties of sediments and minimize well construction interferences with hydraulic tests. Drawdowns measured in stress and observation intervals of multiple tests were simultaneously inverted numerically to reconstruct heterogeneous profiles of K-h, hydraulic conductivity anisotropy (K-v/K-h), and specific storage (S-s). Results were validated by comparison of observed versus predicted drawdowns and with field and laboratory measurements of K-h and K-v made along the tested well. Results indicate that the profile of K-v values obtained with vertical interference slug tests follows a similar pattern with depth than the profile with lab measurements made with a permeameter on soil samples collected in the same intervals as the interference tests, which demonstrates that vertical interference slug tests could provide an efficient method for the field measurement of well-scale K-v values. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.