Influence of seasonal hydraulic head changes on slug tests conducted in shallow low-permeability soils

- Impact of seasonal head changes on slug tests in low-permeability soil is studied.
- Experimental and numerical results are presented.
- Influence of seasonal head changes can be greater than deformation.
- Tests parameters, such as initial hydraulic head, have an influence on test results.

This paper quantifies the influence of seasonal hydraulic head changes in shallow low-permeability soils on the hydraulic conductivity (K) values obtained from slug tests. A total of 61 slug tests with durations between 3 and 6 weeks were conducted between 2007 and 2010 in 17 monitoring wells (MWs) installed in a clay deposit in Lachenaie, Canada. Beginning in 2012, vibrating wire piezometers (VWPs) were sealed in some of the MWs to record the seasonal hydraulic head cycles. To get a better understanding of the influence of seasonal head changes on the variability of K measurements for the Lachenaie tests sites, a series of 936 slug tests were modelled using COMSOL. The seasonal head changes measured with the VWPs were applied as a boundary condition 2 m away from the MW intake zone. Different types of slug test (rising- or falling-head) were launched at different times of the year. The velocity graph method, a graph of the apparent hydraulic head in the MW versus its rate of change, was used to interpret both the experimental and numerical slug test data. Both the experimental and numerical results show that seasonal head changes have a systematic influence upon the middle part of the velocity graph, the part used to calculate K. This influence depends on time of year, type of test (rising- or falling-head), initial hydraulic head difference and clay properties. In Lachenaie, the influence of seasonal hydraulic head changes on K is shown to be at least of the same order (factor 1.1) as the influence of clay deformation.