A numerical study of the influence of sediment compaction caused by groundwater pumping on contaminant transport

Abstract

Studies of clay liners have shown that deformation caused by loading can have an important influence on the transport of contaminants. However, very little comparable work has been carried out on aquifers, even though pumping of groundwater from alluvial aquifer systems often causes sediment compaction and could create a similar effect. In this study, a coupled, multi-field, numerical simulation model was used to investigate the extent to which groundwater pumping affects the porosity and hydraulic conductivity of the aquifer and how such changes may, in turn, modify contaminant transport by altering parameters such as hydrodynamic dispersion. The model was based on aquifer conditions encountered at a representative site in east-central China with pumping introduced at a single well located in the centre of the model domain. Model results for an observation well located 5 m from the pumping well showed that sediment compaction caused by a steady-state drawdown of just 10 m produces a 0.41% reduction in porosity. While a reduction in porosity would normally enhance contaminant movement due to higher seepage velocities and, consequently, higher values of hydrodynamic dispersion, this effect is more than compensated by a reduction in seepage velocity associated with a 1.9% decline in hydraulic conductivity that is also caused by sediment compaction. At the well, where steady-state drawdown approaches 20 m, porosity reduces by 0.77% and hydraulic conductivity declines by over 3%. For the system investigated, the overall effect of sediment compaction is to inhibit the transport of contaminants and significantly reduce the magnitude and areal extent of the contaminant impact.