Sediment mobilisation and release through groundwater discharge to the land surface: Review and theoretical development

The discharge of groundwater to the land surface and to lakes and streams may express subsurface particles. This may lead to preferential pathways and increased fluxes of groundwater, sediment and contaminants, and modified subsurface structures. The current review attempts to describe and categorise the various forms through which sediment may be liberated in areas of groundwater discharge. Forces acting on subsurface particles in areas of groundwater discharge include seepage (drag), buoyancy and particle weight, amongst other, more complex forces. Equations for these can be combined to create formulae for approximating the conditions under which groundwater discharge will transport particles to the surface. Two forms of subsurface sediment transport are considered: (1) flow through an immobile granular matrix (suffusion and suffosion), and (2) flow through preferential pathways (i.e., often treated as pipes). Suffusion involves sediment movement that does not impact the soil's stability, whereas suffosion creates changes to soil stability and, consequently, soil volume. Preferential flow may arise from cracks in cohesive materials or through localised fluidization of non-cohesive soils, leading in some situations to sand boils. Guidance is presented on the minimum theoretical hydraulic gradient required for grains of various sizes to start to rise. New simple formulae are developed that build on existing theory, and these are compared to previous laboratory data, showing that suffusion is more or less predictable using the new simple method. However, experimental sand boils require larger hydraulic gradients compared to theory. The current analysis summarises the state of knowledge and persistent knowledge gaps associated with sediment ejection through groundwater discharge, which we expect has wide-ranging applications in terms of sediment transport in coastal regions and to surface water bodies, and where strong groundwater discharge is known to occur.