dispersiviteit

Dispersiviteit is a concept in hydrogeology describing how solutes spread as they move through a porous medium. The spreading arises from two main processes: mechanical dispersion, caused by velocity variations at the pore scale, and molecular diffusion, driven by concentration gradients. Together they determine how a contaminant plume or a tracer plume broadens as it advects with groundwater.

In many transport models, especially the one-dimensional advection-dispersion equation, the spreading is described by a dispersion coefficient D. D consists of a molecular diffusion part D_m and an advective, velocity-dependent part often expressed through the dispersivity. Longitudinal dispersivity α_L governs spreading along the direction of flow, while transverse dispersivity α_T controls spreading perpendicular to the flow. The dispersion tensor in three dimensions captures anisotropy, with α_T typically smaller than α_L.

Dispersivity is scale-dependent: laboratory measurements on packed columns yield small values (millimetres to centimetres), whereas field measurements can produce dispersivities ranging from metres to tens of metres, depending on the geology and heterogeneity of the aquifer. The ratio α_T/α_L is often less than one and varies with sediment type, porosity, and flow conditions.

Measurement and application involve tracer tests, column experiments, or fitting observed concentration breakthrough curves to transport models. Dispersivity is a key input in groundwater risk assessments, contaminant transport forecasting, and the design of remediation strategies, influencing predictions of contaminant arrival times, concentration plumes, and effective remediation zones.