Solute Migration Mechanisms and Decay Modelling in Dead End Pipes of Water Distribution Networks

Dead-end pipes in water distribution networks are prone to water quality deterioration due to prolonged stagnation, and fluctuating hydraulic conditions can remobilize accumulated contaminants into the main pipe, threatening downstream water quality. However, the mechanisms and concentration dynamics of pollutant migration in such pipes remain insufficiently understood. This study elucidated solute migration mechanisms and developed an exponential solute decay model for dead-end pipes through the integration of full-scale experiments and computational fluid dynamics (CFD) simulations. Velocity and concentration distributions across multiple cross-sections were analyzed, and the mass exchange coefficient was expressed as a function of geometric and hydraulic parameters. Results show that shear forces at the junction between the main and dead-end pipes induce a persistent clockwise vortex on the vertical cross-section, continuously transporting solutes into the main pipe. The interior of dead-end pipes is not fully stagnant but contains weak recirculating flow, with the longitudinal velocity profile exhibiting a distinct three-stage pattern. Although forward velocities in the dead-end pipe are less than 10% of those in the main pipe, advective transport still plays a major role in mass transfer within the velocity disturbance region and the exponential decay region of the dead-end pipe. The proposed decay model accurately captures temporal solute concentration variations, with a relative error below 0.6%. This model provides a robust basis for water quality assessment at dead-end junctions and offers practical guidance for predicting water quality responses to sudden pollutant intrusions.