Abstract: In one aspect, the invention relates to a system for evaluating a fluid system, which includes one or more batches of magnetically-responsive particles and one or more magnetic particle-collectors. In some embodiments, the magnetically-responsive particles have a composition that includes elements that are attracted by magnetic fields, but are not themselves magnetically-attractive. Each batch of magnetically-responsive particles can be released into the fluid system at a location upstream from the magnetic particle-collectors. The particles will follow fluid paths, experiencing effects, such as for example and without limitation, dispersion, diffusion and advection as they move through the fluid system. As particles reach the vicinity of the magnetic particle-collectors, the magnetically-responsive particles will be attracted into and held by the magnetic field and physical structure of the magnetic particle-collectors.