Recently, we demonstrated the potential of synchrotron X-ray micro computed tomography (μ-CT) to visualise the spatial distribution of nanoparticle aggregates inside porous matrices. This paved the way for increasing our understanding of pore-scale nanoparticle retention processes. Here, we present the first 3D timelapse of nanoparticle retention in a sand packed column at the sub-micrometre scale and demonstrate the wealth of information that can be gained through accessing four dimensions, i.e. time and space. This includes i) visualisation of gradual pore space saturation with nanoparticles, ii) localisation of retention growth domains, iii) quantification of the growth of retained nanoparticle clusters, and iv) quantification of the dynamic re-mobilisation processes of retained nanoparticle clusters. The quantification revealed that the retention was primarily controlled by a ripening process but also showed a surprisingly large NP cluster re-mobilisation during injection (i.e., up to 50% of the retained NP were re-mobilised). Our results demonstrate that in situ monitoring of nanoparticle retention is technically feasible and will reveal novel details, likely even processes, of nanoparticle transport and retention at the pore scale.