A filter size of 0.45 microns effectively removes particulate matter from a sample of water that may otherwise impact analytical results, yet allowing for a large enough mesh size so appropriate volumes of water are filtered within a reasonable time (referring to waters that contain large quantities of suspended particulates).
In addition, the optimum size for mobility of particles in ground is approximately 1um, meaning that a 0.45um filter is an adequate size filter.
For more detail on the use of filters for Groundwater sampling, the Connecticut Department of Energy and Environmental protection provides a thorough explanation here.