@koester76-1; @wesemael79-1; @dupuisetal93-2] has been the most widely accepted scenario, despite a number of fundamental problems [e.g. @aannestadetal93-1; @friedrichetal04-1; @farihietal10-2]. However, the rapidly growing number of white dwarfs that are accreting from circumstellar discs [e.g. @becklinetal05-1; @kilicetal05-1; @gaensickeetal06-3; @vonhippeletal07-1; @farihietal08-1; @vennesetal10-1; @dufouretal12-1] unambiguously demonstrates that debris from the tidal disruption of main-belt analogue asteroids or minor planets [@grahametal90-1; @jura03-1], or Kuiper-belt like objects [@bonsoretal11-1], likely perturbed by unseen planets [@debesetal02-1; @debesetal12-1], is the most likely origin of photospheric metals in many, if not most polluted white dwarfs.

Because of the need for high-resolution, high-quality spectroscopy, detailed abundance studies have so far been limited to a handful of white dwarfs [@kleinetal10-1; @kleinetal11-1; @vennesetal11-1; @melisetal11-1; @zuckermanetal11-1; @dufouretal12-1; @juraetal12-1]. For a given abundance and white dwarf temperature, metal lines are stronger in a helium-dominated (DB) atmosphere than in a hydrogen-dominated (DA) atmosphere, as the opacity of helium is much lower than that of hydrogen. Therefore, the small sample of well-studied metal polluted white dwarfs is heavily biased towards DB white dwarfs, which have diffusion time scales of $\sim10^5-10^6$yr. These long diffusion time scales introduce a significant caveat in the interpretation, as the abundances of the circumstellar debris may substantially differ from those in 