  The photospheric metal pollution of white dwarfs is now well-established as
the signature of the accretion of planetary debris. However, the origin of the
trace hydrogen detected in many white dwarfs with helium atmospheres is still
debated. Here, we report the analysis of GD424: a metal-polluted,
helium-atmosphere white dwarf with a large amount of trace hydrogen. We
determined the atmospheric parameters using a hybrid analysis that combines the
sensitivity of spectroscopy to the atmospheric composition,
$\log(\mathrm{H/He})$, with that of photometry and astrometry to the effective
temperature, $T_{\mathrm{eff}}$, and surface gravity, $\log g$. The resulting
white dwarf mass, radius, and cooling age are
$M_{\mathrm{WD}}=0.77\pm0.01\,\mathrm{M}_{\odot}$,
$R_{\mathrm{WD}}=0.0109\pm0.0001\,\mathrm{R}_{\odot}$, and
$\tau_\mathrm{cool}=215\pm10$ Myr, respectively. We identified and measured the
abundances of 11 photospheric metals and argue that the accretion event is most
likely either in the increasing or steady state, and that the disrupted
planetesimal resembles either CI chondrites or the bulk Earth in terms of its
composition. We suggest that the observed $1.33\times 10^{22}$ g of trace
hydrogen in GD424 were at least partly acquired through accretion of water-rich
planetary debris in an earlier accretion episode.
