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The photospheric metal pollution of white dwarfs is now well-established asthe signature of the accretion of planetary debris. However, the origin of thetrace hydrogen detected in many white dwarfs with helium atmospheres is stilldebated. Here, we report the analysis of GD424: a metal-polluted,helium-atmosphere white dwarf with a large amount of trace hydrogen. Wedetermined the atmospheric parameters using a hybrid analysis that combines thesensitivity of spectroscopy to the atmospheric composition,$\log(\mathrm{H/He})$, with that of photometry and astrometry to the effectivetemperature, $T_{\mathrm{eff}}$, and surface gravity, $\log g$. The resultingwhite 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 theabundances of 11 photospheric metals and argue that the accretion event is mostlikely either in the increasing or steady state, and that the disruptedplanetesimal resembles either CI chondrites or the bulk Earth in terms of itscomposition. We suggest that the observed $1.33\times 10^{22}$ g of tracehydrogen in GD424 were at least partly acquired through accretion of water-richplanetary debris in an earlier accretion episode.

GD424 -- a helium-atmosphere white dwarf with a large amount of trace hydrogen in the process of digesting a rocky planetesimal