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Many isolated, old white dwarfs (WDs) show surprising evidence of metals intheir photospheres. Given that the timescale for gravitational sedimentation isastronomically short, this is taken as evidence for ongoing accretion, likelyof tidally disrupted planetesimals. The rate of such accretion, $\dot M_{\rmacc}$, is important to constrain, and most modeling of this process relies onassuming an equilibrium between diffusive sedimentation and metal accretionsupplied to the WD's surface convective envelope. Building on earlier work ofDeal and collaborators, we show that high $\dot M_{\rm acc}$ models with onlydiffusive sedimentation are unstable to thermohaline mixing and that modelswhich account for the enhanced mixing from the active thermohaline instabilityrequire larger accretion rates, sometimes reaching $\dot M_{\rm acc} \approx10^{13} \, \rm g \, s^{-1}$ to explain observed Calcium abundances. We presentresults from a grid of MESA models that include both diffusion and thermohalinemixing. These results demonstrate that both mechanisms are essential forunderstanding metal pollution across the range of polluted WDs with hydrogenatmospheres. Another consequence of active thermohaline mixing is that theobserved metal abundance ratios are identical to accreted material.

Increases to Inferred Rates of Planetesimal Accretion Due to Thermohaline Mixing in Metal Accreting White Dwarfs