  Many isolated, old white dwarfs (WDs) show surprising evidence of metals in
their photospheres. Given that the timescale for gravitational sedimentation is
astronomically short, this is taken as evidence for ongoing accretion, likely
of tidally disrupted planetesimals. The rate of such accretion, $\dot M_{\rm
acc}$, is important to constrain, and most modeling of this process relies on
assuming an equilibrium between diffusive sedimentation and metal accretion
supplied to the WD's surface convective envelope. Building on earlier work of
Deal and collaborators, we show that high $\dot M_{\rm acc}$ models with only
diffusive sedimentation are unstable to thermohaline mixing and that models
which account for the enhanced mixing from the active thermohaline instability
require larger accretion rates, sometimes reaching $\dot M_{\rm acc} \approx
10^{13} \, \rm g \, s^{-1}$ to explain observed Calcium abundances. We present
results from a grid of MESA models that include both diffusion and thermohaline
mixing. These results demonstrate that both mechanisms are essential for
understanding metal pollution across the range of polluted WDs with hydrogen
atmospheres. Another consequence of active thermohaline mixing is that the
observed metal abundance ratios are identical to accreted material.
