  This article studies (optimal) $W^{2m-1,\infty}$-regularity for the
polyharmonic equation $(-\Delta)^m u = Q \; \mathcal{H}^{n-1} \llcorner
\Gamma$, where $\Gamma$ is a (suitably regular) $(n-1)$-dimensional submanifold
of $\mathbb{R}^n$, $\mathcal{H}^{n-1}$ is the Hausdorff measure, and $Q$ is
some suitably regular density. We extend findings in [9], where the
second-order equation $-\mathrm{div}(A(x)\nabla u) = Q \; \mathcal{H}^{n-1}
\llcorner \Gamma$ is studied. As an application we derive (optimal)
$W^{3,\infty}$-regularity for solutions of the biharmonic Alt-Caffarelli
problem in two dimensions.
