  We have studied the single-electron transport spectrum of a quantum dot in
GaAs/AlGaAs resonant tunneling device. The measured spectrum has irregularities
indicating a broken circular symmetry. We model the system with an external
potential consisting of a parabolic confinement and a negatively charged
Coulombic impurity placed in the vicinity of the quantum dot. The model leads
to a good agreement between the calculated single-electron eigenenergies and
the experimental spectrum. Furthermore, we use the spin-density-functional
theory to study the energies and angular momenta when the system contains many
interacting electrons. In the high magnetic field regime the increasing
electron number is shown to reduce the distortion induced by the impurity.
