  We describe and analyze leakage errors of singlet-triplet qubits. Even though
leakage errors are a natural problem for spin qubits encoded using quantum dot
arrays, they have obtained little attention in previous studies. We describe
the realization of leakage correction protocols that can be implemented
together with the quantum error correction protocol of the surface code.
Furthermore we construct explicit leakage reduction units that need, in the
ideal setup, as few as three manipulation steps. Our study shows that leakage
errors can be corrected without the need of measurements and at the cost of
only a few additional ancilla qubits and gate operations compared to standard
quantum error correction codes.
