  Using both muon spin rotation and muon spin relaxation, the superconducting
ground state of the Heusler superconductor {ZrNi$_2$Ga} has been studied. The
temperature dependence of the magnetic penetration depth of {ZrNi$_2$Ga} is
consistent with a single isotropic gap s-wave BCS superconductor. The gap
energy is \Delta(0)=0.44(1) meV and the magnetic penetration depth, \lambda(0),
is 310(5) nm. Furthermore, we show evidence of a possible cross-over from an
square flux line lattice to a hexagonal lattice at low temperatures. No
evidence of time reversal symmetry breaking has been observed as might be
expected for a half metal superconductor.
