A portable nuclear magnetic resonance (NMR) device and a method of determining an oil concentration in water are disclosed. The portable NMR device can include a magnetic field assembly to carry out NMR measurements of water. The portable NMR device can include a housing to at least partly surround the magnetic field assembly and to substantially eliminate a magnetic fringe field generated by the magnetic field assembly outside the housing. The portable NMR device can also include an analysis module to receive the NMR measurement of the water and to determine, based on the received NMR measurements of the water, the oil concentration in the water.