Parts per million is a pseudo-unit that denotes a ratio between two units (which could be mass, volume, etc as long as they're consistent). Mass units in mg/kg can be treated as the same as ppm. A 100 g sample of water is the same as 0.1 kg which contains 0.000325 g or 0.325 mg of calcium. Dividing the mass of the solute over the mass of the solvent: 0.325 mg / 0.1 kg gives us 3.25 mg/kg or 3.25 ppm.
