Milligrams Per Liter – A unit of measurement expressing the concentration of a constituent in solution as the weight (mass) of solute (1 milligram) per unit volume (1 liter) of water; equivalent to 1 part per million (ppm) for a water density. 1 g cm-3. 1 mg/L = 1000 ug/L; 1 g/L = 1000 mg/L.