Like AWK: says, 1 ppm is one part per million, and 1 milligram is 1/1,000,000 of a kilogram. Easy-peasy.
'Course, that get's messy sometimes, million meaning different things in various English speaking countries, and sometimes people use liters of water as equivalent to 1 kg of water, which isn't technically correct. You probably won't have problems if you stick to AWK:'s definition, or at least, define ppm in your report that way, so everyone is on the same page.