Q & A on Measuring Dose
*** Q. 1 ppm for water equal to mg/L, for soil it would be 1 mg/kg. What about gases? Is 1 ppm for gas equal to 1 mg/m^3?
A.. I avoid using ppm for water solutions. In order to use the term "parts per million [parts]", both numerator and denominator must be in the same units. If and only if one liter of the solution weighed exactly one kilogram, 1 ppm equals 1 mg / L. However, in many environmental measurements, the solution is very dilute and one liter weighs just about one kilogram, so ppm and mg/L are used interchangeably. For gases, I added a non-mandatory page about converting ppm to mg / m^3. One ppm of a gas is different than 1 mg / m^3, usually quite different.
** Q. Why do you assert that ppm is not good usage for solid mixtures? I understand the situation for solutions where the units are not the same. But for weight to weight, mass to mass, widget to widget, etc. to etc., the use of parts per whatever is at least technically correct, and is the same as expressing volume to volume gas concentrations on a parts per number basis.
A. I won't argue that point - the usage is common enough. Concentrations are best expressed on a per volume basis. Parts per million of gas in another gas is volume per volume, rationalized by the molar calculations. Soils densities are too variable, of course, for volume to be very accurate, so weight is common.
*Q. Question on the material itself...With regard to chemical units for solid mixtures...the reading states that contaminated soils are described in ug/kg or mg/kg..with the denominator being the weight of both the soil (matrix) and contaminant. But, isn't water almost always present in soil? How does water factor in to these measurements?
A.Yes, and see other closure question on this. The soil would dry out. If the contaminant were volatile, such as benzene, it would evaporate too. If the contaminant were not volatile, like elemental lead, as the soil dried out, its concentration would seem to increase.