The concentration of a chemical in a water body is often expressed in mass per unit volume, e.g., 1 milligram of nitrate (NO₃^–) per liter of water, or 1 mg/L. Since 1 L of fresh water has a mass of 1 kg, a concentration of 1 mg/L is equivalent to 1 mg/kg, so it is also referred to as 1 part-per-million or ppm (since there are a million milligrams in a kilogram). Likewise, 1 ug/L (one microgram per liter) is equivalent to 1 ppb (part-per-billion).

For molecules such as NO₃^–, concentrations can either include the mass of the entire compound (e.g., 1 mg NO₃^–/L) or just the mass of the relevant atom (e.g., 1 mg NO₃^–-N/L, which should be understood as 1 mg of N in the form of NO₃^– per liter of water). Concentrations can also be expressed in molar units, which can be converted to mass-based concentrations using the molecular weight. For example, methane (CH₄) has a molecular weight of 16 g/mole, so 1 mmole/L CH₄ is equivalent to 16 mg/L, as shown in the equation below.

Note: The molecular weight of CH₄ is 16 grams per mole, so 16g/mole can be used as a conversion factor to convert from moles to grams. The crossing out of “mole” on the top and bottom indicates that the units cancel. (The “m” before mole means milli, i.e., 10^-3, so it can simply transfer directly over to stand in front of grams, indicating milligrams, mg.)

Organic pollutants and metals are commonly measured in fish and sediment as well as in water. Concentrations in these media are generally expressed in mg of pollutant per kg of fish or sediment (mg/kg or ppm) or as ug of pollutant per kg of fish or sediment (ug/kg or ppb), usually on a dry-weight basis (i.e., after drying of the sediment or tissue).