ABSTRACT
The amounts of chemical constituents are measured in mass quantities (e.g. mg, kg, or tonnes) or related units (moles, or becquerels (Bq) in the case of radioactive elements). These amounts are expressed as such, or as concentration per unit mass of water, rock, soil, or sediment (mass concentration, e.g. mg kg-1, % (percent), ppm (parts per million), ppb (parts per billion)), or per unit volume (volume concentration, e.g. mg l-1, M (= moles l-1)). Since fresh water has a specific density of 1.00 kg l-1, the mass concentrations in fresh water are approximately equal to volume concentrations. Furthermore, the amount of chemicals can also be expressed per unit area (e.g. Bq m-2, tonnes ha-1) or per unit time (e.g. kg s-1, tonnes y-1). The amount of chemicals or sediment transported per unit time is also referred to as load .
