As a naturally occurring element, chromium is ubiquitous in the environment, with concentrations in uncontaminated waters generally in the low µg/l range. Almost all hexavalent chromium compounds in the environment are a result of anthropogenic activities (WHO 1988). Trivalent chromium sorbs to various ligands to form insoluble entities. Hexavalent chromium forms many soluble salts (Pawlisz et al. 1997). The toxicity of chromium to aquatic organisms is affected by various factors including abiotic factors such as the hardness, temperature, pH and salinity of the water, and biological factors such as species, lifestage and potential differences in local populations (Eisler 1986).
LC50 values for freshwater invertebrates exposed to trivalent chromium range from 2 to 64 mg/l, with a chronic LC50 of 0.066 mg/l. The most sensitive species in both acute and and chronic tests was a cladoceran. Acute LC50 values for fish exposed to trivalent chromium ranged from 33 to 71.9 mg/l, with a chronic LC50 of 1 mg/l. The most sensitive species were guppy (Lebistes reticulates) (acute test) and fathead minnow (Pimephales promelas) (chronic test) (WHO 1988).
The freshwater organisms most sensitive to hexavalent chromium appear to be aquatic algae and plants. Reduced growth of the alga Chlamydomonas reinhardi was reported following exposure to 10 µg/l (US EPA 1980). Reduced fond growth was also reported in duckweed (Lemna minor) exposed to 10 µg/l for 14 days (Mangi et al. 1978). Acute LC50 values for freshwater invertebrates exposed to hexavalent chromium range from 0.067 to 59.9 mg/l. The most sensitive species was scud. Acute LC50 values for fish exposed to trivalent chromium ranged from 17.6 to 249 mg/l, with chronic LC50 values ranging from 0.265 to 2.0 mg/l. The most sensitive species were fathead minnow (Pimephales promelas) (acute test) and rainbow trout (Pimephales promelas) (chronic test) (WHO 1988).
|Habitat/ Ecosystem Type||Critical Load/ Level||Status||Reliability||Indication of exceedance||Reference|
5 to 250 µg/l
|Environmental Assessment Level (EAL)||reliable i.e. a number of published papers of various studies show comparable results||
EALs have been set for protection of salmonid and cyprinid waters at 4 different water hardness concentrations. All values are expressed as dissolved chromium: