Chromium occurs ubiquitously in nature with natural levels in uncontaminated waters ranging from fractions of 1 µg to a few µg l^-1. Sea water contains less than 1 µg chromium l^-1 (US NAS, 1974), but the exact chemical forms in which chromium is present in the ocean, and surface water is unclear. Theoretically, chromium can persist in the hexavalent state (Cr IV) in water with a low organic matter content. In the trivalent form (Cr III), chromium will form insoluble compounds at the natural pH of water, unless protected by complex formation. The exact distribution between the trivalent and hexavalent state is unknown

Almost all the hexavalent chromium in the environment arises from human activities. Chromium compounds are used in ferrochrome production, electroplating, pigment production, and tanning. These industries, the burning of fossil fuels, and waste incineration are sources of chromium in air and water. In the hexavalent oxidation state, chromium is relatively stable in air and pure water, but is reduced to the trivalent state when it comes into contact with organic matter in biota, soil, and water. There is an environmental cycle for chromium, from rocks and soils to water, biota, air, and back to the soil. However, a substantial amount (estimated at 6.7 x 10⁶ kg per year) is diverted from this cycle by discharge into streams, and by run-off and dumping into the sea. The ultimate repository is ocean sediment (WHO 1988).

It appears that the zero-, di-, tri-, and hexavalent oxidation states have biological importance. The effects of the last two oxidation states are so fundamentally different that they must always be considered separately. The trivalent form is an essential nutrient for Man, in amounts of 50 - 200 µg day^-1 (WHO 1988) .

Concentrations of chromium have been measured in water and sediments as part of the National Monitoring Programme at sites throughout the UK in estuaries and coastal waters (MPMMG 1998). The results of the National Monitoring Programme have been summarised in Appendix D. MPMMG (1998) should be consulted for further details.

Grimwood and Dixon (1997) compiled available monitoring data for chromium in water, sediments and biota for marine sites of nature conservation importance in England.

As an example of the recorded levels of dissolved chromium in the marine environment, the following concentrations have been reported by DETR (1998) for some English estuaries (see tables below).

The solubility of trivalent chromium in sea water varies with salinity and depends strongly on the complexes actually present. The loss of Cr III from solution by precipitation is unlikely to occur to any great extent and the main removal process is adsorption to suspended particles. Chromium in the oxidised form (Cr IV) is very soluble and is not adsorbed strongly to suspended particles in natural waters (Mance et al 1984).

Minimum concentration (µg ^l-1) of dissolved chromium in the water column of some English estuaries (from DETR 1998)

Average concentration (µg ^l-1) of dissolved chromium in the water column of some English estuaries (from DETR 1998)

Maximum concentration (µg l^-1) of dissolved chromium in the water column of some English estuaries (from DETR 1998)

An exhaustive literature review on the toxicity of chromium to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Mance et al 1984, Hunt and Hedgecott 1992, Grimwood and Dixon 1997). The most sensitive groups of organisms have been identified.

In 1984, Mance et al reviewed data on the toxicity of chromium to saltwater organisms. The authors found acute toxicity to be extremely variable. Fish appeared to be considerably less sensitive than invertebrates, although the data on fish larvae indicated that these forms were particularly sensitive to chromium contamination.

Mance et al (1984) proposed an EQS for the protection of saltwater organisms of 15 µg l^-1, expressed as a dissolved annual average concentration. (this is currently adopted in UK legislation (HMSO 1989)). The EQS was established by applying an arbitrary factor of 2 to the lowest chronic effects (unspecified) concentration of 30 µg l^-1 reported at that time for the polychaete worm Neanthes arenaceodentata. However, following a review of more recent toxicity data by Hunt and Hedgecott (1992), a more stringent EQS of 5 µg l^-1 was proposed. This value (also expressed a dissolved annual average) is based on effect concentrations of 10 - 30 µg l^-1 reported for a range of organisms tested in the laboratory and corresponds to the application of a factor of 2 to the lower end of this range.

A further review of data on the toxicity of chromium to saltwater organisms (since the report by Hunt and Hedgecott 1992) has been undertaken by Grimwood and Dixon (1997). The authors found no reliable toxicity data that indicated higher sensitivity of saltwater organisms than that previously reported. They recommended that the revised EQS of 5 µg l^-1 (dissolved annual average) was appropriate for the protection of all saltwater life, although where there is concern that the health of communities in sites of nature conservation importance may be compromised as a result of the presence of particularly sensitive species, a lower value may be used as a guideline. However, in the absence of any new toxicity data, it was not possible to make any recommendations for such a value.

Chromium is found in sediments and can pose a hazard to sediment dwelling organisms at concentrations above 52.3 mg kg^-1 according to Canadian interim marine sediment quality guidelines.

Hunt and Hedgecott (1992) reported some information on the bioaccumulation of Cr IV. Laboratory experiments on annelids, crustaceans and molluscs have resulted in bioconcentration factors (BCFs) in the range 158 to 596 for annelids and 383 to 620 (based on dry weights) for molluscs and crustaceans in the laboratory. Much lower BCFs have been calculated in the field: 0.46 to 15 for annelids and <1 for molluscs and crustaceans. These BCFs indicate that chromium is not expected to bioaccumulate under field conditions.

Potential effects include:

• acute toxicity to invertebrates and fish at concentrations of dissolved chromium above the EQS of 5 &micro;g l^-1 (annual average) in the water column;
• toxic effects to sediment dwelling-organisms at concentrations above 52.3 mg kg^-1 according to Canadian interim marine sediment quality guidelines.