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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

The available data indicate no concerns with regard to enrichment in water breathing biota as enrichment compared to the surrounding waters is low and/or decreases with trophic levels.

Key value for chemical safety assessment

Additional information

This endpoint is covered by the category approach for soluble iron salts (please see the section on physical and chemical properties for the category justification/report format).

Testing for this endpoint has been waived as it is considered scientifically unjustified.

General considerations

Generally aquatic biota regulate actively their internal concentrations of metals by active transport, storage or a combination of both. As a result of these processes which do often not discriminate non-essential metals, an inverse relationship gets established between water concentrations and the corresponding accumulation factors (e.g. BCF, BAF) of most metals. This means that organisms accumulate metals unspecifically to meet their metabolic requirements, whereat non-essential metals are moderately accumulated as well. At higher water concentrations, organisms with active regulation mechanisms are able to excrete excess metals or limit their uptake (ICMM 2007, Brix & DeForest 2000). If such mechanisms apply, the typical figures are that metal concentrations in tissue based on a range of exposure concentrations may be quite similar, but the enrichment factors will be quite variable, i.e., higher factors occur at lower exposure concentrations and lower factors at higher exposure concentrations (ICMM 2007, McGeer et al 2002). The inverse relationship between water and enrichment factor values limits the ability to describe hazard as a result of the size of such factors like BCF or BAF, i.e. the most pristine ecosystems have the highest BAFs. A better approach is to directly assess the concentrations of metals at various trophic levels in the ecosystem.

The formation of lipophilic metalorganic species on the other hand would lead to figures as known from organic chemicals. In conclusion not primarily the magnitude of measured enrichment factors (provided the value is not alarmingly high for non-essential metals) is indicative for evaluation but the establishment of an inverse relationship rather than apparent independence or a proportional one. Decreasing values for enrichment in higher trophic levels are indicative for the absence of concern. The latter would be of concern while the former one is just the normal metal behaviour giving no indication for the formation of bioaccumulating species.

Data on enrichment based on the sum of all iron species

The read across from iron determined as sum of all species is justified on the basis that members in the category exhibit similar behaviour and thus share common environmental fate and behaviour. The salts will dissociate and speciate immediately in aqueous solution under normal environmental conditions.

The study of NN (2001) studied the bioconcentration of iron using an appropriate study guideline (OECD TG 305). The study was conducted using ferrous sulphate and it was shown that BCF values were less than 20 for fish (Cyprinus sp) in a 28 day study using flow-through (NN 2001). The BCF values decrease with increasing iron exposure, constituting thus a metal typical inverse relationship.

The low tendency for bioconcentration in fish is supported by another study (Andersen 1997), where the bioconcentration factor of developing stages of Brown trout (Salmo trutta) was investigated. Maybe due to the quick growth of the test organisms and/or due to the fact that both water medium concentrations (0.35 µg Fe/mL and 35 µg Fe/mL) were in excess of the water solubility of ferric iron with the likely consequence of precipitation, most of the BCF were below 1, i.e. depletion compared to the medium level was recorded. BCFs of 0.382-6.63 (egg), 0.008-0.03 (embryo), 0.004-0.028 (larvae), and 0.135-0.917 (fry) were obtained for iron uptake in the high and low dose, respectively.

It is concluded on the basis of these two studies (NN 2001, Andersen 1997) that bioconcentration of iron from water media to fish is low and BCF values are higher at lower water concentrations, which is a metal typical inverse relationship.

Handbook data of Ng et al (1968) indicate BAF of 300 for freshwater and 3'000 L/kg for marine fish. The magnitude of the latter value is irrelevant for the assessment as the BAFs decrease among the food chain, as evidenced by values of 50'000 and 20'000 for marine plants and invertebrates respectively (Ng et al 1968). The same applies to the freshwater food chains as Ng et al (1968) report 5'000 and 3'200 for plants and invertebrates.

Further BAF data are available from environmentally exposed organisms and again the highest values were reported from primary producers, as demonstrated by the measurements of Lopez & Carballeira (1993) and Cameron & Liss (1984) on aquatic bryophytes (mosses). BAFs of 32'391 (Brachythecium rivulare) and even 93'776 (Scapania undulata), respectively, were reported. Nonetheless Rai & Chandra (1992) report a low BAF of 48 for aquatic algae (Hydrodictyon reticulatum).

Primary consumers such as filter feeders may enrich iron as well particularly when exposed to low water concentrations. On one hand Pentreath (1973) reported that a mussel (Mytilus edulis) had highest measured bioaccumulation factors of 2'756 to 9'622 when exposed to initial total iron concentration of 0.009 mg/L (mean seawater concentration: 0.0034 mg/L, Turekian 1968) for 42 days at 10 °C. The results support a sequestering mechanism for active uptake of iron from seawater into the organism, which reflects an adaptation to the generally low iron bioavailability in marine environments rather than an effect of relevance for the hazard assessment. On the other hand Cameron & Liss (1984) found in the Hard calm or Cherrystone (Mercenaria mercenaria) significantly lower iron concentrations as in the surrounding media and calculated BAFs of 0.004 to 0.02. Comparable depletion is reported from Grass shrimps (Palaeomonetes pugio) as well by the same authors, i.e. 0.003 to 0.02.

Another publication confirms the generally low BAF for secondary consumers. Ayyadurai et al (1994) found a BAF of 48 in freshwater finfish (Oreochromis mossambicus). Lappivaara et al (1999) observed no accumulation in 1-year-old whitefish (Coregonus lavaretus) exposed to at 0.1 and 0.35 mg/L dissolved iron added as a 1:1 solution of FeSO4/FeCl2 in a subchronic (30-day) experiment when natural iron-rich humic water was used. In the humic-free water with iron added at 2 and 8 mg/L (0.07 and 0.2 mg l-1 dissolved iron), no accumulation of iron or physiological effects were found at the 2 mg/L added iron concentration. In the humic-free water with 8 mg/L added iron (0.2 mg/L dissolved iron), some accumulation in gills, liver, and gut was found and sublethal effects interpreted as physiological stress response were reported.

Table: Iron BCF/BAFs reported in the literature


Enrichment factor (BCF or BAF)


Trophic level

Aquatic moss

(Scapania undulata)


Cameron & Liss (1981)

Primary producers

Marine plants


Ng et al (1968)

Aquatic moss

(Brachythecium rivulare)


Lopez & Carballeira (1993)

Freshwater plants


Ng et al (1968)

Aquatic algae

(Hydrodictyon reticulatum)


Rai & Chandra (1992)

Marine invertebrates


Ng et al (1968)

Primary consumers

Mussel (Mytilus edulis)

2'756 – 9'622

Pentreath (1973)

Freshwater invertebrates


Ng et al (1968)


(Palaeomonetes pugio)


Cameron & Liss (1984)


(Mercenaria mercenaria)

0.004 to 0.02

Marine fish


Ng et al (1968)

Secondary consumers

Freshwater fish


Fish (Cyprinus sp)

< 20

NN (2001)

Brown trout (Salmo trutta)


Andersen 1997, low water concentrations


Andersen 1997, high water concentrations

Freshwater finfish

(Oreochromis mossambicus)


Ayyadurai et al (1994)

In conclusion a clear trend to decreasing enrichment factors along the aquatic food chains is demonstrated and accordingly no concerns exist with regard to iron bioaccumulation.

The U.K. Environment Agency (Johnson et al 2007) assesses iron bioaccumulation as follows: “Most taxa do not appear to bioaccumulate iron to any significant extent, or if accumulation is evident, associated bioconcentration factors (BCFs) are generally less than 100. An exception to this is algae and higher plants (aquatic mosses and flowering plants), which do appear to have the potential to accumulate high concentrations of iron, though BCFs may also be affected by adsorption to cell surfaces.”

As reported by the U.K. Environment Agency (Johnson et al 2007) the half-life can be assessed on the basis of the publications of Dahlgaard (1981) and Rule (1985) as follows: “Following accumulation by mussels of iron from water and food in the laboratory, the biological half-life for iron from the slow (long retention) compartment was 140–215 days. A biological half-life of 4–7 days was estimated from the medium compartment.”

Biologically, iron is an essential trace element for organisms including micro-organisms, plants and animals. Iron plays an important role in biological processes, and iron homeostasis is under strict control (McCance & Widdowson 1938). Many organisms actively regulate uptake of iron into cells. It is an important factor in oxygen transport in the blood, oxidative metabolism, electron transport, nitrogen fixation and other biological processes in cells. Iron is a constituent of haemoglobin, enzyme systems and chlorophyll molecules essential to life. Iron can be bound into various chelate complexes or proteins in biological material (O'Neil et al 2001, Neilands 1972, WHO 1983).

Iron is a biologically essential metal actively taken up into organisms. The available data on iron concentrations in the environment show the way that organisms display adaptation to the high background concentrations of iron.

As referenced in the section on the natural occurrence of iron, it is the forth-most abundant element in the Earth's crust occurring naturally as almost as oxide and hydroxide. Although it is widely distributed and present in all environmental media the highest levels were in soil. Despite its presence no accumulation in wildlife biota is described in the literature. This supports the suggestion of a low potential for bioaccumulation.

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