In aquatic toxicology, bioconcentration is the accumulation of a water-borne chemical substance in an organism exposed to the water.
There are several ways in which to measure and assess bioaccumulation and bioconcentration. These include: octanol-water partition coefficients (KOW), bioconcentration factors (BCF), bioaccumulation factors (BAF) and biota-sediment accumulation factor (BSAF). Each of these can be calculated using either empirical data or measurements, as well as from mathematical models. One of these mathematical models is a fugacity-based BCF model developed by Don Mackay.
Bioconcentration factor can also be expressed as the ratio of the concentration of a chemical in an organism to the concentration of the chemical in the surrounding environment. The BCF is a measure of the extent of chemical sharing between an organism and the surrounding environment.
In surface water, the BCF is the ratio of a chemical's concentration in an organism to the chemical's aqueous concentration. BCF is often expressed in units of liter per kilogram (ratio of mg of chemical per kg of organism to mg of chemical per liter of water). BCF can simply be an observed ratio, or it can be the prediction of a partitioning model. A partitioning model is based on assumptions that chemicals partition between water and aquatic organisms as well as the idea that chemical equilibrium exists between the organisms and the aquatic environment in which it is found
Calculation
Bioconcentration can be described by a bioconcentration factor (BCF), which is the ratio of the chemical concentration in an organism or biota to the concentration in water:
Bioconcentration factors can also be related to the octanol-water partition coefficient, Kow. The octanol-water partition coefficient (Kow) is correlated with the potential for a chemical to bioaccumulate in organisms; the BCF can be predicted from log Kow, via computer programs based on structure activity relationship (SAR) or through the linear equation:
Where:
at equilibrium
Fugacity capacity
Fugacity and BCF relate to each other in the following equation:
where ZFish is equal to the Fugacity capacity of a chemical in the fish, PFish is equal to the density of the fish (mass/length3), BCF is the partition coefficient between the fish and the water (length3/mass) and H is equal to the Henry's law constant (Length2/Time2)
Regression equations for estimations in fish
Uses
Regulatory uses
Through the use of the PBT Profiler and using criteria set forth by the United States Environmental Protection Agency under the Toxic Substances Control Act (TSCA), a substance is considered to be not bioaccumulative if it has a BCF less than 1000, bioaccumulative if it has a BCF from 1000 to 5000 and very bioaccumulative if it has a BCF greater than 5,000.
The thresholds under REACH are a BCF of > 2000 L/kg bzw. for the B and 5000 L/kg for vB criteria.
Applications
A bioconcentration factor greater than 1 is indicative of a hydrophobic or lipophilic chemical. It is an indicator of how probable a chemical is to bioaccumulate. These chemicals have high lipid affinities and will concentrate in tissues with high lipid content instead of in an aqueous environment like the cytosol. Models are used to predict chemical partitioning in the environment which in turn allows the prediction of the biological fate of lipophilic chemicals.
Equilibrium partitioning models
Based on an assumed steady state scenario, the fate of a chemical in a system is modeled giving predicted endpoint phases and concentrations.
It needs to be considered that reaching steady state may need a substantial amount of time as estimated using the following equation (in hours).
For a substance with a log(KOW) of 4, it thus takes approximately five days to reach effective steady state. For a log(KOW) of 6, the equilibrium time increases to nine months.
Fugacity models
Fugacity is another predictive criterion for equilibrium among phases that has units of pressure. It is equivalent to partial pressure for most environmental purposes. It is the absconding propensity of a material. BCF can be determined from output parameters of a fugacity model and thus used to predict the fraction of chemical immediately interacting with and possibly having an effect on an organism.