Bio-accumulation
The process of bioaccumulation is the gradual build-up (accumulation) of chemicals (these generally consider metals, pesticides and related breakdown products and organic compounds) within an organism. Bioaccumulation is unlike the other soil processes considered in the soil function models, as it occurs passively either through secondary ingestion (i.e. the organisms do not actively consume the chemicals but rather they are eaten in combination with soil particles, organic matter or other organisms) or by absorption (i.e. the chemicals are absorbed from water through the skin, mainly in softer bodied organisms, such as earthworms). Bioaccumulation takes place when the intake or build up these chemicals is greater than the elimination, in that the organism is not able to quickly breakdown or eliminate the chemicals from their body and therefore the toxic chemicals build up over time[1]. The rate of build up is different in different organisms and is also associated to the amount of exposure.
The process of bioaccumulation is important in Water Purification and Regulation, as it contributes to the removal of contaminants from the water in the soil. Contaminants are also taken up by plants and micro-organisms but this takes place through root foraging and food web assimilation.
The measurement of bioaccumulation is not straight forward as the route of accumulation differs by chemical type and by organism. The most well known assessment of bioaccumulation is through single species toxicity assays which are performed primarily under laboratory controlled conditions with bioindicator species for a range of taxa[2]. However these single species laboratory tests do not represent field conditions well and therefore the route of exposure or the rate of uptake may be inadequately assessed. Van Gestel et al.[3] calls for the improvement of “semi-field” tests which use mesocosms to replicate better field conditions and at the same time assessing a community of organisms rather than a single species. At field scale, the process of bioaccumulation is only measured by measuring the presence of the contaminant (chemical) compounds and the impact of the actor in terms of abundance or biomass compared to reference non-contaminated systems.
[1] Mann RM, et al. 2011. Metals and metalloids in terrestrial systems: Bioaccumulation, biomagnification and subsequent adverse effects in Ecological impacts of toxic chemicals. Bentham Science Publishers: 43-62.
[2] Faber J et al. 2019. Priorities and opportunities in the application of the ecosystem services concept in risk assessment for chemicals in the environment. Science of the Total Environment 651:1067-1077.
[3] Van Gestel et al. 2020. Soil Biodiversity: State‐of‐the‐Art and Possible Implementation in Chemical Risk Assessment. Integrated Environmental Assessment and Management. Volume 17, Number 3: 541–551.