The effect of time on bioavailable processes cannot be neglected. Time influences bioavailability processes in several ways. Aging of contaminants affects their bioavail-ability, often decreasing frequency or amount of contaminant available for processes 2, 3, and 4 in Figure 3. Contaminant aging in solid phase environments has been shown to be an important aspect affecting bio-availability. Both organic and metal contaminants typically become less bioavailable with the aging process, as they diffuse or sorb into/onto mineral lattices and organic matrices in soils and sediments. The longer the contaminant is in contact with a sorbent, such as organic matter, the greater is the extent to which these processes occur. However, at present there is little ability to predict the changes in bioavailability with any specific contaminant at any specific site over time.
In general, the longer a contaminant is in the environment the more it is subject to transformations affecting bioavailability. Both metal and organic contaminants may be microbially degraded to different products which may be more or less bioavailable; the microbial degradation process may also be time dependent. For example, micro-bial degradation often transforms inorganic or elemental mercury to methylmercury, a much more bioavailable form of mercury resulting in increased food web transfer and bioaccumulation.
Exposure time is also important when considering organism species in their natural habitats versus organism species in biological tests. Time can be a varying factor in exposure frequency for different organisms. Potential bioavailability and actual bioavailability have been proposed to address this issue of time with regard to an organism's actual exposure time versus potential exposure concentration over a longer period oftime. However, there are considerable ambiguities with the approach, and generalization at the organism species level still affords important predictive bioavailable information. In addition to exposure time, other effects of time on organisms include seasonal habitat changes and life span.
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