Although this has only been a brief and limited study, it has covered a wide range of elements in a variety of natural and anthropogenically modified anomalous systems. For the most part, element behaviour has followed established norms of theoretical resistate or mobile character, though there are important exceptions. The concept of an ‘insoluble’ element may no longer be entirely justifiable, thanks to improvements in analytical methods, but elements such as Nb (and, to a lesser extent, Zr) must still be regarded as essentially immobile under normal surface conditions. In contrast, chloride (irrespective of isotope) is highly mobile and essentially conservative, i.e. non-interacting. Variables such as Eh, pH, organic matter and counter-ions are the dominant controls on element solubility, complexation and mobility. A few elements, such as Sr, are much more mobile than their published thermodynamic Eh-pH diagrams might suggest, whereas others, such as Rb and Cs, are less so. Copper can be mobile in alkali solution if levels of Cl- and organic matter are high, as in coal waste effluents, whereas it is normally immobilised by near-neutral pH levels in natural anomalies. The mobility of molybdenum is likely to be higher in alkaline waters than in acid ones, and radiogenic technetium will probably behave in a similar manner. Regional data for some elements such as Se, Br and I have only recently become available and further improvements in analytical techniques are awaited to confirm the mobility of these elements at low natural concentration levels in solution.
It is important to note that there are limitations to this report. The main limitation is the use of available data with a sampling regime which was not designed with the study of element dispersion as its main aim. If new studies were undertaken to investigate element dispersion specifically, these would require a specialised sampling system which would aim to follow the dispersion of an element from a well-defined bedrock source into soils, groundwater, and surface drainage, following streams at a suitably high sampling rate and analysing sediments and waters to establish the rate of concentration tail-off, and changes in substrate host and element speciation over an area sufficiently large for background conditions to be re-established at its boundaries. This may be an idealised situation, and may not be easily attainable, but should be a ‘gold standard’ to be aimed for.
The transfer of elements from the geosphere to the biosphere is clearly dependent primrily on their mobility, and this has been examined and assessed carefully in this study. The actual mechanisms of bioavailability and element uptake into biota are highly complex. Many academic studies have compared element concentrations in plants and soil organisms and in their host soils, and some empirical data exists on this topic in BGS, but on a very local, not a regional, scale. However, a thorough examination of these processes would require a much more focused and multidisciplinary project than was possible within this desk-study
