The use of atmospheric sea salt deposition effects in the quantification of weathering rates for United Kingdom upland catchments
A method of quantifying the susceptibility of upland catchments to acidification, utilizing the influence of atmospheric sea-salt deposition in maritime dominated climates, is evaluated. Inputs of marine-derived Sodium (Na) provide a scale against which to measure inputs of alternative base cations, principally Ca and Mg, which are released through weathering reactions. Hence, the use of a Na base cation dominance index, as a means of quantifying catchment weathering rates, is proposed. Mineral weathering is important as it provides the only long-term sink for acid inputs including those of anthropogenic pollution. The Critical Loads approach provides a scientific tool with which to address appropriate levels of pollution control legislation by deriving acceptable levels of pollution below which sensitive components of various ecosystems will not be damaged. Critical loads for soils and freshwaters currently rely on methods to determine crucial base weathering rates and associated alkalinity generation. Existing methods are complicated and data intensive and have attracted a level of criticism from scientists in this field. The calibration of the index for streamwaters against both catchment weathering rates and alkalinity production is described across a major river system in Northeast Scotland. Clear differences are demonstrated between relative base cation proportions in streamwaters, soil solutions and on soil exchange sites. Atmospheric base cation inputs are generally shown to exceed those from internal weathering sources. The %Na dominance approach is proposed as being a cost-effective, field-based determination method capable of achieving a greater degree of reliability in the determination of weathering rates. This research has shown that the index has the benefit of providing a unifying approach between catchment soils and surface waters, which is currently lacking in critical loads methodology. The index has potential to describe both spatial and temporal variations in catchment weathering at a range of scales.