A novel index of susceptibility of rivers and their catchments to acidification in regions subject to a maritime influence

A simple, unifying approach to classifying quantitatively the susceptibilit y of catchment soils and surface waters to acidification is suggested. In a reas subject to a strong maritime influence, such as the UK and substantial parts of NW Europe, wherever soil mineral weathering rates are low and soi ls are unfertilised, atmospherically derived base cations of maritime origi ns have a greater effect than those derived from biogeochemical weathering on the exchangeable soil base cations. This is directly reflected in the re lative base cation concentrations of the associated drainage waters, which become increasingly Na-dominated. Using 10 sub-catchments of the River Dee in northeastern Scotland, it is shown here that the extent of Na dominance, the ratio of Na+ to Sigma Na+ +Ca2+ + Mg2+, at any point in a river provid es a quantitative index of the upstream weathering rate and thus of the sus ceptibility of the river concerned to acidification under diverse flow cond itions. Data from a further 58 sub-catchments from the same river system, a nd from 4 other catchments from around Scotland, were used to validate this theory. (C) 1999 Elsevier Science Ltd. All rights reserved.