The role of sea-salts in enhancing and mitigating surface water acidity

Enhancement of the acidity of fresh waters due to sea-salt “episodes” has been reported in western regions of Scotland, Ireland and Norway and eastern regions of Canada and the United States of America. In all cases these short-term pH depressions have been ascribed to cation-exchange processes in catchment soils whereby sodium ions displace acidic cations (H⁺, Al³⁺) resulting in a lowering of the Na/Cl ratio in run-off water. Studies of sea-salt episodes in different catchment types of similar sensitivity in Scotland show significant variations in chemical responses, especially in terms of acidification status. Catchments with high background salt content, in a low sulphur deposition area, show only a small increase in acidity with negligible aluminium release and most of the sodium retention matched by Ca and Mg release. Catchments in intermediate deposition areas, with low background salt levels, exhibit smaller sodium retention with increased leaching of acidity and labile aluminium. High non-marine S deposition sites, with intermediate background salt levels, reveal high levels of acidity and aluminium leaching associated with some calcium leaching and evidence of Mg retention. Mitigation of acidification occurs at sites with high background sea-salt levels (eg north west Scotland) where acidic deposition is selectively retained in catchment soils. Non-marine sulphate values in run-off are therefore much lower (often producing negative values) than those predicted from current S deposition values. Consequently such sites are presently producing false exceedances of freshwater Critical Loads when current S deposition values are used. Future reductions in S deposition will probably affect the adsorption characteristics at these sites with consequent effects on sulphate leaching.