Putting the sodium dominance index to the test as a measure of acid sensitivity across diverse geological conditions and with reference to the influence of plantation forests

Traditionally, alkalinity has been widely used in order to determine the level of acid sensitivity, while pH values were often viewed as an indicator from sensitivity to impact. These methods of assigning acidity classifications are problematic, due to large temporal changes in flow conditions and various geological types, which influence the results. The Sodium Dominance Index (SDI) is essentially a ratio of cations, namely, the concentration of Na⁺, divided by the sum of the concentrations of Na⁺, Ca²⁺, and Mg²⁺ expressed as a percentage. Values of >60 SDI denoted sites 'at high risk' of acidification and were therefore classified acid sensitive. From a total of 248 sites (65 of which were forested) in Ireland, base flow sample results showed significant correlations between SDI, alkalinity and pH levels. Significant relationships for SDI to differing underlying geologies were also noted. The data suggested no effect of coniferous forest on the SDI results. It was found that SDI did not differ significantly between base and elevated flow conditions. It was concluded that a dilution effect of calcium (partially balanced by inputs of sodium and magnesium during storm events) along with a depletion of carbonate sources from soils resulted in small fluctuations in the Index between flow conditions.