RAIN project: Role of organic acids in moderating ph change following reduction in acid deposition

Abstract

The RAIN project (Reversing Acidification In Norway) entails catchment-scale experimental manipulations to investigate the effect on water and soil chemistry of drastic changes in precipitation chemistry. At Risdalsheia in southernmost Norway wet deposition of acid is excluded from a 860-m² headwater catchment by means of a roof and “clean” precipitation is added beneath. Four years of acid exclusion (through June 1988) have resulted in lower concentrations of the strong acid anions NO₃ (from 35 to 7 ueq L^-1) and SO₄ (from 110 to 53 ueq L^-1) in runoff. The decline in strong acid anion concentrations has been compensated partially by a decrease in concentrations of base cations (55%) and partially by an increase in alkalinity (45%). pH has increased only slightly from 4.0 to 4.1. Organic acids have become increasingly important for the pH of runoff. Runoff from the shallow organic soils contains 10 to 20 mg C L^-1 total organic carbon (TOC). The concentration of organic anions (estimated from the ionic balance) has increased from about 22 ueq L^-1 in 1984 to 49 ueq L^-1 in 1987. This increase is due to increased dissociation of organic acids and not to change in TOC concentrations. The organic C in these acid samples apparently has a maximum charge density of about 4.5 ueq mg C^-1 and pK of about 4.