Relations between acid atmospheric deposition and the surface pH of some ombrotrophic bogs in Britain

Analyses are presented of major ion concentrations in surface water samples, and exchangeable cations in peats, collected in November-December 1992 from 16 ombrotrophic bogs in England and Wales and one in Scotland, spanning most of the range from the lowest (Dartmoor, west Wales, west Highlands) to the highest (south Pennines, north Yorkshire) of the mean 1986-88 rainwater concentrations of hydrogen ions and non-marine sulphate(NMS). There was a strong positive correlation (r = +0.89) between surface water pH and mean rainwater pH, and a less close correlation (r = +0.71) between surface water pH and total hydrogen ion deposition. The correlation between surface water pH and mean concentration of rainwater NMS (which may be taken as a surrogate measure of acid deposition) was r = -0.85, and that between surface water pH and total deposition of NMS was r = -0.65. The intercept of the regression of surface water pH on NMS is at pH 4.40 (95% confidence limits 4.22-4.58), suggesting this as the winter pH of an ombrotrophic bog surface in the absence of acid atmospheric pollution. Total sulphur content of intact surface peats from 11 sites correlated well (r = +0.90) with mean rainwater total sulphate concentration. The H(+) ion concentration in wet deposition, plus the net H(+) input resulting from metabolism of deposited NO3(-) and NH4(+), is insufficient to account for the difference between the inferred pH of an unpolluted bog surface and pH values (up to 0.8 pH units lower) measured in the field in polluted sites. Analogy with deposition of NMS suggests that this discrepancy is accounted for quantitatively by seeder-feeder enhancement, cloudwater deposition, and dry deposition of acidity. Ombrotrophic bog surface waters are buffered by the cation-exchange properties of the peat. They are likely to be less sensitive to moderate acid deposition than weakly bicarbonate-buffered near-neutral peats, soils and fresh waters.