Organic acids are generally thought to play a minor role in the episodic acidification of streams in the USA. In this study, we investigated the episodic acidification of a stream at the Hubbard Brook Experimental Forest in New Hampshire with high concentrations of dissolved organic carbon and naturally occurring organic acids. We studied three events in 2001: spring snowmelt, which occurred from 6 April to 14 May and resulted in two distinct melt events; and two rain events, one on 17 June and the other on 17 July. During snowmelt events organic acids were a minor contributor to the short‐term acidification of stream water, with increases in NO3− and dilution of base cations being the dominant mechanisms. During summer rainfall events, however, increases in inputs of organic acids were the dominant mechanism of episodic acidification when soil water was the dominant contributor to stream discharge (59 to 66% of peak stream discharge). We also found that precipitation events occurring after relatively wet antecedent conditions (17 July event) resulted in more severe acid episodes than events that followed drier antecedent conditions (17 June event). The minimum acid neutralizing capacity (ANC) was only −19 µeq l−1 for the 17 June event, whereas the minimum ANC for the 17 July event was much lower (−62 µeq l−1) although the total rainfall amount was similar for the two events. Copyright © 2004 John Wiley & Sons, Ltd.

Abstract:Concentrations of both aluminium (Al) and dissolved organic carbon (DOC) in stream waters are likely to be regulated by factors that influence water flowpaths and residence times, and by the nature of the soil horizons through which waters flow. In order to investigate landscape-scale spatial patterns in streamwater Al and DOC, we sampled seven streams draining the Hubbard Brook valley in central New Hampshire. We observed considerable variation in stream chemistry both within and between headwater watersheds. Across the valley, concentrations of total monomeric aluminium (Al m ) ranged from below detection limits (<0Ð7 µmol l 1 ) to 22Ð3 µmol l 1 . In general, concentrations of Al m decreased as pH increased downslope. There was a strong relationship between organic monomeric aluminium (Al o ) and DOC concentrations (R 2 D 0Ð92). We observed the highest Al m concentrations in: (i) a watershed characterized by a steep narrow drainage basin and shallow soils and (ii) a watershed characterized by exceptionally deep forest floor soils and high concentrations of DOC. Forest floor depth and drainage area together explained much of the variation in ln Al m (R 2 D 0Ð79; N D 45) and ln DOC (R 2 D 0Ð87; N D 45). Linear regression models were moderately successful in predicting ln Al m and ln DOC in streams that were not included in model building. However, when back-transformed, predicted DOC concentrations were as much as 72% adrift from observed DOC concentrations and Al m concentrations were up to 51% off. This geographic approach to modelling Al and DOC is useful for general prediction, but for more detailed predictions, process-level biogeochemical models are required.