EPISODIC ACIDIFICATION OF SMALL STREAMS IN THE NORTHEASTERN UNITED-STATES - IONIC CONTROLS OF EPISODES

As part of the Episodic Response Project (ERP), we intensively monitor ed discharge and stream chemistry of 13 streams located in the Norther n Appalachian region of Pennsylvania and in the Catskill and Adirondac k Mountains of New York from fall 1988 to spring 1990. The ERP clearly documented the occurrence of acidic episodes with minimum episodic pH less than or equal to 5 and inorganic monomeric Al (Al-im) concentrat ions > 150 mu g/L in at least two study streams in each region. Severa l streams consistently experienced episodes with maximum Al-im concent rations > 350 mu g/L. Acid neutralizing capacity (ANC) depressions res ulted from complex interactions of multiple ions. Base cation decrease s often made the most important contributions to ANC depressions durin g episodes. Organic acid pulses were also important contributors to AN C depressions in the Adirondack streams, and to a lesser extent, in th e Catskill and Pennsylvania streams. Nitrate concentrations were low i n the Pennsylvania streams, whereas the Catskill and Adirondack study streams had high NO3- concentrations and large episodic pulses (less t han or equal to 54 mu eq/L). Most of the Pennsylvania study streams al so frequently experienced episodic pulses of SO42- (less than or equal to 78 mu eq/L), whereas the Adirondack and Catskill streams did not. High baseline concentrations of SO42- (all three study areas) and NO3- (Adirondacks and Catskills) reduced episodic minimum ANC, even when t hese ions did not change during episodes. The ion changes that control led the most severe episodes (lowest minimum episodic ANC) differed fr om the ion changes most important to smaller, more frequent episodes. Pulses of NO3- (Catskills and Adirondacks), SO42- (Pennsylvania), or o rganic acids became more important during major episodes. Overall, the behavior of streamwater SO42- and NO3- is an indicator that acidic de position has contributed to the severity of episodes in the study stre ams.