COMPARISON OF EPISODIC ACIDIFICATION OF MID-ATLANTIC UPLAND AND COASTAL-PLAIN STREAMS

Episodic acidification was examined in five mid-Atlantic watersheds re presenting three physiographic provinces: Coastal Plain, Valley and Ri dge, and Blue Ridge. Each of the watersheds receives a similar loading of atmospheric pollutants (SO42- and NO3-) and is underlain by differ ent bedrock type. The purpose of this research was to quantify and com pare the episodic variability in storm flow chemistry in Reedy Creek, Virginia (Coastal Plain), Mill Run and Shelter Run, Virginia (Valley a nd Ridge), and Fishing Creek Tributary and Hunting Creek, Maryland (Bl ue Ridge). Because episodic responses were similar from storm to storm in each of the watersheds, a representative storm from each watershed was discussed. Acidification, defined as the loss of acid-neutralizin g capacity (ANC), was observed in all streams except Mill Run. Mill Ru n chemistry showed little episodic variability. During storms in the o ther streams, pH decreased while SO42-, NO3-, and K+ concentrations in creased. Concentrations of Mg2+ and Ca2+ increased in Reedy Creek and Fishing Creek Tributary, but decreased in Shelter Run and Hunting Cree k. Therefore the net effect of episodic changes on the acid-base statu s differed among the streams. In general, greater losses of ANC were o bserved during storms at Shelter Run and Hunting Creek, watersheds und erlain by reactive bedrock (carbonate, metabasalt); comparatively smal ler losses in ANC were observed at Reedy Creek and Fishing Creek Tribu tary, watersheds underlain by quartzites and unconsolidated quartz san ds and cobbles. Increased SO42- concentrations were most important dur ing storms at Reedy Creek and Fishing Creek Tributary, but organic ani ons (inferred by anion deficit) were also a factor in causing the loss of ANC. Dilution of base cations was the most important factor in the loss of ANC at Shelter Run. Both increased sulfate and dilution of ba se flow were important in causing the episodic acidification at Huntin g Creek. The role of SO42- in contributing to episodic acidification i n these watersheds is similar to that documented in studies conducted in other regions of the United States, Scandinavia, Canada, and the Un ited Kingdom. The importance of SO42- in mid-Atlantic United States st reams contrasts with northeastern United States streams, in which incr eased NO3- derived from snowpack is more important in causing episodic acidification. Results support the importance of shallow subsurface p rocesses in determining storm flow chemistry, regional climatic charac teristics in determining the different sources of acidity during episo des, and the importance of bedrock geology in determining the amount o f ANC loss.