Dr. Dewalle et Br. Swistock, CAUSES OF EPISODIC ACIDIFICATION IN 5 PENNSYLVANIA STREAMS ON THE NORTHERN APPALACHIAN PLATEAU, Water resources research, 30(7), 1994, pp. 1955-1963
Causes of episodic stream acidification and acid-neutralizing capacity
(ANC) declines were investigated in five streams on forested, unglaci
ated catchments on the northern Appalachian Plateau of Pennsylvania, d
uring October 1988 to April 1990. Increased SO4 concentrations during
episodes were the primary cause of ANC declines on three of the stream
s. Episodes on the remaining two streams were dominated by base cation
dilution and/or SO4 increases. Base cation dilution was most importan
t in a stream with initially high preepisode ANC caused by seeps from
limestone outcrops. Bedrock layers on this basin were dipping toward t
he channel, which promoted more rapid movement of low-ANC subsurface s
torm flow water to the channel. A stream with even higher preepisode A
NC, but on a basin with geologic structure that shunted low-ANC subsur
face storm flow away from the channel, exhibited only minor base catio
n dilution. Organic acidity contributed to ANC decreases on all five s
treams, although it was a minor source compared with SO4 and dilution.
NO3 and Cl did not contribute to ANC decreases on any of the study st
reams. Aluminum increases acted to reduce episodic ANC declines especi
ally in the more acidic streams. Episodic acidification on the norther
n Appalachian Plateau in Pennsylvania was similar to that found in mid
-Atlantic streams where SO4 increases play a dominant role, but unlike
glaciated regions in the Northeast where NO3 can be important.