Date of Award

2006

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Stephen A. Norton

Second Committee Member

Ivan J. Fernandez

Third Committee Member

Aria Amirbahman

Abstract

To evaluate the role of stream water and substrates in response to acidification, we experimentally acidified five first-order streams in 2005: East Bear Brook, Hadlock Brook, and Mud Pond Inlet (Maine, USA); Fernow WS3 (West Virginia, USA); and Lesní Potok (Czech Republic). All have forested catchments and low alkalinity water. We evaluated water samples from a reference site above the point of hydrochloric acid addition and from two or three sites located 16 to 94 m downstream. Just before acid addition we collected streambed sediment samples for sequential extraction of metals. Several sediment-water and aqueous processes contributed to neutralization of acid in the streams. Protonation of bicarbonate contributed significantly to neutralization in the relatively high pH Hadlock Brook. Weak organic acids neutralized acid by protonation, most significantly in the streams with relatively high dissolved organic carbon, Mud Pond Inlet and Lesní Potok. Adsorption of sulfate contributed to neutralization in East Bear Brook, Fernow WS3, and Lesní Potok. Neutralization from ion exchange of base cations and aluminum (Al) for protons (H+) and possible dissolution of Al solid phases were the primary neutralization mechanisms in Fernow WS3, East Bear Brook, and Lesní Potok. In all streams, exchangeable calcium (Ca) and magnesium (Mg) were mobilized, with Ca>Mg, followed by mobilization of Al. In Mud Pond Inlet and Hadlock Brook, which had low Al and continued pH depression downstream, Al accumulated in the water column. At the other three streams, where Al was higher and pH increased more dramatically downstream, Al was lost from solution. However, Al was not saturated and did not precipitate, so the declines in Al concentration were likely caused by resorption to streambed sediments as pH increased. Hysteresis in the relative importance of different cations during neutralization and recovery was clear, particularly in East Bear Brook and Lesní Potok. During initial stages of acidification, Ca desorbed preferentially, whereas Al mobilization dominated during later stages. Early in the recovery, adsorption of Ca to the streambed sediments was kinetically favored over adsorption of Al. Trace elements were mobilized during acidification, likely by ion exchange. Trace elements may also be complexed with solid phases of Al, and then be liberated by dissolution, as in Hadlock Brook where dissolved Al and P correlated. P mobilization due to acidification was minimal, indicating that lake productivity is more influenced by influxes of particulate P during high discharge events than by geochemical mobilization. During experimental acidification, the Al:Ca ratio of a stream’s response may indicate the acidification status of the catchment. Accordingly, Fernow WS3 (low Al:Ca ratio) is in a relatively early stage of acidification, despite 17 years of experimental catchment acidification. East Bear Brook, Hadlock Brook, and Lesní Potok catchments are at early to intermediate stages of acidification. The Mud Pond Inlet catchment (high Al:Ca ratio) is in a later stage of acidification. Short-term stream acidification experiments illuminate processes characteristic of episodic stream acidification and of long-term catchment acidification.

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