Title

Controls on dissolved organic carbon at Hubbard Brook Experimental Forest and its effects on stream acid-base chemistry

Date of Award

8-2002

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Charles T. Driscoll

Keywords

Dissolved organic carbon, Stream, Acid-base chemistry, Episodic acidification

Subject Categories

Biogeochemistry | Civil and Environmental Engineering

Abstract

In this study, I investigated the factors that control the spatial and temporal variations in stream and soil solution dissolved organic carbon (DOC) concentrations and composition within a north-facing watershed (Watershed 9) and across the forested valley at the Hubbard Brook Experimental Forest, New Hampshire.

Soil solutions displayed horizontal patterns in DOC fractions with hydrophobic acids dominating in the Oa horizon. Hydrophobic acids were also the dominant fraction in stream water DOC. Temporal variations in the proportion of DOC fractions were mainly evident in the Oa soil horizon leachate and stream water. Stream DOC fractions varied spatially, with increases in the concentration of both hydrophobic and hydrophilic acid fractions with increasing elevation, however hydrophobic acids increased more markedly. Various factors were related to spatial variations in stream DOC concentration and composition including variations in forest floor C, topographic index, soil depth, and slope. However, forest floor C concentrations exhibited the strongest relationship. During hydrologic events, hydrophobic acids dominated DOC increases at peak discharge. During both base flow and at peak discharge during events, the ratio of hydrophobic acids to hydrophilic acids suggested that DOC in the stream largely originated from a wetland at the headwaters of Watershed 9.

Organic acids are generally thought to play a minor role in the episodic acidification of streams in the U.S., mainly because most streams studied to date are characterized by low concentrations of DOC. I hypothesized that in streams with high concentrations of DOC, episodic acidification will be controlled by increases in naturally occurring organic acids. During snow melt events, organic acids were a minor contributor to the short-term acidification of stream water, with increases in NO 3 - 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. Also precipitation events occurring after relatively wet antecedent conditions were likely to result in more severe acid episodes than events that followed drier antecedent conditions.

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