Date of Award

May 2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Charles T. Driscoll

Keywords

dissolved organic carbon, hydrologic flowpaths, nitrate, soil water, watershed, winter climate change

Subject Categories

Engineering

Abstract

This dissertation is a two-phase study of the hydrochemical dynamics of drainage waters at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA, that aims to improve understanding of changes in water quality associated with winter climate variation. The first phase was an analysis of the long-term stream and soil water chemistry dataset from Watershed 6, the biogeochemical reference watershed of the HBEF. The second phase was a series of field measurements designed to evaluate variation in the chemistry and hydrology of stream and soil water across a natural gradient of winter climate at the HBEF.

Thirty years (1982-2011) of stream and soil water chemistry data were analyzed to assess the trends of overall recovery from acidification, as well as a trends associated with the snowmelt periods of the record, which are characterized by seasonal and episodic acidification of stream runoff. Trends varied by landscape position, but the analysis generally revealed slow increases in the pH and acid neutralizing capacity (ANC) in stream water that were associated with decreases in atmospheric deposition of acid anions, sulfate (SO42-) and nitrate (NO3-). Trends during snowmelt were similar to the whole-year record, including ANC recovery. Nitrate concentrations in streamwater during snowmelt decreased more rapidly than the whole-year record. Dissolved organic carbon (DOC) concentrations have declined significantly in most forest floor soil waters, apparently driving a small overall decrease in streamwater DOC at the base of the watershed. The DOC concentration decreases in streamwater occurred primarily in the first 15 years of the record.

Soil water chemistry was monitored for two years (2011 and 2012) at a series of 20 plots across the Hubbard Brook valley located to capture variability in winter climate. Variables such as maximum soil frost depth and winter soil temperature variability were positively correlated with increased leaching of DOC, but not NO3-, during the early growing season (May-July). The DOC mobilization was primarily observed in the soil waters draining the forest floor (Oa horizon), and less in the mineral soil (Bs horizon). No effect of winter soil conditions was noted during the late growing season.

Daily streamwater sampling during snowmelt was conducted in two south-facing catchments (Watershed 3 and Watershed 6) for three years (2010-2012), and in one north-facing catchment (Watershed 7) for two years (2011-2012). Streamwater concentrations of NO3- and DOC varied among the watersheds and among the years. Nitrate was flushed in high concentration early in snowmelt, prior to dilution. Nitrate was exported in highest concentrations from Watershed 7 during each year, presumably the result of higher microbial nitrogen mineralization and nitrification rates. The highest NO3- concentrations in each watershed occurred during snowmelt of 2012, following a winter with low snowpack and above average temperatures. DOC concentrations were largely determined by changes in hydrologic flow, increasing during snowmelt events. The DOC concentration varied among the watersheds and was highly correlated to the winter climate variables for each of the watersheds.

End-member mixing analysis (EMMA) revealed differences in hydrologic flowpaths related to the presence of soil frost. Flow through preferential flowpaths in the forest floor was reduced during days with extensive soil frost. Direct contribution of snow or precipitation water to stream flow water was not markedly increased during times when the soils were frozen, indicating that the soil frost was likely granular and soils retained permeability.

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