Stormwater and organic matter in the urban stream continuum

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2017

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Abstract

Dissolved organic matter (DOM) is present in all natural waters and modulates

aquatic ecosystems by absorbing light and heat, and because it comprises a complex

mixture of organic molecules including amino acids, sugars, fulvic acids, and humic

material. DOM is derived from dissolution of organic matter and can be altered by

both biotic and abiotic processes that may its structure or mineralize it to CO2.

Urbanization is a widespread agent of landscape change that can alter DOM

regimes by changing the amount and types of organic matter in the catchment and

by changing the way that water moves through the landscape (transporting DOM

from land to stream). This dissertation examines DOM in urban stream networks,

exploring its sources, bioavailability, and broad patterns throughout the continental

United States.

We determined the role of impervious infrastructure as a proximate source of

DOM to stormwater by a) constructing an annual carbon budget for the roof of a

house as a small catchment nested within the 60 h catchment of an urban headwater

stream and b) comparing the estimated fluxes of solutes and stormwater from imper

vious infrastructure in the catchment. We found that roofs convert nearly one-third

of the leaf litter carbon they receive into dissolved organic carbon (DOC), which

leaves through downspouts. On the event scale, we estimated fluxes of DOC and

total dissolved nitrogen from impervious surfaces that generally exceed the fluxes

that leave the catchment in stream stormflow.

When we compared the chemical composition of runoff from impervious surfaces

to stream stormflow, we found them to be distinct, despite the fact that the we

estimated a volume of runoff from impervious surfaces that generally matched the

volume of water flowing through the stream during storms. Our findings suggest

that a water source other than baseflow and impervious runoff contributes to stream

stormflow, and that a considerable proportion of impervious runoff is lost before it

reaches the catchment pour point.

An experimental incubation of potential DOM sources in the urban landscape and

DOM in stormwater showed that urban DOM is highly bioavailable. The composition

of DOM also became more homogeneous over the course of processing.

Finally, we examined continental-scale patterns and long-term trends in riverine

DOC. Unlike the widespread ’browning’ trends observed in far northern aquatic

systems, we did not find evidence for long-term increases in DOC throughout most

of the U.S. Instead, we both decreases and increases in long-term DOC concentrations

that differed among regions and generally seemed to be driven by changes in weltand

cover. We also found evidence for a marginal effect of impervious surfaces that

increases DOC concentrations at high canopy cover, consistent with our observations

that urban infrastructure can contribute considerable DOM loads in storm runoff.

Together, this research shows that urban stormwater infrastructure functions

as the ephemeral headwaters of the urban stream network. In catchments with

significant canopy cover, these ’engineered headwaters’ collect and transform organic

matter between storms and transport DOM during stormflow.

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Fork, Megan L. (2017). Stormwater and organic matter in the urban stream continuum. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/16399.

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