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Letter
Non-Point Source Contributions Drive Elevated Major Ion And Dissolved Inorganic Carbon Concentrations In Urban Watersheds Joel Moore, Darcy L. Bird, Seth K. Dobbis, and Gregory Woodward Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.7b00096 • Publication Date (Web): 21 Apr 2017 Downloaded from http://pubs.acs.org on April 29, 2017
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Environmental Science & Technology Letters
Non-Point Source Contributions Drive Elevated Major Ion And Dissolved Inorganic Carbon Concentrations In Urban Watersheds
Joel Moore*a,b, Darcy L. Birdb, Seth K. Dobbisa, Gregory Woodwardb a
Department of Physics, Astronomy, and Geosciences, 8000 York Road, Towson University, Towson, MD 21252 USA
b
Environmental Science and Studies Program, 8000 York Road, Towson University, Towson, MD 21252 USA
* Corresponding author:
[email protected] Tel: 1-410-704-4245 Fax: 1-410-704-3511
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Abstract
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The rapid expansion of urban land cover is associated with negative impacts on stream
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ecosystems. Elevated specific conductance (SC) and major ion concentrations are increasingly
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documented in urban streams. However, the degree to which non-point sources contribute to
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elevated ion concentrations is unclear. We characterized SC and major ion concentrations in five
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small watersheds along a forested-to-urban gradient with impervious surface cover (ISC) ranging
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from 0–25%, no major point sources such as wastewater treatment plants, and similar bedrock
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chemistry. Ion concentrations increase by an average of 27x along the forested-to-urban gradient,
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including an increase of 10x in dissolved inorganic carbon (DIC). Inputs from road salt caused
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large increases in Na+ and Cl– concentrations as well as high seasonal variability. Significant
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increases in Ca2+, Mg2+, DIC, and SO42– concentrations in watersheds with high ISC provide the
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best evidence to date that concrete is a substantial non-point source contributor to urban stream
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chemistry. Elevated Ca2+, Mg2+, DIC, and SO42– concentrations in urban watersheds due to non-
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point source contributions have wide ranging implications, including potential reduction of Cl–
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toxicity, changes in metal speciation and toxicity, a shift to waters saturated with respect to
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calcium carbonate, and altered carbon and sulfur cycling.
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Introduction
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Urban areas and associated impervious surface cover (ISC) are among the fastest growing land
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cover types.1 The negative effects of urban land cover on stream ecosystems are frequently
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called urban stream syndrome with the common symptoms of highly variable discharge, altered
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geomorphology, increased nutrient loads, and reduced biological “richness”.2,3 While not
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explicitly part of urban stream syndrome, specific conductance (SC) in urban streams is often
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high,4-7 and an increasing number of studies report elevated major ion concentrations.5-11 As a
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result, ecologists are beginning to hypothesize about the implications of high ion concentrations
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for the adaptability and health of urban stream ecosystems.12,13 However, the degree to which
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non-point source inputs such as road salt and weathering of concrete and other anthropogenic
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materials versus point source inputs contribute to elevated ion concentrations in urban streams is
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poorly understood.
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Use of NaCl as a de-icing salt has been increasingly documented as a non-point source
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contributor to elevated Cl– and Na+ concentrations and SC. Road salt use has increased 40%
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faster than urban land cover from 1990–2011 in the northern US14 and corresponds with
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elevated, and increasing, Cl– and Na+ concentrations in streams with a wide range of ISCs.14-25
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Elevated Cl– has known negative implications for stream ecosystems, including reduced viability
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or altered community structure for daphnia, amphibians, benthic macroinvertebrates, and fish
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based on field studies,26-28 laboratory experiments,29-33 or both.34 Despite the essential role that
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other major ion concentrations play in controlling the abundance and composition of freshwater
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organisms,35 they have been much less studied than Cl–in urban streams. For example, Cl– is less
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toxic to daphnia as Ca2+ concentrations increase and the proportional differences in concentration
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affect major ion toxicity.36,37
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Previous studies of the factors contributing to elevated ion concentrations in urban streams
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have been unable distinguish between anthropogenic and geological contributions to stream
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chemistry9-11 because of watersheds underlain by multiple types of bedrock with substantially
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differing chemistry (particularly carbonate minerals), the lack of a forested reference watershed,
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or both. Additionally, determining the main contributors for individual ions in urban streams has
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been challenging because of unknown contributions from large point sources (e.g., wastewater
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treatment plants [WWTP] or industrial facilities) and the unknown chemistry of non-point
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sources (e.g., impervious surface weathering and degradation, road salt addition, irrigation
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water).9-11,38 For example, concrete has been suggested as a major non-point source for Ca2+ and
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alkalinity (or dissolved inorganic carbon, DIC).9-11,39 However, the magnitude of concrete
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contributions remains unclear because Ca2+ and alkalinity also result from weathering of
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carbonate bedrock and alkalinity can be contributed by WWTPs. Thus, limited information is
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available about the controls on major ion concentrations as urban land cover increases.
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To address this gap, we investigated the role that non-point source contributions play in
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determining major ion chemistry in five watersheds along a forested-to-urban gradient. The
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watersheds contain no major point sources and have similar bedrock geology. Non-point source
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contributions in urban watersheds cause substantial increases in major ion concentrations even at
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1% ISC.
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Materials and Methods
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Site Description. Samples were collected from 5 small watersheds in the Maryland Piedmont
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along a forested-to-urban gradient (Figure S1) with ISC ranging from 0–25%40 (Table S1, see
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Supporting Information for more detail). All watersheds are underlain by felsic, silica-rich
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metamorphic bedrock.41-45 The study watersheds contain neither carbonate bedrock or veins41-43
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nor any large point sources such as WWTPs.46
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Sample Collection and Analysis. Sampling occurred at or near base flow conditions with 22
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samples collected from each stream from 2014–2016. Conductivity, pH, and temperature data
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were collected in the field with a Thermo Orion A325 meter calibrated before each round of
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sampling. Before sampling for dissolved organic carbon (DOC) analysis, amber glass vials were
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heated at 500°C for >4 hours to combust any organic carbon. Water samples were field-filtered
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(0.45 µm). After triple-rinsing with sample water, samples for alkalinity and for cation and anion
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analysis were collected in LDPE bottles. Alkalinity and DOC samples were collected with no
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headspace. Samples were stored at 4°C. Alkalinity was determined via the Gran titration method;
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uncertainty was 10% from expected, samples were reanalyzed. Concentrations of
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NH4+ and PO43– were below 0.3–0.4 mg/L, the approximate detection limit, for all streams.
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Statistical Analyses. Statistical analyses were conducted with R.47 Concentrations showed non-
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normal distributions, and thus a non-parametric statistical approach was used with Mann-
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Whitney-Wilcoxon tests used to determine if concentration differences between watersheds or
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seasons were statistically significant. Winter samples were defined as those collected from
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January through March, representing the months with 83% of average annual snowfall for 1981–
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2010; in 2014–2016, December snowfall was trace–0.2 inches.48 Calcite saturation indices were
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calculated with PHREEQC.49
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Results and Discussion
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Median Stream Chemistry. The forested stream (watershed #1) has the lowest pH and lowest
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median ion concentrations and SC (Figures 1 and 2). Mean pH and concentration values for
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watershed #1 are within 1 standard deviation or 1 mg/L of values measured at the same location
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in the mid-1960s.50 Similar concentrations indicate that no major regional-scale changes in
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stream chemistry have occurred over the last 50 years.
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Suburban and urban streams (watersheds #2–5) have higher median pHs than watershed #1
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(Figure 2A) with these and all concentrations described hereafter as different demonstrating
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statistical significance at p
