Quantifying the effects of stream channels on storm water quality in a semi-arid urban environment Erika L. Gallo a,⇑ , Kathleen A. Lohse b,1 , Paul D. Brooks a , Jennifer C. McIntosh a , Thomas Meixner a , Jean E.T. McLain c,2 a Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721, United States b School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721, United States c USDA-ARS, US Arid-Land Agricultural Research Center, Maricopa, AZ 85138, United States article info Article history: Received 19 September 2011 Received in revised form 29 July 2012 Accepted 17 August 2012 Available online 31 August 2012 This manuscript was handled by Laurent Charlet, Editor-in-Chief, with the assistance of Eddy Y. Zeng, Associate Editor Keywords: Urban Runoff quality Nitrogen Carbon Ephemeral stream Stream channel summary Stormwater drainage systems can have a large effect on urban runoff quality, but it is unclear how ephemeral urban streams alter runoff hydrochemistry. This problem is particularly relevant in semi-arid regions, where urban storm runoff is considered a renewable water resource. Here we address the ques- tion: how do stream channels alter urban runoff hydrochemistry? We collected synoptic stormwater samples during three rainfall–runoff events from nine ephemeral streams reaches (three concrete or metal, three grass, three gravel) in Tucson, Arizona. We identified patterns of temporal and spatial (lon- gitudinal) variability in concentrations of conservative (chloride and isotopes of water) and reactive sol- utes (inorganic-N, soluble reactive phosphorous, sulfate-S, dissolved organic carbon (DOC) and nitrogen, and fecal indicator bacteria). Water isotopes and chloride (Cl) concentrations indicate that solute flushing and evapoconcentration alter temporal patterns in runoff hydrochemistry, but not spatial hydrochemical responses. Solute concentrations and stream channel solute sourcing and retention during runoff were significantly more variable at the grass reaches (CV = 2.3  144%) than at the concrete or metal (CV = 1.6  107%) or gravel reaches (CV = 1.9  60%), which functioned like flow-through systems. Stream channel soil Cl and DOC decreased following a runoff event (Cl: 12.1–7.3 lgg 1 soil; DOC: 87.7– 30.1 lgg 1 soil), while soil fecal indicator bacteria counts increased (55–215 CFU g 1 soil). Finding from this study suggest that the characteristics of the ephemeral stream channel substrate control biogeo- chemical reactions between runoff events, which alter stream channel soil solute stores and the hydro- chemistry of subsequent runoff events. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction In water limited areas such as the semi-arid southwestern Uni- ted States (US), urban runoff is a water resource that, through ac- tive management as focused recharge, can augment limited water supplies and enhance localized renewable groundwater re- sources (Hoffmann et al., 2007). Urbanization in arid and semi-arid regions can negatively impact stream and drinking water quality by increasing loads of nutrients, metals and organic pollutants to surface and ground water (Asaf et al., 2004; Barber et al., 2006; Carlson, 2004; Gallo et al., 2012; e.g. Hamilton et al., 2004; Lewis and Grimm, 2007), yet little is known regarding how the character- istics of the stormwater drainage system, and specifically the char- acteristics of the stream channel substrate, alter urban runoff hydrochemistry. In the semi-arid Southwest, increasing imperviousness has been shown to enhance partitioning of rainfall into runoff, resulting in higher runoff ratios and more frequent and longer runoff duration (Gallo, 2011; Gallo et al., in review; Kennedy, 2007). However, the characteristics of the stormwater drainage system appear to be more important in controlling urban runoff hydrochemistry than catchment imperviousness (Gallo et al., in review; Grimm et al., 2005; Lewis and Grimm, 2007; Meierdiercks et al., 2010). It is likely that in semi-arid regions, increases in runoff frequency and duration promote ephemeral stream channel wetting and expand the spatial and temporal distribution of biogeochemical processes. Urban ephemeral channels may therefore act as biogeochemical 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2012.08.047 ⇑ Corresponding author. Present address: Department of Biological Sciences, Idaho State University, Pocatello, ID 83209-8007, United States. Mobile: +1 520 390 5770; fax: +1 520 621 1422. E-mail addresses: elgallo@email.arizona.edu (E.L. Gallo), klohse@isu.edu (K.A. Lohse), brooks@hwr.arizona.edu (P.D. Brooks), mcintosh@hwr.arizona.edu (J.C. McIntosh), tmeixner@hwr.arizona.edu (T. Meixner), jmclain@cals.arizona.edu, Jean.McLain@ars.usda.gov (J.E.T. McLain). 1 Present address: Department of Biological Sciences, Idaho State University, Pocatello, ID 83209-8007, United States. 2 Present address: Water Resources Research Center, University of Arizona, Tucson, AZ 85719, United States. Journal of Hydrology 470–471 (2012) 98–110 Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol