Monsoonal-type climate or land-use management: Understanding their role in the mobilization of nitrate and DOC in a mountainous catchment Svenja Bartsch a , Stefan Peiffer a , Christopher L. Shope a,f , Sebastian Arnhold b , Jong-Jin Jeong c , Ji-Hyung Park d , Jaesung Eum e , Bomchul Kim e , Jan H. Fleckenstein a,g,⇑ a Department of Hydrology, Bayreuth Center of Ecology and Environmental Sciences – BayCEER, University of Bayreuth, Bayreuth, Germany b Department of Soil Physics, Bayreuth Center of Ecology and Environmental Sciences – BayCEER, University of Bayreuth, Bayreuth, Germany c Department of Forest Environment Protection, College of Forest and Environmental Sciences, Kangwon National University, Chuncheon, Republic of Korea d Department of Environmental Science and Engineering, Ewha Womans University, Seoul, Republic of Korea e Department of Environmental Science, Kangwon National University, Chuncheon, Republic of Korea f US Geological Survey, Utah Water Science Center, Salt Lake City, UT, USA g Department of Hydrogeology, Helmholtz Centre for Environmental Research – UFZ, Leipzig, Germany article info Article history: Received 12 August 2013 Received in revised form 7 October 2013 Accepted 10 October 2013 Available online 18 October 2013 This manuscript was handled by Geoff Syme, Editor-in-Chief Keywords: Nitrate Dissolved organic carbon Monsoonal-type climate Land-use type River-aquifer exchange dynamics Topography summary The linkage between hydrologic dynamics and the delivery of nitrate and DOC (dissolved organic carbon) to streams was studied in the Haean catchment, a mixed land-use mountainous catchment in South Korea. Three monsoonal precipitation events were analyzed, which varied in total rainfall amount (39– 70 mm) and intensities (mean: 1.6–5.6 mm h À1 ), by high-resolution (2–4 h interval) stream water-qual- ity sampling along the topographic elevation gradient of the catchment, from an upland deciduous forest stream, over areas intensively used for agriculture (dryland farming and rice paddies) down to the catch- ment outlet. The dynamics of river-aquifer exchange were investigated at two piezometer transects at mid and lower elevations. DOC and nitrate sources and their transport pathways to the receiving surface waters differed between the forested and the agricultural stream site. In the forest stream, elevated DOC concentrations (max: 3.5 mgC l À1 ) during precipitation events were due to hydrologic flushing of soluble organic matter in upper soil horizons, with a strong dependency on pre-storm wetness conditions. Nitrate contributions to the forested stream occurred along shallow subsurface transport pathways. At the agricultural sites stream DOC concentrations were considerably higher (max: 23.5 mgC l À1 ) supplied from adjacent rice paddies. The highest in-stream nitrate concentrations (max: 4.1 mgN l À1 ) occurred at river reaches located in the lower agricultural part of the catchment, affected by groundwater inputs. Groundwater nitrate concentrations were high (max: 7.4 mgN l À1 ) owing to chemical fertilizer leaching from dryland fields forced by monsoonal rainfalls. Overall, this study demonstrates that the hydrologic dynamics resulting from the monsoonal climate drive the in-stream DOC dynamics in the forested 1st-order catchment whereas sources and mobilization of DOC in downstream agricultural areas are mainly controlled by the prevailing land-use type and irri- gation management. Nitrate dynamics in higher order agricultural streams and their connected aquifers reflect combined effects of land-use type and monsoonal hydrology. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction In recent years, there has been considerable interest in under- standing the processes controlling the delivery of nitrate and dis- solved organic carbon (DOC) to surface waters as well as identifying the sources for these substances. Knowledge of DOC and nitrate dynamics in surface waters is imperative since both DOC and nitrate, are critical components of the global nitrogen and carbon cycles (Wagner et al., 2008). Elevated nitrate concen- tration in rivers and streams can cause phytoplankton growth and algal blooms, which in turn reduces the dissolved oxygen con- tent of surface waters and can lead to eutrophication (Royer et al., 2006; Howarth, 2008). Dissolved organic carbon facilitates the transport of a variety of elements, ranging from nutrients to toxics such as heavy metals and pesticides (i.e.: Åkerblom et al., 2008; Bolan et al., 2011). Elevated DOC concentrations in surface waters are often a concern because it complicates water treatment, which increases drinking water supply costs. 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.10.012 ⇑ Corresponding author. Address: Department of Hydrogeology, Helmholtz Centre for Environmental Research – UFZ, Permoserstr. 15, 04318 Leipzig, Germany. Tel.: +49 (0)341 235 1207. E-mail address: jan.fleckenstein@ufz.de (J.H. Fleckenstein). Journal of Hydrology 507 (2013) 149–162 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol