Determining Sources of Dissolved Organic Carbon and Nutrients in an Urban Basin Using Novel and Traditional Methods



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Water quality in urban ecosystems is sensitive to localized disturbances potentially affecting those mechanisms which influence nutrient cycles. The Carters Creek Basin has been reported to have elevated concentrations of dissolved organic carbon (DOC). In combination with high terrestrial nutrient export from non-point sources and point source effluent discharge, this has been suggested to contribute to E.Coli recovery and regrowth. Spatial identification of loading ?hot-spots? or locations of elevated nutrient concentrations of non-point, terrestrial sources may provide critical information necessary for appropriate mitigation efforts and watershed management. This study used traditional and novel methods for source tracking nutrients and dissolved organic carbon in small urban and rural watersheds in Brazos County, Texas. A nested watershed approach allowed identification of problem areas of nutrient loading. A novel cost-effective technique using diffuse reflectance near-infrared spectroscopy was used to identify sources of DOC. Monthly stream sampling was conducted at 12 sites from 2012 to 2013.

Impacts of human activity on landscape features determining source pathways for nutrient retention, transport, and conversion were identified in this study. Higher nitrate-N (0.12-22.8 mg L-1), orthophosphate-P (0.11-3.60mgL-1), and DOC concentrations (18.6-68.1 mg L-1) were found across the watershed than in 2007. Factors such as increased erosion, sodic soil dispersion, land use, and flow conditions wereidentified as possible causes for increased carbon (C), nitrogen (N) and phosphorus (P) observed in the basin.

This study supported the use of near-infrared spectroscopy to elucidate watershed sources of carbon. The major sources of DOC into the Carter Creek basin watersheds were leachate from soil and turfgrass. Rural subwatersheds had less complicated source signatures than their urban counterparts. Urban impervious runoff signatures also clustered with stream water signatures, especially during high flow in October and September. These results indicate that specific vegetation such as turfgrass used for landscapes in urban watersheds coupled with sodic irrigation may alter traditional nitrogen, phosphorus and carbon cycling in urbanizing watersheds. Spatial source tracking will enable efficient pollution mitigation and protect water quality as a result of this study.