Desalination 226 (2008) 13–20 Presented at the 10 th IWA International Specialized Conference on Diffuse Pollution and Sustainable Basin Management, Istanbul, Turkey, 18–22 September 2006. 0011-9164/06/$– See front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.0000.00.000 Nitrate pollution of surface water induced by agricultural non-point pollution in the Pocochay watershed, Chile L. Ribbe a *, P. Delgado b , E. Salgado b , W.-A. Flügel c a Cologne University of Applied Sciences, Institute for Technology in the Tropics, Betzdorfer Str. 2, D-50679 Cologne, Germany Tel. +49 221 8275 2125; Fax +49 221 8275 2736; email: lars.ribbe@fh-koeln.de b Faculty of Agronomy, Catholic University of Valparaiso, Quillota, Chile c Department of Geoinformatics, Institute for Geography, Friedrich Schiller University Jena, D-07743 Jena, Germany Received 21 December 2006; revised accepted 30 January 2007 Abstract Diffuse nitrogen pollution caused by irrigated agriculture is quantified for the Pocochay stream located in the Aconcagua watershed in the Valparaíso region, Chile. The study area has an extent of 93 km 2 with 36 km 2 under irrigation. It is representative for the central part of Chile where intensive irrigation is prevailing and predomi- nantly taking place in the alluvial plains of river systems with water sources from the Andes Mountains. Water quality and discharge were monitored biweekly at several points of the Pocochay main stream from December 2003 to June 2006. The study was supported by an agricultural survey and remote imagery interpretation in order to retrieve data on cropping pattern and actual fertilizer uses in the sub-watersheds. Nitrate – N concentration in the Pocochay stream ranged from 1.3 to 10.1 mg L -1 with a mean of 4.8 mg L -1 . Other nitrogen species showed very low concentrations. At average, around 52 kg N ha -1 a -1 are drained from the study area as compared to 167 kg N ha -1 a -1 applied as fertilizer to the fields. Even though nitrate concentrations in the Pocochay do not reach alarming levels yet, the impact of irrigated agriculture on surface water quality is obvious and makes a case for introducing best management practices to avoid aggravation of contamination. Keywords: Nitrate; Irrigated agriculture; Surface water contamination; Monitoring; Chile 1. Introduction Agriculture can have a significant impact on the quality of surface and ground water. Nitrate pollution of surface water is of high concern as it may have negative impacts on water supply and on riverine and coastal ecosystems. The impact of intensive agriculture on nitrate pollution has been reported in several studies and review articles [1–3]. High nitrate concentrations in water serving as a source for drinking water represent *Corresponding author.