1 Spatial patterns and temporal trends in the water quality of the Tuul River in Mongolia Ochir Altansukh (Corresponding author) School of Earth Sciences, National University of Mongolia Ulaanbaatar 210646, Mongolia E-mail: altansukh@num.edu.mn Paul Whitehead, John Bromley School of Geography and the Environment, University of Oxford, Oxford, OX1 3QY, UK The research is financed by Open Society Foundation (OSF). Abstract The purpose of this research is to assess spatio-temporal variability of water quality determinands of the Tuul River in surrounding area of Ulaanbaatar city, Mongolia using an extensive dataset between 1998 and 2008. It presents the spatio-temporal assessment and seasonal pattern of 14 hydro-chemical determinants at 15 monitoring sites in the study area. According to the Mongolian water quality classification system, all sections of the Tuul River and its tributaries in the surrounding area of Ulaanbaatar city belong to moderately and heavily polluted waters due to high concentration of ammonium. In accordance with European Union water quality standard, the downstream section of the Tuul River fails. In order to change this situation, operation enhancement of wastewater treatment plants and artificial increment of dissolved oxygen concentration become crucial to improve the water quality significantly. Perhaps a new wastewater treatment plant is needed for Ulaanbaatar city. Keywords: Tuul River, water quality, pollution source, dissolved oxygen, water quality map, Mongolia 1. Introduction Unpolluted waters in rivers are a vital natural resource, providing drinking and irrigation water for humans, livestock and agriculture. However, water quality in many large river waters has deteriorated significantly worldwide due to anthropogenic activities in the past two-three decades (Ferrier, et al., 2001). It is also widely accepted that discharges from sewage treatment plants provide major fluxes of P and N to rivers, predominantly in populated urban areas (Jarvie, Neal, & Withers, 2006; Neal, et al., 2005). Nutrient enrichment can result in excessive growth of aquatic plants and reductions in dissolved oxygen (Neal, et al., 2002; Whitehead, Johnes, & Butterfield, 2002). Rising pollution levels and the increasing demand for water and the associated increased discharges of pollutants are having significant impacts on the water cycle and water quality (Whitehead, Wilby, Battarbee, Kerman, & Wade, 2009; Whitehead, Wilby, Butterfield, & Wade, 2006). Climate change is also starting to have some effects with increasing temperatures and changed rainfall patterns. The increasing air temperatures and decreasing river flows in warmer months are the main concerns, and intensive water use is often constrained by the lack of natural low flow, and low flow rivers are more affected by effluent discharges from cities, industries, and agriculture (Johnes, 2007; Mainstone & Parr, 2002). Surface waters in Mongolia have tended to decrease in recent years due to the combined effect caused by the decrease of precipitation and the increase of potential evaporation as a result of rising air temperature. This situation indicates that droughts may occur more frequently due to the effects of global warming (Sato, Kimura, & Kitoh, 2007). Over the last decade, rapid urbanization and increased industry have had significant impacts on the water quality and chemical composition of rivers in the surrounding area of Ulaanbaatar city (Javzan, Sauleguli, & Tsengelmaa, 2004). Air and soil pollution as well as accumulated wastes in the catchment area, are being transferred by surface runoff and flood events into the local river systems and having a significant impact on the river water quality. Major causes of the water pollutants are mining industries in the lower basin of the Tuul River. More than 180 licensed mining companies are operating in 145 km 2 areas of the basin (MNE, 2006). Water demand of the city had increased by 20% from 1998 to 2005. Population growth, urbanization and