Communicated by Y. Oikawa Received Mar. 24, 2017 Accepted Mar. 7, 2018 * Corresponding author mia@nodai.ac.jp Effects of Agriculture on the Water Quality of the Cisadane River System in West Java, Indonesia Luqman Arif LUBADA 1 , Kenji YOKOTA 2 , Arief HARTONO 3 , and Rie MIYAURA 4, * 1 Graduate School of Agriculture, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan 2 Faculty of Applied Biosciences, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan 3 Faculty of Agriculture, Bogor Agricultural University, Dramaga, Bogor, West Java 16680, Indonesia 4 Faculty of International Agriculture and Food Studies, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan Abstract This study investigates the impact of agriculture on the water quality of the Cisadane River system in West Java, Indonesia by focusing on one stream flowing from an upland area through an intensive agricultural area before merging with the rest of the river system downstream. In total, 25 water samples were collected from this river system over a straight-line distance of ~70 km and an altitudinal difference of 750 m during the dry season of 2015 and the rainy season of 2016. The samples were analyzed to measure the pH, EC, and concentrations of NO 3 -N, NH 4 -N, and PO 4 -P. The average contamination in the water samples during the dry and rainy seasons was 0.61 and 0.45 mg NO 3 -N L -1 , respectively, 0.56 and 0.28 mg NH 4 -N L -1 , respectively, and 0.64 and 0.55 mg PO 4 -P L -1 , respectively. The results revealed that water was contaminated by inorganic P even in the upper stream. Moreover, the spring water in the agriculture-intensive areas of a rural community, which is believed by the locals to be sufficiently clean for use, was found to more be contaminated by nitrogen than the water in the lower part of the river. As the flow of contaminated water can cause eutrophication in estuaries, it is important to characterize the contamination of water systems to encourage the implementation of sustainable water management strategies. Key words: Eutrophication, Fertilizer, Nitrogen, Phosphorus, Water quality Introduction Asia accounted for ~60% of the world fertilizer consumption in terms of nitrogen (total N nutrients), phosphate (total P 2 O 5 nutrients), and potassium (total K 2 O nutrients) in 2014 (FAO, 2017). In Asia, after China and India, Indonesia stands fourth in the consumption of nitrogen and phosphate and third in the consumption of potassium. The consumption of these fertilizers in Indonesia has drastically increased between 2002 and 2014: nitrogen consumption increased by 47%, phos- phate consumption increased by 201%, and potassium consumption increased by 400%, whereas the increases in the world were 32%, 35%, and 41%, respectively. More than 55% of the total Indonesian population of 260 mil- lion resides in Java, a highly populated island. Intensive farming predominates in small local farm households. Nowadays, it is easy for local farmers to access chemical fertilizers in these rural areas. However, agriculture can impact the water quality by introducing non-point source pollution; this is a major issue facing society today (Carpenter, 1998). Past studies have reported that eutrophication of freshwater streams due to various agriculture activities has caused algal bloom in the mouths of rivers, e.g., in lakes, reservoirs (Chrost et al., 1984; Makarewicz et al., 2007; Qin et al., 2013), and estuaries (Conley et al., 2009). In Indonesia, particularly in Jakarta Bay, harmful algal blooms (HABs) caused by nutrient runoff (Prayitno, 2011) occurred sev- eral times in early 2000s. Jakarta Bay is an estuary where 13 rivers that flow through the West Java Province to the DKI Jakarta Province meet the Java Sea. The study by Prayinto (2011) also showed that Jakarta Bay was in a eutrophic state due to the abundance of phosphate and nitrate contaminants, although the contamination was not significant enough to cause HAB. It is known that small-scale intensive agriculture is supported by the fertile volcanic soil at the foot of 2000-3000-m-scale mountains and the abundant rainfall throughout the year in the western region of Java. The Cisadane River is one of the main rivers in the West Java and Banten Provinces. It covers an area of 1,100 km 2 and is 80 km in length, flowing from the water sources of Mt. Salak (2,211 m above sea level) and Mt. Gede (2,958 m)/Pangrango (3,019 m) to the Java Sea through Bogor Regency, Bogor City, Tangerang City, and Tangerang Regency. The estuary of the Cisadane River is located in Trop. Agr. Develop. 62 （2）：94 － 103，2018