Surface water chemistry, particularly concentrations of NO - 3 and DO and d 15 N values, near a tea plantation in Kyushu, Japan Hiroyuki Ii a , *, Tatemasa Hirata a , Hiroshi Matsuo b , Masataka Nishikawa c , Norio Tase d a Faculty of Systems Engineering, Wakayama University, 930, Sakaedani, Wakayama-city, Wakayama 640, Japan b Fukuoka Prefecture Institute of Health and Environment, 39, Mukaisano, Dazaihu-city, Fukuoka 818-01, Japan c National Institute for Environmental Studies, 16-2, Onogawa, Tsukuba-city, Ibaraki 305, Japan d Institute of Geoscience, University of Tsukuba, 1-1-1, Tennodai, Tsukuba-city, Ibaraki 305, Japan Received 30 October 1996; accepted 30 May 1997 Abstract To investigate the influence of chemical fertilizers on drinking water neighboring tea plantations in Kyushu, Japan, time series of chemical concentrations and nitrogen isotopic ratios within local ponds were studied. Results indicated that the main dissolved components of spring and river water originating from a tea plantation catchment were Ca 2+ , Mg 2+ , NO - 3 , and SO 2 - 4 . Furthermore, pH values were below 7 due to plantation soil enrichment using nitrogen fertilizers and calcium and magnesium carbonates. Although acidic spring water flowed into a local pond, the pH value of the pond water remained mostly above 7. Occasionally, excessive spring water influx after heavy rains caused the pond water pH to dip below 7. The pond water showed temporal variations of pH ranging over 4 orders of magnitude within a 3–4 month period. The increase in pH values within the pond was associated with an increase in dissolved oxygen (DO) and d 15 N values and the partial disappearance of NO - 3 . This is explained by nitrogen assimilation and production of oxygen during photosynthesis by algae.  1997 Elsevier Science B.V. Keywords: Nitrogen isotopes; Nitrogen contamination; Nitrogen fertilizer; Nitrification; Nitrogen assimilation 1. Introduction NO - 3 , a possible carcinogen, and its concentration in surface waters has become an important environmental issue in Japan. Contamination of surface waters over the last 0022-1694/97/$17.00  1997 Elsevier Science B.V. All rights reserved PII S0022-1694(97)00076-0 Journal of Hydrology 202 (1997) 341–352 * Corresponding author. Fax: + 81 734 54 0134; e-mail: hiro@sys.wakayama-u.ac.jp