In the cultivation of tea in Japan, compared with other crops, large amounts of nitrogen have been applied as fertilizer, resulting in an increase in nitrate-nitrogen (NO3-N) concentrations in the groundwater around intensive tea-growing areas. To address this problem, the amounts of nitrogen fertilizer applied to tea fields have been reduced in recent years. It has been reported that the amount of fertilizer used in 1993 was more than 1,000 kg N ha−1, but had decreased to 600 kg N ha−1 in 2002 in Shizuoka Prefecture. In the present study, the effect of a reduction in nitrogen fertilizer use in an intensive tea-growing area in Shizuoka Prefecture on surrounding water systems was assessed. A seasonal Mann–Kendall test, one of the most widely used non-parametric tests, was applied to 10-year datasets of monthly pH and NO3-N concentrations from 16 sites, including drainages, springwater, groundwater and streams, for trend analysis. The results showed that there were significant downward trends (two-sided test at a 5% level) in NO3-N concentrations at most of the sites, and the seasonal Kendall slope estimator β, which represents the magnitude of the trend, ranged from −2.9 to −0.8 in drainages, from −2.6 to −1.8 in springwater and from −0.6 to −0.3 mg N L−1 year−1 in streams. The results of the present study suggested that the efforts made by farmers and authorities to reduce nitrogen fertilizer use in tea fields have resulted in improvements in NO3-N concentrations in the surrounding water systems in the study area. In contrast, for pH, significant downward trends were observed at nine of the 16 sites, indicating that water systems in this area have been acidifying, despite a reduction in nitrogen fertilizer use.