Nitrogen contamination from fertilizer in tea plantation, Kiku River basin, Japan

摘要

The relationship between the amount of fertilizer used and concentrations of nitrate and metals in well and river waters was studied on a Japanese tea plantation. The amount of nitrogen fertilizer used decreased from112 kg N/10a in 1993 to 65 kg N/10a in 2000. Nitrate and metal concentrations of some heavily contaminated well water decreased considerably. However reduced concentrations were still high and concentrations in most contaminated wells were unchanged. Therefore, a greater reduction of nitrogen fertilizer used is still needed. Irrigation water from the Ohi River basin flows into the Kiku River basin. Delta ~(18)O and delta D values were used for estimating the mixing rate between the two basin waters. Average irrigation water was calculated to be more than 40% of total flow in the Tanno River of the Kiku River basin. Therefore irrigation was very effective to reduce nitrate and metal concentrations in the Kiku River basin.