Ministry of the Environment (MOE) added Nitrate and Nitrite Nitrogen (Nitrate-N/Nitrite-N) to the Environment Quality Standards (EQS) for groundwater to protect human health in February 1999, considering that Nitrate-N/Nitrite-N has been detected at a high level in groundwater all over the country. Since then, local governments such as prefectures and cities designated based on the law have been conducting monitoring survey of groundwater quality, and it is confirmed that the ratio of wells exceeding EQS have been highly maintained. In this context, it is required to promote measures to protect groundwater quality from Nitrate-N/Nitrite-N pollution.
There are three main causes of Nitrate-N/Nitrite-N pollution, namely 1) chemical fertilizer and compost, 2) livestock excreta and 3) domestic waste water. And these cause widespread pollution, different from other pollution by artificial materials from factories. Therefore, different approach is needed to solve this Nitrate-N/Nitrite-N problem, considering natural and social characteristics of the polluted area.
One of the effective measures for Nitrate-N/Nitrite-N pollution is to set up forum or organization of stakeholder for sharing the relevant information and discussing the causes of the pollution and possible measures. Some prefecture and city governments prepared "promotion program" including same purposes.
MOE also has been making efforts to promote the measures for Nitrate-N/Nitrite-N pollution for several years and now considering the method to support local government through the technical based approach at the model local area and legislative approach emphasizing incentive to farmers and other communities.

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Groundwater contamination by nitrogen has been a serious problem in the world. Although many measurements such as the enactment of environmentrelated laws, proper application of fertilizers, and proper treatment of animal wastes have been taken, improvement or restoration of environments are not necessarily notable, especially remediation of contaminated groundwater has hardly progressed yet. Establishment of effective in-situ or on-site remediation methods for restoring groundwater environments is urgently needed. Good examples of remediation of nitrate-contaminated groundwater have to be accumulated.
Permeable reactive barrier (PRB) is one of the possible in-situ methods to remediate contaminated groundwater by many contaminants including nitrate with relatively low cost of construction and maintenance. There have been only several cases of PRB applied to nitrate-contaminated groundwater in Japan, which could show their possibility. In order to make many good examples of applications, there are several essential conditions such that a method has a clear chemical remediation process and it has to be applied at good field condition such as good permeable aquifers with a proper hydraulic gradient. Dimensions of the structure of PRB are also important, especially a length of the barrier is large enough to treat contaminated groundwater.

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Nitrate-nitrogen concentrations of groundwater and paddy field water were observed for five years (2005 to 2009) and the potential for purifying of itrate-nitrogen (denitrification) of paddy fields located arround Lake Inbanuma was estimated.
Consequently, it became clear that the paddy fields surrounding the Lake Inbanuma were able to purify 286ton of nitrate-nitrogen per year, which corresponds to 24% of the nitrogen load discharged from the Lake Inbanuma basin per year.

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[in Japanese]

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To identify the nitrate source and spatial distribution of nitrogen isotope values originated from forest and agricultural lands, fifty-seven stream water samples from Fuji River Basin were analyzed for nitrate concentration and nitrate-nitrogen (δ¹⁵N-NO₃^-) and oxygen (δ¹⁸O-NO₃^-) isotope values. The δ¹⁸O-NO₃^- showed that the nitrate originated through nitrification. These values were plotted against fraction of each land use category in the study catchments. The δ¹⁵N-NO₃^- was positively correlated with the fractions of agricultural and residential areas in the catchments. The increasing of δ¹⁵N-NO₃^- with fraction of agricultural area converged to 5.4‰ which was similar to a reported nitrate isotope value of groundwater at orchard area in Kofu-Basin. The increasing δ¹⁵N-NO₃^- with more than 1% residential area suggested that potential source of nitrate contamination was domestic aste water. An increase of δ¹⁵N-NO₃^- in the catchments with less than 98% forest area suggested the sources of nitrate contamination were thropogenic. The estimated value of δ¹⁵N-NO₃^- in forest catchment was 0.9±1.2‰.

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Denitrification is one of important processes of nitrate attenuation in groundwater. We reviewed published studies on effect of groundwater flow conditions for the denitrification process and discussed future directions of this study topic. The results were summarized as follows: 1) significant denitrification zone in groundwater flow system mainly corresponds to discharge area of shallower groundwater including topographic transition area and deeper aquifers (both confined and unconfined aquifers), 2) residence time of groundwater controls the amount of nitrate removed from the groundwater flow system by the denitrification process, 3) groundwater velocity (e.g, Darcy flux) might be a critical factor controlling occurrence of the denitrification process in the flow system.
On the basis of these results, we conclude that a future direction of the research topics is the ""quantification and scale-up of the quantitative evaluation of the denitrification process"" by considering spatial and temporal variations in groundwater flow conditions. For the quantification, estimation of critical velocity of groundwater required for controlling the denitrification process would be important. For the scale-up of the quantitative evaluation, it would be necessary to clarify the relation between geographical factors such as topographic gradient and the denitrification process in groundwater.

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