本井戸は，東京湾西岸地域の沖積低地に位置しており，品川区には過去に生活用水として使用されていた手押しポンプ付きの井戸が多数残されている．本井戸の深さは，9.1 mであり，スクリーンの幅は不明である．本井戸は現在手押しポンプが故障中であり，そのためオレンジ色の布を被せている．

 東京都品川区の公共下水道普及率は99.5%（東京都下水道局¹⁾）にも関わらず，δ¹⁵N-NO₃と窒素濃度を用いた混合解析（EMMA）結果，本井戸から採水される地下水には下水漏水の混入が示唆され，周辺の地下水と比較しても高いCl⁻濃度を有している．一方，NO₃⁻濃度は低く（n.d. ～ 5 mg/L），HCO₃⁻濃度は高く（120 ～ 200 mg/L），ORP_SHEは低い（50 ～ 200 mV）ことから，嫌気的な環境下における脱窒反応が生じていると考えられる（伊東ほか，2023）．

（撮影：伊東優希；説明：李　盛源）

注

1) 東京都下水道局下水道100％普及達成年次表

 https://www.gesui.metro.tokyo.lg.jp/living/a2/spread/tasseinennzi/index.html [Cited 2023/1/28].

 Nitrogen (N) provides great benefits as a fertilizer for food production and as a material for industrial production. However, its use simultaneously induces N pollution, which threatens the health of human beings and ecosystems due to low N use efficiency in the anthroposphere. This is the N issue—a tradeoff between large benefits and threats. It is our responsibility to have future generations inherit sustainable N use by resolving the N issue. Groundwater nitrate pollution is a form of N pollution that should be addressed to prevent harm to human and animal health through drinking water and aquatic ecosystems through excess eutrophication. Links between groundwater nitrate pollution in N cycling and the N issue are reviewed to provide information for international and domestic actors to address the N issue, and to indicate groundwater nitrate pollution research needed to comprehensively address the N issue. Concretely, global and local N cycling and links with groundwater N processes are overviewed. The status of the N issue is summarized and a comprehensive framework to grasp the entire issue is introduced, i.e., the driver–pressure–state–impact–response (DPSIR) framework. Programs and projects tackling the N issue in Japan and around the world are summarized. Research required on groundwater nitrate pollution that contributes to resolving the N issue is discussed, and future expectations are indicated.

 Among all substances, nitrate nitrogen exceeds groundwater environment standards the most. The average excess rate of 87,000 samples collected across the nation so far is 4.5%. Efforts implemented by the Ministry of Environment to address such groundwater pollution caused by nitrate nitrogen are described. The Ministry of Environment has established guidelines for comprehensive regional measures to improve nitrate groundwater pollution. The guidelines include methods for investigating nitrate groundwater pollution, measures for reducing nitrogen loads, and methods for simulating groundwater numerical analyses to confirm the effects.

 The rapid growth in livestock farming since the late 1960s has resulted in problems managing excess waste excreted from livestock. A study on the water quality of groundwater, river water, and soil solution clarified that groundwater pollution by nitrate nitrogen was caused by nitrogenous compounds permeating livestock wastes under inappropriate waste management, including dumping and/or digging in. The appropriate treatment of livestock waste as compost and liquid fertilizer improved nitrate nitrogen pollution. The full-scale enforcement of “The Law Concerning the Appropriate Treatment and Promotion of Utilization of Livestock Manure” in 2004 reduced inappropriate waste management practices such as dumping and/or digging in. In 2019, all livestock farms installed comprehensive waste management facilities for composting, wastewater treatment, and other measures. However some issues remain to be resolved relating to the inappropriate operation of facilities and the existence of facilities aged over 20 years, which may cause pollution problems. In addition, elution of nitrate nitrogen from place of use digging in requires care even now. In 2021, an investigation report on livestock waste management was published by the Ministry of Agriculture, Forestry and Fisheries. Most solid livestock waste matter is converted into compost, which can be widely distributed on croplands of agricultural farms. Broiler litter is used for combustion energy. Liquid dairy cattle waste is applied on croplands. Wastewater from pig farms is purified with an activated sludge process to obtain clear water that complies with effluent standards and then is discharged into rivers and public waters. An appropriate application rate of compost on croplands is recommended to promote the recycling of organic and nitrogen resources and to control nitrate nitrogen in groundwater.

 Nitrate nitrogen pollution in groundwater can be understood from three viewpoints: generation sources, mobility, and elimination effects (natural purification effects). Alluvial fans are often utilized for farmland where nitrate fertilizer (pollutant source) may be used. Nitrate pollutants readily spread in groundwater at alluvial fans where two controlling factors for pollutant mobility are high: permeability and hydraulic gradient of groundwater. Moreover, the denitrification process, which is one of the most important natural purification effects, is not expected to occur readily in alluvial fan groundwater, which is seldom under anoxic conditions and contains little organic matter. In numerous previous studies, groundwater at alluvial fans has been researched using four main methods: water quality monitoring, calculations based on pollutant load per unit activity of source, stable isotopic tracers, and computed simulation models. As a result, processes and reactions of nitrate nitrogen in groundwater can now be evaluated quantitatively and visualized. Of these, because an oxidation reaction of ammonium sulfate, which is one of the main sources of nitrate nitrogen, multiplies the effects on groundwater chemistry, it is noted that NO₃⁻ : SO₄²⁻ molar ratios and δ¹³C_DIC values are secondary indicators for identifying this process in terms of groundwater pollution.

 Shallow groundwater in the densely populated Shinagawa area (Kita-Shinagawa and Minami-Shinagawa), central Tokyo, was sampled from 10 shallow wells (less than 12 m deep) in February (cold dry season) and July (hot wet season) 2019. The concentrations in seven groundwater samples from Kita-Shinagawa in February and July were 1.6-34.1 mg/L and not detected −34.8 mg/L for NO₃⁻, and 17.4-31.9 mg/L and 15.7-42.3 mg/L for Cl⁻. The measured isotopic ratios were 11.9-23.8‰ and 12.3-21.8‰ for δ¹⁵N-NO₃⁻, and 5.1-11.8‰ and 0.8-19.9‰ for δ¹⁸O-NO₃⁻, respectively. Shallow groundwater with elevated NO₃⁻ and Cl⁻ concentrations was probably contaminated by sewage leaking from damaged sewers. Although the wells are near each other in a small area of about 100 m (E–W) and about 60 m (N–S), shallow groundwater in Kita-Shinagawa showed a wide range of chemical concentrations and stable isotopic ratios, indicating sewage leakage as a source of groundwater contamination. Among groundwater samples collected once every two months in Kita-Shinagawa from January 2019 to February 2020, δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ values generally plot along the trendline with a slope of 0.5. This indicates that the study area had suitable conditions for denitrification to occur, although the degree of denitrification depended on the season and relative location of each well in the study area. The observed NO₃⁻ concentration and seasonal variability in the shallow groundwater were attributed to the mixing of three groundwater sources: 1) rainfall infiltration (natural recharge), 2) water-supply leakage, and 3) sewage leakage, with subsequent denitrification.

 Kumamoto City has been known as the “Water City” since ancient times, and it has also earned the title “Japan's Number 1 Groundwater City” because 100% of the water used by all 740,000 residents is supplied entirely by groundwater. Previously, the city implemented various measures to protect water quality by monitoring wastewater from industries and purifying groundwater polluted by organochlorides. However, in recent years, there has been an increasing trend of nitrate nitrogen in groundwater in the eastern and central parts of the city, and creating countermeasures for this problem has become an urgent issue. In order to preserve the quality of groundwater for future generations, the city began to implement countermeasures against nitrate nitrogen pollution by establishing the Kumamoto City Tobu Compost Center in order to properly treat domesticated animal manure, which is the main cause of nitrate nitrogen pollution in the eastern part of the city. Countermeasures will continue to be developed against nitrate nitrogen pollution through cooperation among citizens, private cattle farmers, and municipal governments.

 Numerous groundwater problems involving nitrate contamination, mainly derived from fertilizer and livestock waste, are reported worldwide. In Japan, although livestock wastes thrown in unlined pits has been banned by a law established in 1999, unlined pits still remain, causing high nitrogen discharges into groundwater. At the Tsukuba Plateau Ibaraki Pref., groundwater with more than 100 mgNO₃⁻/L is observed just below an old unlined pit with 4.1 kgN/year of nitrogen estimated to be released from the pit. In addition, a large part of the released nitrogen is denitrified in a percolation process through a clay layer beneath the pit.

 ネパール・カトマンズ盆地における硝酸イオン（NO₃⁻）およびアンモニウム（NH₄⁺）による高濃度の地下水汚染について，窒素の起源とその分布の要因について調査を行なった。2009年と2010年の雨季（8月）に，丸井戸と掘り抜き井戸から計36の浅層地下水の試料を採取した結果，高濃度のNO₃⁻（最大：63.9 mg/L）ならびにNH₄⁺（最大：36.7 mg/L）が検出された。多くの丸井戸から採取した地下水については全溶存無機窒素に対するNO₃⁻の含有量が多く，不飽和帯から流入するNO₃⁻または酸素の供給により井戸内部で硝化が生じている可能性が示唆された。NO₃⁻の窒素および酸素の安定同位体比（δ¹⁵Nとδ¹⁸O）の観測の結果，浅層地下水の主要な窒素汚染源は下水の漏水による窒素負荷であることが示された。また，NO₃⁻濃度の減少に伴うNO₃⁻-δ¹⁵N値の指数関数的な増加がみられたことや，δ¹⁸O値とδ¹⁵N値の間に得られる相関の近似式の傾きがおよそ0.5を示したことから，浅層地下水中において脱窒反応が生じでいることが考えられた。加えて，NO₃⁻濃度と溶存有機炭素の間には負の相関関係がみられ，溶存有機炭素が脱窒反応の進行に強く寄与していることを示した。