This well is located in an alluvial lowland area in the western coastal part of Tokyo Bay. In Shinagawa area, there still remain a number of wells with hand pumps, which were used as domestic water sources in the past. This well is 9.1 m deep and its screen width is unknown. The hand pump on this well is covered with an orangecloth because it is currently out of order.

 Although the public sewerage coverage ratio of Tokyo's Shinagawa area is 99.5% (Bureau of Sewerage Tokyo Metropolitan Government¹⁾), the results of an end member mixing analysis (EMMA) based on the δ¹⁵N-NO₃ and nitrogen concentration suggest sewage is seeping into the groundwater sampled from this well. And its Cl⁻ concentration is higher than that of groundwater sampled in nearby areas. Meanwhile, other water quality results indicate a lower NO₃⁻ concentration (n.d. −5 mg/L), a higher HCO₃⁻ concentration (120-200 mg/L), and a lower ORP_SHE (50-200 mV), despite seasonal variations, which are signs of denitrification under an anaerobic environment (Itoh et al., 2023).

(Photograph: Yuki ITOH; Explanation: Seongwon LEE)

Note

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.

 The high contaminated shallow groundwater by nitrate (NO₃⁻) and ammonium (NH₄⁺) in the Kathmandu Valley, Nepal were investigated along with their nitrogen sources and distribution mechanisms. Thirty-six shallow groundwater samples were collected from the tube- and dug- wells during the monsoon season (August) of 2009 and 2010. High levels of NO₃⁻ (maximum: 63.9 mg/L) and NH₄⁺ (maximum: 36.7 mg/L) concentrations were detected in shallow groundwater. NO₃⁻ represented a significant contribution to the total dissolved inorganic nitrogen in most the dug wells; these wells were characterized by unprotected inner walls, indicating the possible influx of nitrified NO₃⁻ from the unsaturated zone to the insides of the wells. The dual isotopic (δ¹⁵N and δ¹⁸O) NO₃⁻ approach suggested that sewer leakage is a major source of nitrogen contamination in shallow aquifers. In addition, the exponential increase in NO₃⁻-δ¹⁵N accompanied by NO₃⁻ reduction with the distinct δ¹⁸O/δ¹⁵N slopes of NO₃⁻ (∼0.5) indicated denitrification. Finally, we established a negative nonlinear relationship between NO₃⁻ and the dissolved organic carbon, suggesting that organic carbon strongly affects denitrification reactions in shallow groundwater.