Effective use of farmland soil samples for N and O isotopic source fingerprinting of groundwater nitrate contamination in the subsurface dammed limestone aquifer, Southern Okinawa Island, Japan
Effective use of farmland soil samples for N and O isotopic source fingerprinting of groundwater nitrate contamination in the subsurface dammed limestone aquifer, Southern Okinawa Island, Japan
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DOI：
10.1016/j.jhydrol.2023.129364
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发表时间：
2023-04
影响因子：
6.4
通讯作者：
Oktanius Richard Hermawan;T. Hosono;J. Yasumoto;Ko Yasumoto;Ke Song;Rio Maruyama;Mariko Iijima;Mina Yasumoto-Hirose;Ryogo Takada;Kento Hijikawa;R. Shinjo
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作者：
Oktanius Richard Hermawan;T. Hosono;J. Yasumoto;Ko Yasumoto;Ke Song;Rio Maruyama;Mariko Iijima;Mina Yasumoto-Hirose;Ryogo Takada;Kento Hijikawa;R. Shinjo
由于农业活动期间施肥管理和控制不当而造成的地下水硝酸盐污染是一个不断升级的世界性问题。许多研究报告称，冲绳岛南部的琉球石灰岩含水层也存在类似污染。该研究区域具有独特的环境，地下水坝将地下水保留在其隔离墙后面，可能保留了受污染的水。这项研究利用含水层水中溶解硝酸盐（ẟ15NNO3 和 ẟ18ONO3）的稳定 N 和 O 同位素比率，提供了有关污染源的最新见解。为了在源指纹评估中提供更好的参考，特别强调了肥料与施用相应肥料的农田土壤之间的同位素比较。为了确定水源，通过几次空间和每月采样活动总共收集了 176 个地下水样本。我们的同位素比较检测到施用化肥和粪肥后，由于硝化和挥发作用，土壤样品中15N硝态氮的富集。与之前的研究结果不同，大多数地下水样品的ẟ15NNO3组成并不属于主要受化肥控制的土壤范围（+3‰至+5‰），而是与受化肥和粪肥混合肥料影响的土壤（+7‰至+9‰）相当。未经处理的粪便储存场下方的土壤具有最高的ẟ15NNO3值（从+18‰到+20‰），影响了一些地下水，显示出更高的值（> +9‰）。受污染的水域往往聚集在地下水坝附近，这种趋势在科梅苏和吉萨这两个地下水坝设置之间没有差异。然而，由于粪便储存场的影响较大，Komesu 地区地下水的 δ15NNO3 值往往略高于吉萨地区。我们的研究证明了土壤样品对于农田地区精确同位素源识别的有用性。
Nitrate contamination of groundwater due to improper management of fertilizer application and controls during agricultural activities is an escalating worldwide issue. Numerous studies have reported similar contamination in the Ryukyu limestone aquifer in the southern part of Okinawa Island. The study area has a unique setting where the subsurface dams hold the groundwater behind their cut-off walls, probably holding contaminated waters. This study provides updated insights on the source of contamination using stable N and O isotope ratios of dissolved nitrate (ẟ15NNO3and ẟ18ONO3) in the aquifer waters. To propose a better reference in the source fingerprinting assessment, a special emphasis was placed on the isotopic comparison between the fertilizers and farmland soils where the respective fertilizers were applied. A total of 176 groundwater samples were collected from several spatial and monthly sampling campaigns for the purpose of source determination. Our isotopic comparison detected the enrichment of15N of nitrate-nitrogen in the soil samples after the application of both chemical fertilizers and manure due to nitrification and volatilization. Unlike results from previous studies, the ẟ15NNO3compositions of most groundwater samples did not fall into the range of soils mainly controlled by chemical fertilizers (from +3‰ to +5‰), but were comparable to the soils affected by mixed fertilizers including both chemical fertilizers and manure (from +7‰ to +9‰). The soils beneath untreated manure storage sites characterized by the highest ẟ15NNO3values (from +18‰ to +20‰) affected some groundwater, which shows an even higher value (> +9‰). The contaminated waters tend to be gathered close to the subsurface dams and this tendency did not differ between two subsurface dam settings, Komesu and Giza. However, due to more impact from manure storage sites, the groundwater in the Komesu area tends to show slightly higher δ15NNO3values than those in the Giza area. Our study demonstrated the usefulness of soil samples for precise isotopic source identification in farmland areas.