Department of Geography, Geology and Planning, Missouri State University, Springfield, MO 65897, USA.
Department of Chemistry, Missouri State University, Springfield, MO 65897, USA.
Sci Total Environ. 2018 May 15;624:1513-1522. doi: 10.1016/j.scitotenv.2017.12.252. Epub 2017 Dec 28.
Nitrate concentration in most aquifers in arid and semi-arid areas has increased in the past several decades as a result of human activities. Under the predominantly oxic conditions of these aquifers, denitrification is inhibited, allowing nitrate, a soluble and stable form of nitrogen (N), to accumulate. Because of its close association with municipal and agricultural wastes, nitrate is commonly used as an indicator of anthropogenic contamination. Aquifers affected by agricultural waste may contain salts from irrigation returns and herbicides in addition to nitrates. Preventing leakage from soil to deeper parts of the aquifer is thus a priority in the sustainable management of aquifers in arid and semiarid areas. Studies report a wide range of nitrate concentrations distributed non-uniformly within the aquifer, with roughly 40% and 20% of sampled wells exceeding 50mg/L nitrate in shallow and deep parts of the aquifer respectively. In aquifers at risk of becoming contaminated, nitrate isotopes (δN, δO, ΔO) can be used to identify the source of nitrogen as mineral or organic fertilizer, sewage, or atmospheric deposition. A variety of mathematical models (crop, hydrological, geochemical, or a combination of them) have been successful in identifying best practices that minimize N leakage without negatively affecting crop yield. In addition, field research in crop management, e.g., conservation agriculture, has yielded promising results in determining the adequate dosage and time of application of fertilizers to reduce N losses. Examples of key dryland aquifers impacted by nitrate are discussed, and some of the most pressing challenges to achieve sustainability are presented.
在过去几十年中，由于人类活动，干旱和半干旱地区的大多数含水层中的硝酸盐浓度都有所增加。在这些含水层主要为氧化条件下，反硝化作用受到抑制，导致硝酸盐（一种可溶性和稳定的氮形式）积累。由于硝酸盐与城市和农业废物密切相关，因此通常将其用作人为污染的指标。受农业废物影响的含水层除了硝酸盐外，还可能含有灌溉回水和除草剂中的盐分。因此，防止土壤渗漏到含水层的较深部位是干旱和半干旱地区含水层可持续管理的首要任务。研究报告称，在含水层中，硝酸盐浓度分布不均匀，浅层和深层含水层中约有 40%和 20%的采样井硝酸盐浓度分别超过 50mg/L。在有污染风险的含水层中，可以使用硝酸盐同位素（δN、δO、ΔO）来确定氮的来源是矿物或有机肥料、污水还是大气沉降。各种数学模型（作物、水文、地球化学或它们的组合）已成功用于确定最佳实践，以最大限度地减少氮泄漏，而不会对作物产量产生负面影响。此外，在作物管理方面的田间研究，例如保护性农业，在确定减少氮损失的肥料适当剂量和施用时间方面取得了有希望的结果。讨论了受硝酸盐影响的一些主要旱地含水层的实例，并提出了实现可持续性所面临的一些最紧迫的挑战。
