Tracing nitrate pollution sources and transformation in surface- and ground-waters using environmental isotopes
Introduction
Nitrate contamination is a pervasive problem in China and all over the world (Erisman et al., 2013, Ju et al., 2009). The concentration of nitrate nitrogen (NO3−–N) in most rivers in populated regions reaches levels that are up to seven times higher than the maximum contaminant level (10 mg/L) allowed by the Global Environment Monitoring System database (He et al., 2011). High NO3−–N concentrations in drinking water increase the risk of diseases, such as certain types of cancers, methemoglobinemia, spontaneous abortions, diabetes, and thyroid disorders (Dan-Hassan et al., 2012). In addition to the health risk, once water resources are polluted by nitrate, recovery is associated with high costs related to a series of physical, chemical, and biological reactions used to decrease NO3−–N concentrations (Jiang et al., 2011).
A prerequisite for controlling and managing nitrate pollution is to identify sources of nitrate. Possible sources of nitrate contamination in water include agricultural N fertilizer, industrial wastewater discharges, urban domestic sewage, septic systems, human waste lagoons, animal feedlots, native soil organic matter, and atmospheric N deposition. It has been well established that nitrate dual isotope (15N–NO3− and 18O–NO3−) analysis is an effective approach for providing information on nitrate sources because of their distinct N and O isotopic compositions (Nestler et al., 2011). Nitrogen can be transformed through microbial processes, such as nitrification and denitrification, which are accompanied by isotopic fractionation (Kendall, 1998). In the denitrification process, NO3− is reduced to NOx or N2 by bacteria, leaving the residual NO3− enriched in δ15N–NO3− and δ18O–NO3−. A crossplot of concentration and isotopic composition of NO3− can be used to evaluate mixing processes within hydrological systems (Kendall, 1998, Li et al., 2010b). No distinct positive or negative relationship between δ15N–NO3− or δ18O–NO3− vs. NO3− suggests that there is no simple mixing or only one biological process is responsible for the shifting of isotopic values. Nitrate originating from precipitation, synthetic nitrate fertilizer, or nitrification of reduced N fertilizer shows much lower δ15N values (from − 5‰ to + 5‰) than from soil organic N (from 2‰ to + 9‰), and manure and sewage (from 8‰ to + 20‰). Although nitrate derived from precipitation, synthetic nitrate fertilizer, or produced by nitrification of reduced N compounds in fertilizer shows almost the same range of δ15N–NO3− values, they can be distinguished by their δ18O values. Values of δ18O–NO3− in nitrate are usually between + 20‰ and + 70‰ in precipitation, between + 17‰ and + 25‰ in synthetic nitrate fertilizer, and lower than + 15‰ in nitrification of reduced N fertilizer (Bateman and Kelly, 2007, Silva et al., 2002).
A growing number of studies have used dual isotopic analysis on nitrate to trace the main sources and transformations of nitrate from polluted rivers all over the world. For example, findings show that nitrification and urban sewage effluent are the major sources of nitrate in the Changjiang River in China (Li et al., 2010a); nitrification and denitrification contribute to nitrogen removal in the wetland area along a discontinuous tributary with slow water transport in the Tuul River (Itoh et al., 2011); the main nitrate source is atmospheric deposition or soil organic matter in the North Han River, but is manure or sewage in the South Han River in Korea (Lee et al., 2008); manure, fertilizer, and sewage are the major nitrate sources in a lowland agricultural catchment in eastern England (Wexler et al., 2009); the nitrate load in the Marano Lagoon in northeast Italy was derived not only from agriculture activities but also from other sources such as urban wastewaters, in situ nitrification, and atmospheric deposition (Saccon et al., 2013); and wastewater was an important nitrate source in urbanized streams in the Baltimore metropolitan area (Kaushal et al., 2011). However, there have only been a small number of studies on nitrate pollution in irrigated agricultural regions (Almasri and Kaluarachchi, 2004, Zhang et al., 2013a).
In agricultural regions, fertilizers and irrigation are the primary factors that contributed to increase world crop production. The use of fertilizers accounts for approximately 50% of the yield increase, and greater irrigation for another substantial part (Nellemann, 2009). China is the world's greatest producer and consumer of fertilizers using about 31% of the total amount of fertilizers used worldwide (Su et al., 2013). This large-scale use of chemical fertilizers has led to increasing water nitrate pollution. The North China Plain (NCP), which is the largest alluvial plain in China, provides about 61% and 31% of the nation's wheat and maize production, respectively (Zhao et al., 2013). Agricultural fertilization is a common practice in this region, which caused severe nitrate contamination of the local rivers and aquifers (Ju et al., 2009). In addition, irrigation is used to supplement seasonally deficient precipitation and to decrease soil salinity in the NCP (Liu and Luo, 2010). Approximately 75% of the agricultural land is irrigated, and it consumes 70% to 80% of the total water resources allocation (Chen et al., 2003). The City of Yucheng typifies the NCP's landscape, water, and soil resources (Jia et al., 2010). The nature and extent of agricultural development and industrialization are also typical of the wider NCP. The average NO3−–N concentration in surface water is 6.57 mg/L and in groundwater 10.23 mg/L (Chen et al., 2007, Chen, 2010). Nearly 39% of surface water samples (totally 31 samples) have nitrate levels in excess of the average value, and the percentage of groundwater samples with high levels are even greater in the Yellow River irrigation region (Wu et al., 2011). Chen et al. (2005) determined the distribution of nitrate pollution in various hydrogeological zones of the NCP's regional groundwater system. However, the sources and fate of nitrate in the NCP are still largely unknown. Also, advanced research into nitrate pollution due to changes in seasons in agricultural regions has been scant.
In this study, nitrate concentration, dual isotopic analysis of nitrate and water are used to (1) identify nitrate pollution in both surface and groundwater in irrigated agricultural regions, and (2) trace the main sources and transformations of nitrate in different seasons. This study provides useful information to evaluate the water quality and to promote a more sustainable water management in irrigated farming regions.
Section snippets
Study region description
The NCP is located in the eastern part of China (35°00′–40°30′ N, 113°00′–119°30′ E) and represents a very important agricultural region, because of intense wheat, maize, cotton, and vegetable farming. The study region (36°41′13″–37°12′13″ N, 113°22′11″–116°45′00″ E) is in the central part of the NCP (Fig. 1) and hosts the Yucheng Comprehensive Experiment Station (YCES), Chinese Academy of Sciences (CAS). The study region has approximately 500,000 permanent residents, a total area of 990 km2,
Water chemistry
The EC and pH values of surface and groundwater samples are listed in Table 1. EC values ranged from 170 to 2340 μS/cm in surface water and from 880 to 4270 μS/cm in groundwater. As shown in Table 1, the pH values of surface water ranged from 7.7 to 9.4, indicating a somewhat alkaline nature. However, all groundwater samples had pH values (7.0–7.7) lower than in surface water. High pH values were mainly observed in surface water collected in July and August (Table 1). The DO level gradually
Concentrations of forms of nitrogen
In this study, concentrations of NO2−–N in all samples were below the detection limit (0.002 mg/L). Therefore, only two nitrogen compounds (NO3−–N and NH4+–N) were compared. The NO3–N concentration was higher than the NH4+–N concentration in both surface water and groundwater (Table 1). This may be attributed to two reasons. One is the application of nitrogen fertilizers at most monitoring sites, which were located in agricultural regions. Nitrogen fertilizers are generally the primary source of
Conclusions
In the NCP, concentrations of NO2−–N in all samples were all below the detection limit (0.002 mg/L), so only results for NO3−–N and NH4+–N were reported. Concentrations of NO3−–N were significantly higher than for NH4+–N. NO3−–N pollution is more serious in surface water than in groundwater. Approximately 46.7% of the surface water samples and 10% of the groundwater samples showed exceeded nitrate concentrations according to the drinking water standard for NO3−–N established by the WHO.
Nitrate
Conflict of interest statement
No conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all authors for publication. On behalf of my co-authors, I would like to declare that the work described was an original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.
Acknowledgments
This work was support by the Key Knowledge Innovation Project of Chinese Academy of Sciences (Grant No.: KZCX2-EW 310), the National Natural Science Foundation of China (Grant No.: 41271047) and 100-Talent Project of Chinese Academy of Sciences. Thanks to Dr. Sun Zhenzhong for assistance in field sampling. Also thanks to Marc for providing polishing the English. Special thanks are expressed to four anonymous reviewers and editors.
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