Use of multiple isotopic and chemical tracers to identify sources of nitrate in shallow groundwaters along the northern slope of the Qinling Mountains, China

Highlights

• Levels of nitrate observed in the groundwater exceed an unusually high 850 mg/L.

• The centrally located Jijiahe Reservoir and Shidi River have been a major source of aquifer recharge.

• The shallow aquifer nitrate contamination is a legacy of historical discharge of high-nitrogen wastewater.

• The shallow aquifer continues to be impacted by other anthropogenic activities.

Abstract

Isotopes and major ion hydrochemistry are used to investigate potential sources of nitrate contamination in shallow aquifers along the northern slope of the Qinling Mountains, west-central China. Locally, nitrate concentrations exceed a remarkable 850 mg/L, over 40 times background levels observed in fresh mountain springs, and a knowledge and understanding of the nitrate pollution sources is essential for the development of aquifer management and protection strategies that will allow the problem to be effectively controlled. Presently, none of the suspected pollutant sources generates concentrations of nitrate that even approach levels observed in the groundwater, and this is clearly a challenge in terms of assigning responsibility for the contamination and developing appropriate management plans. However, stable isotopes of hydrogen and oxygen (δD and δ¹⁸O) in study area waters demonstrate that the centrally located Jijiahe Reservoir and Shidi River have been a major source of aquifer recharge in recent times and are an obvious source of the contamination. Moreover, δ¹⁵N–NO₃^– and δ¹⁸O–NO₃^– determinations suggest that, while nitrate concentrations in the river and reservoir are currently relatively low, a local, long-established fertilizer factory that has likely released high-nitrogen waste to these nearby receptors in the past, is the primary pollutant source. On a positive note, it would appear that the majority of the nitrate contamination, albeit serious in terms of drinking water quality, represents a legacy of past practice and should show gradual improvement due to modern-day environmental protection initiatives, improved equipment, better monitoring and significantly enhanced methods of wastewater treatment. However, the isotope data do indicate that the aquifer continues to be impacted by other anthropogenic activities, including agriculture and livestock raising, downstream of the industrial area and in the eastern part of the study area, meaning that monitoring and active remediation strategies need to be continued with vigilance.

Introduction

Nitrate contamination of groundwater associated with anthropogenic activities has been a global issue (Benkovitz et al., 1996; Keeler and Polasky, 2014; Kendall and Aravena, 2000; Stadler et al., 2008) for well over fifty years (Howard, 1985). The primary concern is that nitrate ingested through drinking water may lead to methemoglobinemia, a blood disorder that may develop following the bacterial reduction of nitrate to nitrite in the human intestinal tract (Zhang et al., 2017). Nitrite enters the blood stream and combines with the haemoglobin (Hb) to form methaemoglobin (MetHb) which diminishes the capacity of the blood to transport oxygen (Addiscott and Benjamin, 2004; Weitzberg and Lundberg, 2013). The problem is particularly acute in very young children and babies (where the condition is often referred to as “blue baby syndrome”) as their gastric juices have a pH closer to neutrality, thus providing a better environment for nitrate-reducing bacteria to flourish. It is due to this prevailing health concern, that the World Health Organization establishes the maximum permissible limit for nitrate in drinking water at 50 mg/L (10 mg/L as NO₃⁻-N). The WHO guideline for nitrite, a reduction product of nitrate, is 3 mg/L. Concern over nitrate is not limited to health. Nitrate is also widely recognized as a threat to the environment since elevated nitrate in surface water can lead to an excessive abundance of nutrients (known as eutrophication) which can result in the dense growth of plant life, and the death of aquatic animals due to a resulting lack of oxygen (Smolders et al., 2010).

The presence of nitrate in groundwater is normally the result of human “anthropogenic” activities. It is commonly associated with agriculture (chemical fertilizers and manure applied to cropland) (Babiker et al., 2004; Dahan et al., 2014; Liao et al., 2012; Nikolenko et al., 2018) but non-agricultural releases of nitrate (industrial effluents, atmospheric deposition of airborne industrial pollutants, and domestic sewage) are also common (Wakida and Lerner, 2005). Nitrate is highly soluble in water and migrates readily. Once it enters the aquifer system, it can remain stable for decades in oxidizing environments (positive redox potential) and can accumulate gradually where persistent sources of nitrate load the aquifer on an annual basis (Gutiérrez et al., 2018).

Managing the groundwater nitrate problem is a constant challenge. Effective management requires a knowledge of the nitrate distribution together with a full understanding of the chemical sources and the chemical transport processes (Chen et al., 2018). As a starting point, the hydrochemical characteristics of the groundwater as a function of their geological and hydrogeological setting must be established (Chen et al., 2017; Kaown et al., 2007; Schaider et al., 2016). Routinely, this can be approached using various methodologies including multivariate statistics and graphical analysis (e.g. Piper and Durov diagrams and contour maps) (Busico et al., 2018; Chen et al., 2013; Kim et al., 2009; Li et al., 2018; Matiatos, 2016; Mattern et al., 2009; Mfumu Kihumba et al., 2015). However, considerably added benefit can be derived from the stable isotopes of nitrogen, oxygen and hydrogen, which can provide more definitive indications of both water and nitrate sources (Wang et al., 2016; Stellato et al., 2016).

For example, elevated nitrate in groundwater can derive from multiple sources, each of which has experienced a unique sequence of chemical and biogeochemical processes such as fixation, ammonification, nitrification, and denitrification (Sebilo et al., 2006). The stable isotopes of nitrogen and oxygen (in nitrate) can be used to reveal those processes, thereby “fingerprinting” the source or sources (Ding et al., 2014; El Gaouzi et al., 2013; Hao et al., 2018). Xue et al. (2009) have shown the representative ranges of nitrate isotopes for different sources. The isotopic comparison between the water samples and possible sources is commonly used by many researchers to estimate the contamination sources (Li et al., 2007, 2010; Lohse et al., 2013). However, it becomes difficult to distinguish the manure and sewage which have the similar isotopic signatures (Minet et al., 2017). Good understanding of the recharge, discharge and regional groundwater flow of aquifer is also very important to reduce uncertainties (Hosono et al., 2013). In addition, boron has distinct isotopic signature in uncontaminated groundwater, seawater (De Giorgio et al., 2018) and artificial sources, such as animal manure, fertilizer and sewage (Di Fusco et al., 2017). As boron isotopes are not affected by denitrification, the combined use of δ¹¹B and δ¹⁵N can provide a method to distinguish nitrate sources in groundwater and allows a semi-quantification of nitrate sources contributions (Seiler, 2005).

Stable isotopes of oxygen and hydrogen can provide a valuable insight to the origin, behavior and mixing history of the water (H₂O) in which the nitrogen compounds are dissolved and transported (Qian et al., 2007). The ¹⁸O and ²H are the intrinsic components of water molecules and retain the primary compositions of precipitation and recharge. Although groundwater retains the ¹⁸O and ²H signals of precipitation with some fidelity, it can possibly be modified by mixing of recharge of different origins. Groundwater mixing ratios based on conservative ¹⁸O and ²H are useful for understanding the major recharge of groundwater since it commonly controls the water chemistry, allowing the nitrate sources to be further confirmed.

In the study presented here, the dual stable isotopes of nitrate (δ¹⁵N–NO₃^– and δ¹⁸O–NO₃^–)¹ (Coplen, 2011) are used in combination with more conventional hydrochemical and isotope techniques to investigate sources of severe nitrate contamination of shallow groundwater in aquifers along the northern slope of the Qinling Mountains, China. This area has been subject to intense anthropogenic activity in recent decades. As environmental protection policies have been implemented, surface water quality has been remarkably improved in a short time. The subsurface environment, however, may need a longer recovery time. The knowledge and understanding of the nitrate pollution sources is essential to develop aquifer protection strategies that will allow this problem to be effectively managed. Long term monitoring is the most efficient method, but it is also challenging to use in developing areas due to the lack of monitoring system in the past. The present study focuses on providing a feasible method using integrated application of nitrate isotopes and groundwater recharge, to trace potential nitrate sources with lower uncertainties in complex groundwater nitrate issues.