Natural and anthropogenic nitrate (NO3-N), nitrite (NO2-N) and ammonia (NH4-N) in groundwater represents vital environmental and health concern issue globally. Here, we present data and discuss sources of nitrogen compounds in the groundwater that accounts for two-thirds of the total water supply of the Haihe River Plain with a population of over 100 million. The spatial and temporal distribution of the nitrogen compounds (NO3-N, NO2-N, NH4-N) in the groundwater are linked to a variety of sources, such as fertilizers, domestic sewages, industrial wastewater and precipitation. About 12.64%, 53.90% and 16.73% of the investigated groundwater wells in the Haihe River Plain have NO3-N, NO2-N and NH4-N concentrations above permissible values for drinking water, respectively. Comprehensive actions such as changing farming methods, applying fertilizer at suitable times and appropriate irrigation pattern for the Haihe River Plain are required to reduce the nitrogen pollution in the future.
Groundwater pollution is a growing concern everywhere in the world, especially from intensive human activities, including agriculture (Chen et al. 2007; Landon et al. 2011; Bonton et al. 2012) and industrial production (Farshad & Imandel 2003; Zakhem & Hafez 2015). The new established sub-capital of China (Xiong'an New Area), located in the Haihe River Plain (Figure 1). The annual average groundwater supply is about 20 billion m3, accounting for two-thirds of the total water supply. A large portion of groundwater is utilized in the Haihe River Plain for grain production with output reaching 109,370,000 tons in 2015, accounting for 22% of China.
Several studies reported groundwater nitrogen pollution which may threaten drinking water resources in the Haihe River Plain. Pollution sources of nitrogen compounds included septic waste (Gao et al. 2011), organic matter, animal manure and chemical fertilizer (Fang et al. 2015). Past research lacks many aspects in terms of spatial coverage, choice of pollution indices, effects on human health, comprehensive analysis (NO3-N, NO2-N, NH4-N) and possible sources of pollutants. To overcome some of these problems, a large spatial coverage of the Haihe River Plain was achieved here through seasonal analyses of groundwater from a large number of actively used wells in 2014. The data are used to provide information regarding the distribution levels, sources and environmental and health hazard impacts of the nitrogen compounds in the region.
MATERIALS AND METHODS
The Haihe River Plain (112°30′-119°30′ E, 34°46′-40°25′ N) has a total surface area of 136,189 km2 (Figure 1). The unique geology and geomorphology play an important role in controlling the movement and distribution of groundwater (Figure S1). The Quaternary deposits consist of sand, gravel and clay with the amount of gravel fraction decreasing from the Piedmont Plain to the Central Plains and into the Coastal Plain (Figure S1). The hydrogeological conditions in the region are strongly controlled by the geological elements where the groundwater level is lower in the middle and eastern parts (Figure S2). (Figures S1 and S2 are available with the online version of this paper.)
Groundwater samples were collected from 269 wells at a quarterly period in 2014. The sampling was performed after letting the well pump for a period of at least 1 h to avoid a stagnation effect. Total dissolved solids (TDS) and pH were determined in the field using a calibrated multi-parameter water quality detector (YSI6820). 2.5 L of water was collected from each well in tightly capped high-density polyethylene (HDPE) bottles. These samples were analysed for NO3-N, NO2-N, NH4-N, and MnO4 by continuous flowing analysis (AA3) and Cl by ion chromatograph (ICS-1100). The analyses were done following protocols and testing methods approved for industrial standards for quality control. For all water samples, ions equilibrium balance errors (IBE) were <10%, and most of them were <5%. The geostatistics module in ArcGIS10.2 was used for spatial analysis (Javad et al. 2016).
RESULTS AND DISCUSSION
Data on TDS, Cl and MnO4 were used here to provide information about water salinity, seawater intrusion and oxidation potential of the groundwater. The complete seasonal results of the groundwater analyses are presented in Table S1. A summary of the data is presented in Table 1 which indicates TDS range of 168–4,775 mg/L, and places the groundwater in the fresh to brackish water. Using annual average distribution pattern, a few parts of the region show fresh drinkable water (TDS < 300 mg/L) while most of the central and coastal parts indicate rather high salinity water (Figure 2). The groundwater TDS seems also to depend on the seasons (Qin et al. 2013) where the northward extension of high salinity groundwater during winter moves further south and to the central part during spring through autumn (Figure S3). Variability in the Cl concentrations is rather large stretching from about 2 mg/L to 3,480 mg/L and the concentrations increase seawards (Figure 2 and Table 1). The seasonal Cl concentrations indicate some reduction in the high concentration zone during winter (Figure S4). The MnO4 distribution (0.5–51 mg/L) seems different from the other compounds (Figure 2) where high concentration occurs near to the seaside and is maintained through spring and autumn (Figure S5). (Table S1 and Figures S3-S5 are available with the online version of this paper.)
|.||TDS .||Cl .||MnO4 .||NO3-N .||NO2-N .||NH4-N .|
|.||TDS .||Cl .||MnO4 .||NO3-N .||NO2-N .||NH4-N .|
aThe permissible limit is for drinking water according the World Health Organization (WHO 2011).
bThe permissible value is for drinking water according to the General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China.
The variability in the NO3-N is at 0.8–301 mg/L and that of NO2-N is at 0.001–6.65 mg/L while the NH4-N occurs at concentration of 0.1–10.9 mg/L (Figure 3 and Table 1). The seasonal distribution in these compounds seems unique to each of them with patches that contain high concentration seawards and at the southern tip of the region for the NO3-N (Figure S6). The NO2-N seasonal changes appear retaining high concentrations during winter compared to other seasons (Figure S7). The NH4-N concentrations show occurrence of high values during all seasons with slight difference in the distribution patterns (Figure S8). (Figures S6-S8 are available online.)
Liner correlation matrix of the whole data set (Table S2) shows that the most significant correlation for the nitrates is between NO3-N and MnO4, with R value = 0.63. The correlation coefficient between NO3-N and Cl is R = 0.46. The rest of the correlation values are <0.4. All of the p values are <0.01. The correlation coefficients between the different parameters and nitrogen compounds did not change significantly when each season data is treated separately (Table S2). Frequency histograms and probability plots (Figure S9) indicate mainly non-normal distribution of the nitrogen compounds and the rather limited range of significance within the t-test confidence range of >95%. (Table S2 and Figure S9 are available online.)
Variability in the distribution of the analyzed nitrogen compounds of the groundwater reflects complex influence of spatial and temporal factors that are related to sources of nitrogen, aquifer characteristics, landscape features, and seasons as discussed below.
Sources of nitrogen compounds
The amount of chemical fertilizer used in the Haihe River Plain during 2015 at 837.7 × 104 t (Zhao et al. 2015) with an average nitrogen content of 35% (Gao et al. 2009) was considered. Estimates of nitrogen fertilizer utilization and nitrogen volatilization rates were approximately 40% (Ju et al. 2009) and 36% (Wang et al. 2014a, 2014b), respectively. Variation range in infiltration rate was estimated to between 0.06 and 0.3 and is considered here at 0.18 (Cao et al. 2013). The amount of nitrogen produced from the fertilizer application during 2015 is estimated at 12.6 × 104t. When the nitrogen entered the groundwater through the aeration zone, part of it will be adsorbed to particles and according to studies carried out in the Haihe Plain, about 20% of the nitrogen may infiltrate into the groundwater (Min et al. 2015). Therefore, the amount of nitrogen in groundwater sourced from fertilizer is estimated at 2.52 × 104 t in 2015. The amount of chemical fertilizer applied by most farmers in the Haihe River Plain exceeds 200 kg/hm2 and may reach 500 kg/hm2 in a winter wheat and summer maize rotation system (Zhao et al. 2015). An estimate of ratio for the nitrogen, phosphorus and potassium fertilizers is about 5:3:1. The high amounts of nitrogen fertilizers far exceeded the nutrient consumption of crops in the same season. In addition, the loss of farmland nitrate nitrogen with runoff was relatively small (Zhou et al. 2011) that results in increasing content of nitrate and consequently infiltration into groundwater.
In summary, among the anthropogenic nitrogen sources (agricultural chemical fertilizer, industrial wastewater and domestic sewage), the application of agricultural chemical fertilizer represents the most significant source of nitrogen pollution in the Haihe River Plain (Figure 5). Another signature of the fertilizers-induced nitrogen is the source apportion of the different nitrogen compounds as illustrated by the frequency and probability diagrams (Figure S9). The data indicate rather non-normal distribution for nitrogen compounds that implies un-equilibrium conditions created by the variable sources (anthropogenic and natural). Factor analysis was used here to define correlated grouping of comparable variables, natural and/or anthropogenic (Figure S10, available online). Factor loading is used to show which variables load into each factor (Love et al. 2004). Despite the relatively far from normal distribution shown by the data, factor analysis can indicate approximate relationship between the parameters. The data reveal comparable trends for TDS and Cl thus suggest linking to mainly common sources, which can be mainly a mixture of natural (aquifer and seawater) and anthropogenic ones. The nitrogen compounds and MnO4 show attributes of load and trends supporting source apportions that is likely dominated by anthropogenic additives such as fertilizers. The rather separated trends of NH4-N and NO3-N (Figure S10) suggest supply of these nitrogen forms under different temporal and/or chemical conditions. The relatively higher amount of NO3-N in the seasonal pattern (Figure S6) compared to NH4-N in the southern part of the region points out differential nitrogen compounds discharge from industrial wastewater as the cause of the rather separate trend in the factor analysis data (Figure S10).
In most areas nitrogen fertilizer and household livestock or poultry appear to be the most common pollution source (Zhao et al. 2008). Nitrate concentration in the Hetao Irrigation District shows much higher NO3-N in the fertilized compared to the non-fertilized area (Feng et al. 2003). The groundwater NO3-N content was different among different land use types in the Chao Lake watershed, Anhui province. This pattern was proven to be related to effects of seasons and amount of fertilizer applied (Wang et al. 2014a, 2014b). In the central Sichuan basin, NO3-N concentration was greatest in the rainy season (Chen et al. 2006) which was likely related to that fertilizer's nitrogen stored in the soil infiltrates with the rain into the groundwater. The urbanization rate and sewage treatment rate in the study area were higher than other parts of China (China Statistics Yearbook 2015), leading to a possible less impact of domestic sewage on the groundwater in the Haihe River Plain (Figure S11, available online).
The permissible limits of nitrogen and other compounds in drinking water set by the Chinese Quality Supervision and the World Health Organization (WHO 2011) are shown in Table 1. There are 12.64%, 53.90% and 16.73% wells in the Haihe River Plain, showing values above the permissible values of NO3-N, NO2-N and NH4-N, respectively. The relatively greater than permissible values of NO2-N (>50%) in the groundwater suggest that many of the wells produce water which is not suitable for direct drinking.
The results presented here indicate that nitrogen compounds, NO3-N, NO2-N and NH4-N, in the groundwater of the Haihe River Plain were mainly derived from excessive use of fertilizers during agricultural production. The excessive concentration of NO2-N was the highest, which indicates that the Haihe River Plain groundwater nitrogen pollution is still ongoing. The Xiong'an Area, the sub-capital of China, is presently located in zones where the level of permissible limits for nitrogen compounds has been exceeded. Remediation actions, such as changing farming methods, applying fertilizer at suitable times and appropriate irrigation pattern, will result in reducing infiltration of the nitrogen compounds into the groundwater and thus create a more sustainable and less polluted environment.
This work was supported by the Public Welfare Industry Research Funds (Grant No. 201501008), Ministry of Water Resources of People's Republic of China.