Hydrochemical characteristics of natural water and selenium-rich water resources in the Northern Daba Mountains, China

Trace elements play an important role in human health, especially selenium (Se), arsenic (As), and fluorine (F), and excessive or deficient quantities may cause endemic diseases (Tan et al. 2002; Rayman 2006; Selinus et al. 2013). Moderate Se intake facilitates thyroid hormone secretion, and has been demonstrated to have an anti-oxidation and anti-cancerous function, as well as improve the human immune system. Conversely, excessive Se intake (>400 μg/d) may lead to selenosis (WHO 2004; Fordyce 2007). Long-term excessive F intake can cause dental as well as skeletal fluorosis. Excessive As exposure can lead to chronic health problems and skin lesions (Selinus et al. 2013).

Drinking water is important in the human intake of trace elements (Bleiman & Mishael 2010). Se, F, and As contents in drinking water can have an important effect on human health. Certainly, too low a Se content in drinking water is not good for human health (Fordyce et al. 2000). However, Se concentrations in natural water are generally low, rarely exceeding 10 μg/L, with the majority of concentrations below 3 μg/L (Plant et al. 2014). The average Se content of global river waters is only 0.2 μg/L (Selinus et al. 2013). A high As content in drinking water represents one of the most serious threats to human health. The arsenic content of most natural waters is low (usually <10 μg/L, with the average ∼5 μg/L), but variations in As concentrations between surface water and groundwater are significant (∼0.5–5,000 μg/L) (Selinus et al. 2013). Drinking water with F concentrations >1.5 mg/L will probably lead to dental fluorosis, while concentrations >4.0 mg/L may cause skeletal fluorosis and other health problems (Dissanayake 1991). Usually, the F concentration in natural water is 0.1–10 mg/L and <300 μg/L in surface water. It is easy for F to accumulate in groundwater, with the F content in high fluorine groundwaters potentially reaching several hundred mg/L (Selinus et al. 2013).

Ankang City, Ziyang County, and Lan'gao County in the Northern Daba Mountains (NDM) in Shaanxi Province, China, are famous for being Se-rich (Combs 2001; Hartikainen 2005). However, the population of this region also suffers from fluorine and arsenic poisoning (Luo 2011; Xu & Luo 2012). The prevalence rate of dental fluorosis in residents of this region is >50% (Chen et al. 2003), and even exceeds 90% in some areas (Wen et al. 2011). In addition, there is serious arsenic-based pollution of the environment between the Qinling and Daba Mountains. Significant arsenism is found in nine counties, including Pingli, Lan'gao, and Zhenba, affecting more than one million inhabitants (Li et al. 2004; Xu et al. 2008).

There have been many studies of Se distribution and the provenances of soil and rock in the NDM (Luo et al. 1994, 1995, 2001, 2002a; Luo & Jiang 1995; Chen & Luo 1996; Luo 2003, 2011; Li et al. 2005; Xu & Luo 2012). Se contents were found to be generally >4.0 mg/kg in the Lower Cambrian and Daguiping Formations of the Silurian, with some even reaching, or exceeding, industrial grade (15 mg/kg). Although more widely Se-enriched strata exhibit contents too low to be mined, they nonetheless enrich crops and the bodies of animals with Se (Luo et al. 1994, 1995, 2001, 2002a; Luo & Jiang 1995; Chen & Luo 1996; Luo 2003, 2011; Li et al. 2005; Xu & Luo 2012). Further, the Early Paleozoic strata of the Daba region are generally rich in fluorine, with F contents usually >800 mg/kg in the carbonaceous slates, black shales, and phosphate rocks of the Early Cambrian Lujiaping Formation. Fluorine content is approximately several times higher than the average value found in the continental crust (Luo et al. 1995; Xu & Luo 2012). Moreover, the As content of the stone-like coals found in this area are as high as 277 mg/kg (Luo 2011), ∼100 times higher than the average As content of the continental crust (Selinus et al. 2013). Since we find Se, F, As, and other trace elements in these concentrations in the rocks of the NDM, what are their contents in the area's natural waters? There has, to date, been no systematic investigation of this, and studies of the hydrochemical characteristics and trace element contents of the natural waters of this area are very rare (Luo et al. 2002b, 2002c). In particular, there has been a paucity of systematic studies of the trace element content of drinking water in the NDM.

The Qinling-Daba Mountain (QDM) system is not only the water source of the Chinese South-to-North Water Transfer Project (Middle Route), but is also the poorest area of China (Cao et al. 2015; Liu et al. 2015). Any research into the hydrochemical characteristics and trace element contents of this area therefore is of considerable scientific and practical significance. We collected and analyzed fissure water and river water samples from the NDM to investigate the hydrochemical characteristics and trace element contents of natural waters in this area, and thereby attain a better understanding of the Se, F, and As concentrations found in drinking water in this Se-rich, F- and As-high region. To monitor the stability of content of elements in water, we collected and analyzed fissure and river water samples in May, July, and September, 2015, respectively. Finally, we aimed to quantify, at least approximately, the Se-rich water resources of the NDM.

The NDM belongs to the southern margin of the Qinling Fold Belt, covering Ziyang, Lan'gao, and Pingli counties. The strata generally lie in a northwest–southeast direction, with repeated sequences of faults, folds, and strata (Figure 1(c)). The Late Neoproterozoic and Early Paleozoic strata are well developed in this area. The Late Neoproterozoic strata mainly contain volcanic rocks. The Cambrian stratigraphy is dominated by thick carbonaceous strata, containing carbonaceous siliceous slate and carbonate rock, marl, limestone, and anthracite. The Ordovician stratigraphy is composed of carbonaceous argillaceous clastic rocks and carbonate rocks. Carbonaceous slate, sandy slate, and silty fine sandstone dominate the Silurian strata (Luo et al. 1994).

Lan'gao County is a fluorosis high-incidence area, and experiences high levels of arsenic poisoning as well as being the focus of local Se poisoning (Luo et al. 2004a, 2004b; Li et al. 2005). It is also a key region in the Chinese poverty alleviation program (Cao et al. 2015). The water system is well-developed in this region (Figure 1(b) and 1(c)). The natural bedrock fissure water and river water are major local sources of drinking water.

Sixty-two water samples, comprising 47 fissure water and 15 river water samples, were collected from the NDM in May 2015. Forty-nine (31 fissure water and 18 river water samples) and 61 samples (47 fissure water and 14 river water samples) were also collected at the same area in July and September 2015, respectively. Residential drinking water at present includes untreated fissure water or river water, so these samples reflected NDM drinking water to some extent (Table S1, available with the online version of this paper).

The collection of samples was conducted in accordance with the Monitoring and Analytical Methods for Water and Waste Water (MEP 2002). Samples were analyzed at the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences. The pH and total dissolved solids (TDS) were determined using a Switzerland Mettler Toledo pH tester (SevenGo SG2) and a Switzerland Mettler Toledo Ec tester (SevenGo SG3). The total hardness (TH) was calculated using the measured values of Ca²⁺ and Mg²⁺ concentrations. The alkalinity (as HCO₃⁻) was measured using acid–base titration (MH 1985; MEP 2002).

Dionex ICS-900 ion chromatography was used to analyze the chloride (Cl⁻) and fluoride (F⁻) concentrations. Se and As concentrations were determined using a hydride Generation Atomic Fluorescence Spectrometer (MH 1985; MEP 2002). The concentrations of the major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺, P, Sr, B, and SiO₂) and the SO₄²⁻ anion were determined using an Elan DRC-e Inductively Coupled Plasma Optical Emission Spectrometer. The analyses of trace elements (Li, Zn, U, Rb, Ba, Bi, Co, Cs, Ga, In, Ti, V, Ag, Al, Be, Cd, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, and Tl) were performed by a Perkin-Elmer Optima 5300DV Inductively Coupled Plasma Mass Spectrometry. Parallel samples and repeated measurements were used. The percentage errors in all the samples were relatively low, and mostly <± 5% (Figure S1, available with the online version of this paper).

The box plot was plotted by Origin 8.0 software based on the data of element concentrations in the water. In addition, the 1st, 5th, and 25th percentiles, median, mean, and the 75th, 95th, and 99th percentiles were indicated in each box plot, respectively. The Pearson correlation coefficients analysis was carried out using IBM SPSS 20 software.

Due to the fact that the sampling points of the three times are in the same area, and the main characteristic of the water is consistent, the detailed analysis focused on the May samples.

With reference to toxic elements, the concentrations of Be, Cr, Mn, Cu, As, Mo, Ag, Sb, Hg, and Pb were lower than both CNS and INS stipulations (Table 2, Figure 4). The arsenic content was in the range of 0.33–6.29 μg/L (As_avg = 2.88 μg/L). Al concentrations in seven fissure water and two river water samples exceeded CNS limits (200 μg/L). The Fe concentrations of five samples (326.74–484.48 μg/L) were higher than CNS guidelines (300 μg/L). The Ni content of five fissure water samples (20.32–24.58 μg/L) was slightly higher than CNS limits (20 μg/L), but lower than INS guidelines (70 μg/L). Cd content was lower than CNS guidelines, while five fissure water samples (3.37–4.41 μg/L) showed concentrations slightly higher than CNSM limits (3 μg/L). The Tl content of 11 fissure water samples (0.11–0.49 μg/L) was higher than CNS limits (0.1 μg/L) (Table 2, Figure 4).

The water of the study area was mainly neutral to weakly alkaline. The TDS, TH, and major ion concentrations were lower than the limits outlined by CNS (MH 2006) (Figures 2 and 3, Table 3). The low concentrations of F and As found in natural water in the study area indicate that these waters are not the source of the region's endemic fluorosis and arsenicosis, and that the water is safe for drinking (Tables 1 and 2).

The Se content of eight samples (10.16–23.58 μg/L) and the Sr concentrations in eight samples (0.21–0.45 mg/L), all met CNSM standards (GAQS 2008) (Table 1, Figure 4). Of these, the Se (13.47 μg/L, 12.89 μg/L) and Sr contents (0.21 mg/L, 0.21 mg/L) of Sample Nos. 2 and 3 from Candle Mountain met the CNSM guidelines (GAQS 2008); their toxic element levels were lower than CNS and INS limits (MH 2006; WHO 2011) (Figure 4).

The World Health Organization has stated that Se is an indispensable nutrient and essential to human life (WHO 1996). The Chinese Nutrition Society has also listed Se as a vital dietary nutrient (Cheng et al. 2009). However, Se is a scarce resource. About 0.5–1 billion people in the world are affected by Se deficiency (Haug et al. 2007). Nearly 300 million people in China are affected by a lack of Se in their diet (Yang 1994; Winkel et al. 2011; Zhang et al. 2014).

Drinking water is crucial for human survival. However, water quality survey data show that Se concentrations are often very low (<1 μg/L) in natural water, and only ∼0.02–10 μg/L in freshwater (Schutz & Turekian 1965; Bowen 1966). For instance, Se content is 0.14 μg/L in the Mississippi River, 0.21 μg/L in the Brazilian Amazon River, and 0.04–5 μg/L in the rivers of China (Wang & Gao 2001).

Europe has the longest commercial mineral water history in the world (Wynn et al. 2009). A recent study found that, following European Union legislation, 58.1% of 571 bottles of mineral water from 23 European countries could be defined as ‘suitable for a low-sodium diet’, while only 1.2% of the total samples exceeded the Se limit (>10 μg/L) (Bertoldi et al. 2011). As of February 2015, the total output of the Chinese mineral water market was 5.36 million tons, giving the trade a market value of ¥15.2 billion RMB (DFCMW 2015). However, metasilicate, strontium, and carbonated mineral water types accounted for 90% of that market (Wang 2015). Ninety percent of the remaining categories of mineral water were of lithium, zinc, and iodine types. Se-bearing mineral waters were even rarer (An 1992; Zhang 2009; DFCMW 2015).

CNSM guidelines stipulate that Se content should be 10–50 μg/L (GAQS 2008). At present, only one brand of Se-rich mineral water, i.e., ‘Rare Water’, whose Se content is >10 μg/L (ranging from 25 to 37 μg/L), has been developed for the Chinese market. Its costs ¥13RMB per 380 mL, which is about ten times the price of ordinary mineral water (¥1–2RMB per 500 mL). The development of natural Se-rich mineral water would clearly have both economic benefits and market prospects. The discovery of high-Se water sources is therefore of important practical significance.

In November 2011, the China Rural Poverty Alleviation and Development Program (2011–2020) was promulgated. This focused on the need for a clear assessment of all ecological assets as well as improvements in ecological and environmental protection work in poverty-stricken areas. The QDM region is typical of such impoverished and underdeveloped areas (Cao et al. 2015). Our systematic investigation of high-Se water resources in the QDM region has focused on both the evaluation of ecological assets and poverty alleviation. The results of this study show that Se and Sr concentrations in natural fissure water from the Lan'gao area were moderate in value. The Se content of eight water samples was 10.16–23.58 μg/L. Of these, the Se (13.47 μg/L, 12.89 μg/L) and Sr contents (0.21 mg/L, 0.21 mg/L) of two samples from Candle Mountain, and the Se content (14.79 μg/L) of one sample from Minzhu Town met the CNSM guidelines (GAQS 2008), with limited element concentrations lower than both the CNS and INS measures (MH 2006; WHO 2011). The average Se content of river water was 2.82 μg/L, 14 times higher than that of global surface water (0.2 μg/L).

However, the concentrations of some limited elements should be considered during the exploitation of high Se-rich mineral water in the NDM. Cd, Ni, Tl, Fe, and Al concentrations correlated significantly with Se content (r= 0.73, 0.71, 0.51, 0.36, and 0.36; P < 0.01) (Table 4). Of the eight Se-rich fissure water samples (Se >10 μg/L), the Ni content of two samples (22.78, 22.95 μg/L) and the Tl content of five samples (0.11–0.2 μg/L) were slightly higher than the CNS guidelines (Ni < 20 μg/L, Tl < 0.1 μg/L) (Figure 4), suggesting that the concentrations of these elements in potable water should be taken into account.

Human activities and climate change have brought about significant changes to water resources and sea level (Xu 2005; Baba et al. 2011). However, although some trace elements, such as selenium, are essential for human health and the ecosystem of rivers, few researchers have paid attention to this, which might be due to trace elements in water being difficult or expensive to measure accurately (Selinus et al. 2013).

The NDM region remains relatively ecologically intact, with a high forest coverage (70% in the hinterland), no large industrial or mining enterprises, good atmospheric quality, and unpolluted natural soil and biological resources (Wang 2004; Liu et al. 2015). Consequently, element content in the Lan'gao area has been principally affected by natural factors, with relatively weak human influence. Therefore, it is the ideal area to study element content variation and its responses to climate change.

The contents of the limited elements were relatively stable. The Sr concentrations gradually increased from May to September (Figure 8). The mean concentrations of Cd, Ni, Fe, and Al were stable and lower than the INS and CNS (Figure 8). The Tl content was a little higher in July and September than in May, therefore the long-term toxic impact of Tl in drinking water on the health of local people should be noted.

The population of the NDM of Shaanxi Province suffers from endemic fluorine and arsenic poisoning (Luo 2011; Gong et al. 2012). Drinking water is one of the important ways of trace element intake for humans (Bleiman & Mishael 2010). In order to verify the stability of fluorine and arsenic contents in water, six field trips were conducted in the study area from May 2015 to March 2016. A total of 218 water samples were collected in May, July, September, October, and December in 2015, and March in 2016 (Tables S3 and S4, available with the online version of this paper). The results indicated that the arsenic and fluoride concentrations in natural water were lower than the CNS and INS standards. Hence, F and As in natural water were not the sources of fluorosis and arseniasis in the NDM.

Stone coal (combustible black shale) is widely used for domestic purposes in the NDM (Luo 2011). The contents of F, As, and Se in the stone coal are about 10–50 times more than in common humic coal (Luo et al. 2004a; Zheng et al. 2007; Luo 2011; Xu & Luo 2012), and higher than the average contents of the continental crust (Selinus et al. 2013). Moreover, F and As emission rates are about 80% and 70%, respectively, during stone coal burning indoors (Ren et al. 2006; Yu et al. 2009).

There have been some studies of Se, As, and F content, distribution, and occurrence status in rock and soil in the NDM (Luo et al. 2004a; Li et al. 2005; Xue et al. 2010; Gao et al. 2011; Xu & Luo 2012; Chen et al. 2014). Se-enriched strata are exhibited in the Lower Cambrian and Daguiping Formations of the Silurian (Chen & Luo 1996; Li et al. 2005; Luo 2011). Fluorine and arsenic contents of the Early Paleozoic strata, especially in the black shales and phosphate rocks of the Early Cambrian Lujiaping Formation of the Daba region, are approximately several times higher than the average value found in the continental crust (Luo et al. 1995; Xu & Luo 2012).

Based on a systematic investigation of the hydrochemical characteristics and trace element contents of fissure and river water in the NDM, this study found that drinking the natural water is not the reason for the endemic fluorine and arsenic poisoning. According to previous research, high fluorine and arsenic content in burning stone coal might be the main explanation of endemic fluorosis and arsenic poisoning in the NDM (Luo 2011; Gong et al. 2012). Therefore, to establish the relationship more accurately between indoor combustion of high fluorine and arsenic content in stone coal and the health effect in this region will be the aim of further work.

The natural water in the Lan'gao area of the NDM is of relatively good quality and is suitable for drinking. F and As concentrations in natural water were lower than CNS and INS guidelines, indicating that fluorine and arsenic in drinking water are not the source of the area's endemic fluorosis and arsenicosis.

The average Se content in fissure water was 5.52 μg/L. There were eight samples where Se content reached the CNSM guidelines (>10 μg/L). The Sr concentrations of eight samples also met the CNSM guidelines (>0.2 mg/L), with the highest value being 0.45 mg/L. It is noteworthy that the Se (13.47 μg/L, 12.89 μg/L) and Sr contents (0.21 mg/L, 0.21 mg/L) of two samples met the CNSM guidelines, but these limited elements were at the same time lower than both the CNS and INS guidelines.

The Se and Sr contents are relatively stable and reveal an increasing tendency along with the increase of humidity. The high quality low-sodium Se-rich mineral water of the NDM should be able to be commercially exploited, but prior to this, the hydrological geology of this area will require further detailed study.

This research was supported by the National Natural Science Foundation of China (Grant No. 41172310, 41472322), and the National Key Basic Research Program of China (973 Program, Grant No. 2014CB238906). We gratefully acknowledge the editor and anonymous reviewers for their constructive comments and suggestions. The authors thank Zhong Yougen, Duan Wenxun and the staff members of the Guangdong Hillpower Technology Co. Ltd for their active assistance in collecting the samples and Tian Xinglei, Wang Shaobin, and Pu Haixia for their cooperation in drawing technologies and language modifications.