Comparative study of physico-chemical parameters of drinking water from some longevity and non-longevity areas of China

Water is an important component of the human body, providing many essential macro- and micro-elements and playing an important role in human health (Nkono & Asubiojo 1997). Hamilton et al. (1973) found a similarity between the abundance of elements present in human tissues and average crystal rocks. Drinking water is also important for the environmental influences of human health (Nkono & Asubiojo 1997).

Increasing attention is being paid to the quality of drinking water worldwide. Water quality can be affected by the natural background and anthropogenic activities. Natural sources of elements are largely the result of chemical weathering of rock or volcanic activity, both of which can have considerable spatial variability (Ritter et al. 2002). In some rural areas in China, there is no official tap water and local residents usually drink water from mountain springs or river water. The quality of drinking water in these areas has a long-term impact on local residents’ health.

The chemical and physical properties of elements influence their availability for uptake by biological organisms and their potential to be toxic to organisms, as well as transport mechanisms in natural waters (Ritter et al. 2002). Drinking water contains various micro-elements, some of which have been widely implicated in human health and disease (Kumar et al. 2016). Elements such as fluorine (F), iron (Fe), manganese (Mn), copper (Cu), cobalt (Co), molybdenum (Mo), zinc (Zn), chromium (Cr), selenium (Se), nickel (Ni), vanadium (V), and silicon (Si) are essential for the growth of human and other living organisms. Even though some of the micro-elements are essential to man, at elevated levels both essential and non-essential elements can pose a hazardous risk to humans (WHO 1996). Elevated level of F in drinking water can result in problems of increased dental caries and of the development of bone deformities (Zhang et al. 2007), and long-term exposure to arsenic in drinking water at elevated levels can result in skin problems, kidney and lung cancers (Smith et al. 2002).

There is an obvious regional longevity phenomenon in China (Xin-min 2006). The distribution of centenarians in China is very uneven, and the longevity counties are geographically concentrated in South China. Bama, Donglan, Fengshan, Rugao, Chengmai, Lingao, and Sanshui are famous longevity counties in South China. The occurrences of centenarians (OC) (the number of centenarians per 100,000 inhabitants) in these counties are higher than the average level of China and most of the other counties in China. The elemental contents of drinking water in longevity counties in China have been studied by many researchers, but the study areas have always been limited to one longevity county. Until now, the characteristics of elemental contents of drinking water in longevity areas of South China and the difference to non-longevity areas in China have not been clear. Therefore, this study was carried out to investigate the elemental contents of drinking water in the longevity areas of South China and the differences compared to the non-longevity counties in China.

China is divided into South and North China by Kunlun-Qinling Mountain and the Huaihe River. In South China, the annual precipitation is higher than 800 mm, and the climate belongs to the subtropical monsoon generally. The annual precipitation in North China is lower than 800 mm and the climate belongs to the temperate continental monsoon generally. The study areas are located in South China, except for Huxian, Xinxiang, and Xilinhot counties which are located in North China. More details of the natural geography of those counties are shown in Table S1 (available with the online version of this paper).

A field survey was conducted to collect the drinking water samples from longevity and non-longevity counties of South China. For comparative analysis the drinking water was also collected from the non-longevity counties of North China. First, the percentage of centenarian to county levels was calculated from the national population census (NBS 2010). A total of 128 samples (Table 1) was collected that included 86% of groundwater (spring water and well water) and 14% of surface water (lake water, river water, and reservoir water). Samples collected from longevity areas accounted for 31% and the other 69% was collected from non-longevity areas.

The samples were collected in colorless polythene plastic bottles that had been washed once with distilled water and three times with sample water; this way the chances of contamination in the sample collection would be small. All water samples were stored in clean plastic bottles at 4 °C before further analysis. Details of the sampling method can be found in Methods for Chemical Analysis of Water and Waste Water (MEPPRC 2002).

The concentrations of Ca, Mg, Na, K, SO²⁻₄, SiO₂, B, P, Sr were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (PerkinElmer, Optima 5300 DV) and elements Al, Ba, Be, Cd, Co, Cr, Cu, Li, Mn, Ni, Pb, Tl, V, Cs, Ga, In, Rb, U, Bi, Mo, Zn, and Fe were tested by ICP-mass spectrometry (ICP-MS) (PerkinElmer, Elan DRC-e). The pH, electrical conductivity (EC), and oxidation–reduction potential (Eh) were determined in situ using a SevenGo SG2/3 analyzer (Mettler Toledo Company, Shanghai, China). Bicarbonate (HCO⁻₃) was determined using an acid–base titration method within 3 days of sampling. Nitrate (NO⁻₃) and chlorine (Cl) concentrations in water were determined by ion chromatography (Dionex Inc., Dionex-ICS-900). Total dissolved solids (TDS) were equal to the sum of ions except bicarbonate (HCO⁻₃) and half of HCO⁻₃. Total hardness (TH) was calculated by the concentration of Ca and Mg.

For determining selenium (Se), 10 mL samples and 5 mL concentrated hydrochloric acid were added into 20 mL glass test tubes and heated in a boiling water bath for 40 minutes. For determining arsenic (As), 10 mL samples, 1 mL concentrated hydrochloric acid, and 1 mL reducing agent (2.5% thiourea + 2.5% ascorbic acid) were added, and allowed to stand for 40 minutes. Both Se and As were determined by hydride generation atomic fluorescence spectrometry (HG-AFS; Beijing Haiguang Instruments Co. Ltd).

The concentrations of elements were calculated by a five-point calibration curve with linearity of all target elements higher than 0.999. The precision of analyses was tested at regular intervals by running duplicate analyses of selected samples and internal standards. The precision and reproducibility were found to be within the confidence limit of 98%. The detection limit of ICP-MS was <0.1 μg/L and that of ICP-OES and ion chromatography <0.1 mg/L. The detection limit for As and Se was <0.01 μg/L. All the samples were determined in the Geochemistry Laboratory of the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences.

The Mann–Whitney U nonparametric test was applied as a hypothetical test because the distribution of data did not follow normal distribution or logarithmic distribution. Statistical Package of Social Science version 20 and Excel 2010 were used for the statistical analysis.

The OC (the number of centenarians per 100,000 inhabitants) is an important indicator of the longevity (Magnolfi et al. 2007; Stefanadis 2010; Willcox et al. 2010). However, OC was stongly affected by migration and birth rate (Magnolfi et al. 2007; Liu et al. 2014). To maintain OC, four new indices, namely, LI% (ratio of ultranonagenarians to those above 65 years old), CI% (ratio of centenarians within the ultranonagenarians), UOI% (ratio of people above 80 years old), UC (number of centenarians per 10,000 over 65 year-old subjects) were applied.

The longevity indices of OC, CI%, and UC in Guangxi province and Hainan province are significantly higher than those of other counties of China (Table 2). Bama and Lingao are typical longevity counties of China as almost all their indices are relatively higher. The differences of longevity indices are apparent and all five longevity indices of South China are all higher than those of North China. A thought-provoking phenomenon is that OC, CI%, and UC increase consecutively from north-west to south-east in the order of Huaiji, Guangning, Sihui, and Sanshui respectively. The OC, CI% and UC in Sanshui are 12.1-fold, 9-fold, and 14.2-fold, respectively, as compared to northern Huaiji.

Table 3 shows regular parameters and macro-elements of drinking water in longevity areas and non-longevity areas in South China as well as non-longevity areas in North China.

The pH value of drinking water in longevity areas of South China are 6.62 ∼ 8.01 with a median of 7.42, indicating drinking water in longevity areas of South China are slightly alkaline. The TDS of drinking water in longevity areas in South China ranges between 35.43 and 1,014.66 mg/L, varying from fresh water (TDS < 1,000 mg/L) to brackish water (1,000 < TDS <2,000 mg/L). The TH of drinking water is 20.14 ∼ 594.13 mg/L, varying from soft water (TH < 75 mg/L) to high-grade hard water (TH > 450 mg/L). The order of relative abundance of major cations of drinking water in longevity areas of South China is Ca > Na > Mg > K (on a mg/L basis), while that of anions is HCO⁻₃ > SO²⁻₄ > NO⁻₃ > Cl. The concentration of micro-elements varies greatly (Table 4). The sum concentration of micro-elements (SCME) is 138.26 ∼ 2,269.66 μg/L with a median of 358.28 μg/L, and the top three micro-elements in decreasing order according to median value are Fe (108.66 μg/L) > Sr (82.1 μg/L) > Zn (7.97 μg/L). The ranges of the sum concentration of essential micro-elements (SCEE: Fe, Zn, Cu, Mn, Cr, Mo, Co, Ni, V, Se) and sum concentration of hazardous elements (SCHE: As, Cd, Pb) are more than one magnitude with a median of 160.17 μg/L and 1.26 μg/L, respectively. The ranges of the ratio of SCEE in SCME (SCEE_SCME) and the ratio of SCHE in SCME (SCHE_SCME) are within two magnitudes with a median of 0.4 and 0.035, respectively.

The pH value of drinking water in non-longevity areas in South China is 5.17 ∼ 9.18 with a median of 7.07, indicating drinking water in non-longevity areas in South China is slightly alkaline or weakly acidic. The samples of drinking water in non-longevity areas in South China are all fresh water with TDS ranges between 18.41 and 719.28 mg/L. The TH of drinking water is 0.63 ∼ 471.15 mg/L, varying from over two orders of magnitude from soft water to hard water. The order of relative abundance of major cations of drinking water in non-longevity areas in South China is Na > Ca > Mg > K (mg/L), while that of anions is HCO⁻₃ > Cl > SO²⁻₄ > NO⁻₃. The ranges of SCME, SCEE, SCHE, SCHE_SCME are no less than two magnitudes with a median of 204.53 μg/L, 75.16 μg/L, 0.96 μg/L, 0.004, respectively. The ranges of SCEE_SCME are within one magnitude with a median of 0.35. The top three micro-elements in decreasing order according to median value are Fe (42.04 μg/L) > Al (26.99 μg/L) > Ba (22.71 μg/L).

The pH value of drinking water in non-longevity areas in North China is 7.42 ∼ 8.38 with a median of 7.71, indicating drinking water in longevity areas in North China is slightly alkaline. The TDS of drinking water in longevity areas in North China ranges between 172.16 and 1,725.65 mg/L, varying from fresh water to brackish water. The TH of drinking water is 59.2 ∼ 755.81 mg/L, varying from over two orders of magnitude from soft water to high-grade hard water. The order of relative abundance of major cations of drinking water in non-longevity areas in North China is Na > Ca > Mg > K (mg/L), while that of anions is HCO⁻₃ > Cl > SO²⁻₄>NO⁻₃. The concentration of micro-elements varies greatly. The ranges of SCME, SCEE, SCHE, SCEE_SCME are within one magnitude with medians of 1,171.79 μg/L, 158.65 μg/L, 17.12 μg/L, 0.14, respectively. The range of SCHE_SCME is within one magnitude with a median of 0.01 and the top three micro-elements in decreasing order according to median value are Sr (748.65 μg/L) > B (90.75 μ/L) > Fe (80.1 μg/L).

The quality of drinking water was evaluated based on recommended standards of the World Health Organization (WHO 2011) and National Standards for Drinking Water Quality of the People's Republic of China (MHPRC 2006).

In the longevity areas of South China, TDS, TH, As, Cr, Tl, Zn, and NO⁻₃ are 2.5%, 7.5%, 7.5%, 2.5%, 2.5%, 5%, and 7.5%, respectively, higher than the highest limits set by the National Standards for Drinking Water Quality of the People's Republic of China (MHPRC 2006), while their highest values are 1.01-fold, 1.32-fold, 5.63-fold, 1.01-fold, 1.98-fold, 1.95-fold, and 2.75-fold, respectively, as compared with the National Standards for Drinking Water Quality of the People's Republic of China (Tables 3 and 4).

In non-longevity areas of South China, TH, As, Mn, Tl, and NO⁻₃ are 1.35%, 12.16%, 4.05%, 6.76%, and 11.63%, respectively. These parameters in drinking water samples were higher than the highest limits of the standards set by the National Standards for Drinking Water Quality of the People's Republic of China (MHPRC 2006), while their highest values are 1.05-fold, 5.64-fold, 12.13-fold, 1.77-fold, and 3.86-fold respectively, as compared with the National Standards for Drinking Water Quality of the People's Republic of China. The pH in 20.27% of drinking water in non-longevity areas in South China is lower than the lowest limit of 6.5, and the highest and lowest values are 1.08-fold and 0.88-fold, respectively, by comparison with the highest limit (pH = 8.5) and lowest limit (pH = 6.5).

In non-longevity areas of North China, TDS, TH, As, Fe, Na, SO²⁻₄, Cl⁻ and NO⁻₃ are 14.29%, 28.57%, 78.57%, 7.14%, 14.29%, 14.29%, 14.29%, and 11.11% of drinking water samples are higher than the respective limits set by the National Standards for Drinking Water Quality of the People's Republic of China (MHPRC 2006) and their highest values are 1.73-fold, 1.68-fold, 4.06-fold, 1.41-fold, 1.21-fold, 2.56-fold, 1.6-fold, and 1.16-fold, respectively, when compared with the National Standards for Drinking Water Quality of the People's Republic of China.

The parameters of drinking water from longevity areas and non-longevity areas in South China varied greatly. Mann–Whitney U nonparametric test was applied and the results showed significant difference (p < 0.05) in pH (1.05-fold, median value of longevity areas in South China was 1.05-fold that of non-longevity areas in South China; p = 0.011), EC (8.1-fold, p < 0.001), major iron HCO⁻₃ (8.79-fold, p < 0.001), Ca (19.36-fold, p < 0.001), Mg (1.63-fold, p < 0.001), SiO₂ (0.56-fold, p = 0.014), SO²⁻₄ (4.38-fold, p < 0.001), Cl (1.75-fold, p = 0.005), NO⁻₃ (3.31-fold, p = 0.028), and micro-elements Se (0.25-fold, p < 0.001), As (1.58-fold, p = 0.018), Al (0.22-fold, p = 0.002), B (BD, median value of longevity areas in South China was below detection, p < 0.001), Ba (0.29-fold, p < 0.001), Co (2.88-fold, p < 0.001), Cu (BD, p < 0.001), Ni (BD, p = 0.002), P (BD, p < 0.001), Tl (0.29-fold, p = 0.004), V (BD, p < 0.001), Cs (0.25-fold, p < 0.001), Ga (0.21-fold, p < 0.001), Rb (0.37-fold, p = 0.003), U (6.55-fold, p = 0.006), Sr (11.02-fold, p < 0.001), Fe (2.58-fold, p < 0.001), SCME (1.75-fold, p < 0.001) and SCEE (2.13-fold, p < 0.001).

Mann–Whitney U nonparametric test was applied on parameters of drinking water from longevity areas in South China versus non-longevity counties in North China, and the results showed significant difference (p < 0.05) in pH (0.96-fold, median value of longevity areas in South China was 0.96-fold that of non-longevity areas in North China, p < 0.001), EC (0.47-fold, p = 0.004), HCO⁻₃ (0.56-fold, p = 0.018), Mg (0.09-fold, p = 0.006), Na (0.09-fold, p < 0.001), SiO₂ (0.51-fold, p = 0.003), SO²⁻₄ (0.21-fold, p = 0.009), NO₃ (9.03-fold, p = 0.022), Se (0.36-fold, p = 0.003), As (0.06-fold, p < 0.001), B (0.05-fold, p < 0.001), Ba (0.13-fold, p < 0.001), Be (0-fold, p = 0.026), Cd (0.52-fold, p = 0.034), Cu (0.26-fold, p = 0.009), Li (0.06-fold, p = 0.001), P (0.62-fold, p = 0.007), Pb (3.7-fold, p = 0.012), Tl (BD, median value of non-longevity areas in North China was below detection, p = 0.043), V (0.01-fold, p < 0.001), Cs (1.72-fold, p = 0.014), Ga (0.15-fold, p < 0.001), Rb (3.22-fold, p = 0.001), U (0.27-fold, p < 0.001), Sr (0.11-fold, p < 0.001), Mo (0.03-fold, p < 0.001), SCME (0.31-fold, p < 0.001), SCHE (0.07-fold, p < 0.001), SCEE_SCME (2.86-fold, p < 0.001), and SCHE_SCME (0.33-fold, p < 0.001).

Mann–Whitney U nonparametric test was also applied on parameters of drinking water from non-longevity areas in South China versus non-longevity counties in North China, and the results showed significant difference (p < 0.05) in pH (0.92-fold, median value of non-longevity areas in South China was 0.92-fold that of non-longevity areas in North China, p < 0.001), EC (0.06-fold, p < 0.001), Eh (1.12-fold, p = 0.007), HCO⁻₃ (0.06-fold, p < 0.001), Ca (0.05-fold, p < 0.001), Mg (0.06-fold, p < 0.001), Na (0.06-fold, p < 0.001), Cl (0.05-fold, p = 0.021), SO²⁻₄ (0.05-fold, p < 0.001), As (0.04-fold, p < 0.001), B (0-fold, p < 0.001), Ba (0.44-fold, p < 0.001), Be (0-fold, p = 0.02), Cd (0.39-fold, p = 0.007), Co (0.36-fold, p = 0.001), Cr (0.34-fold, p = 0.002), Cu (0-fold, p < 0.001), Li (0.09-fold, p < 0.001), Ni (0-fold, p = 0.006), P (0-fold, p < 0.001), Pb (5.15-fold, p < 0.001), Tl (0-fold, p < 0.001), V (0-fold, p < 0.001), Cs (6.83-fold, p < 0.001), In (0-fold, p = 0.006), Rb (8.6-fold, p < 0.001), U (0.04-fold, p < 0.001), Sr (0.01-fold, p < 0.001), Mo (0.01-fold, p < 0.001), Fe (0.52-fold, p = 0.009), SCME (0.17-fold, p < 0.001), SCEE (0.47-fold, p < 0.001), SCHE (0.06-fold, p < 0.001), SCEE_SCME (2.5-fold, p < 0.001), and SCHE_SCME (0.37-fold, p < 0.001).

Drinking water samples from both longevity areas in South China and non-longevity areas were mainly hard water. The percentage of very soft water (TH <75 mg/L), soft water (75 < TH <150 mg/L), hard water (150 < TH <450 mg/L), and very hard water (TH >450 mg/L) in longevity areas in South China were 22.5%, 22.5%, 47.5%, 7.5%, and that of non-longevity areas in South China and non-longevity areas in North China were 78.4%, 6.8%, 13.5%, 1.4%, and 14.3%, 28.6%, 28.6%, 28.6%, respectively.

The concentrations of As were decreased gradually in Huaiji, Guangning, Sihui, and Sanshui in the order of north-west to south-east, which were in agreement with OC, CI%, and UC, while Cl was the opposite (Table S2, available with the online version of this paper). The concentrations of As and Cl in drinking water in Sanshui were 0.3-fold and 12.35-fold that of Huaiji, respectively.

The drinking water of both longevity areas in South China and non-longevity areas in North China was mainly hard fresh water, while that in non-longevity areas in South China was mainly soft fresh water. Most median concentrations of elements in non-longevity areas in South China were lower than those of longevity areas in South China and non-longevity areas in North China. Generally, by comparison with non-longevity areas in China, the drinking water in longevity areas of South China has high value SCEE_SCME and low value SCHE_SCME. Compared with non-longevity areas in South China, the concentrations of Sr and TH in drinking water in longevity areas of South China were significantly higher, while the same trend was found in non-longevity areas in North China. In the longevity areas of South China, the concentration of TH and Sr was observed at 157.82 mg/L and 82.1 μg/L, respectively, and considered as the optimum level for human health. The elemental concentrations of As were decreased gradually in Huaiji, Guangning, Sihui, and Sanshui in the order of north-west to south-east in agreement with OC, CI% and UC, while Cl^- was the opposite.

Support for this research was provided by the National Natural Sciences Foundation of China (Grant no. 41472322, 41172310) and the National Basic Research Program of China (973 Program) (Grant no. 2014CB238906). The authors declare that they have no conflict of interest.