4.1. Temporal and Spatial of the Longevity Population in Hechi
Data from the National Population Census in 1982, 1990, 2000, and 2010 showed that Hechi tended to have the leading centenarian prevalence in China and there were stable areas of centenarian clustering in Hechi across the decades. Larger proportions of centenarians tended to be located in areas of longevity in southwestern Hechi, including Bama, Donglan and Fengshan counties, especially Bama, than in other areas. However, the socio-economic index (mainly referring to the average GDP per capital) in the Hechi area was comparatively lower than that of other cities in Guangxi [
35,
36]. In addition, the socio-economic index in the longevity counties in the southwestern area of Hechi was lower than that of other non-longevity counties from 1982 to 2010. The results indicate that from a long-term perspective, the number of centenarians in Hechi may be more influenced by local environmental factors than by economic factors. From 1990 to 2010, Moran’s
I values for the centenarian prevalence and longevity index in Hechi were greater than 0.3, with significant spatial autocorrelations. Hechi is located in a relatively remote mountainous area that is isolated from the outside world, suggesting that the natural environment has a significant impact on regional longevity [
24,
35,
36]. Within this context, an excellent natural environment is possibly the key contributor to a large elderly population, especially regarding the distribution of centenarians, as reported by many studies on longevity [
3,
4]. Apart from the environmental factors, some additional factors might be also important for longevity in isolated mountainous areas. For example, Pes et al. considered the lifestyle such as suitable amount of physical activity in mountainous terrain might be a reason that reduce the associated illness and promote longevity in Sardinia, Italy; and that could be important insight to understand the longevity phenomenon in mountainous areas [
37].
4.2. Trace Element and Mineral Levels in Drinking Water and the Soil Environment
Based on this study, the high quality of the drinking water and soil environment may be two important factors contributing to high longevity prevalence in Hechi from the environmental perspective. The average contents of test quality indicators satisfied both Chinese drinking water quality guidelines (GB5479-2006) and WHO guidelines. The drinking water in Hechi exhibited a weakly alkaline range, with a pH of approximately 7.25, and it was rich in trace elements and minerals such as H2SiO3, Ca, Fe, Na, Mg, Li, Mo and Se and low in heavy metals such as Pb and Cd. As a result, the drinking water in Hechi was generally good. The soil environment in Hechi was also generally good; however, Cd, Zn, Cu, Cr, Pb and Ni levels exceeded the soil standard in some samples (GB15618-2008). The reason is that the high-value areas are located in the northeastern region within the non-ferrous metal vein areas of Hechi. In those areas, the background concentrations of heavy metals were higher than in other areas, as non-ferrous metal exploitation has increased the levels of these metals in soil.
4.3. Relationship between Regional Longevity and Trace Element and Mineral Levels in Drinking Water and the Soil Environment
According to correlation analysis, the longevity index in Hechi were significantly correlated with H2SiO3, Ca, and Fe levels in drinking water and positively correlated with Na, Mg, Li, Mo, and Se levels. In contrast, Pb, Cd, SO42−, Mn, and Zn had a negative correlation with the two longevity indicators. Regarding soil, there was a significant negative correlation between Sr and each of the longevity indicators. In addition, there were positive correlations between each of Mo, Fe, Co, Zn, and Cu and the longevity index regarding the results of Pearson correlation analysis, the effect of trace elements in drinking water on regional longevity is more direct and obvious than the effect of trace elements in the soil environment.
From a drinking water perspective, the alkalinity and trace element and mineral levels of drinking water in Hechi was closely related to the regional longevity observed. Many studies have confirmed that slightly alkaline drinking water, close to the level in the human body and the plasma pH, can keep blood vessels elastic and maintain blood pressure, which is beneficial for the cardiovascular system [
16,
17]. Regarding the trace elements, mineral H
2SiO
3 (silicate) can maintain blood vessel health and prevent arteriosclerosis, coronary heart disease and hypertension [
38]. Fe is a component of myoglobin and hemoglobin; Fe is also a component of catalase and peroxidase, affects energy metabolism and may have a positive effect on human health. Additionally, moderate levels of Ca and Mg in drinking water can help to avoid geriatric diseases and conditions such as cardiovascular disease [
39], hypertension and osteoporosis and thereby promote longevity [
17]. For example, the population in Nicoya Peninsula has been regarded as a longevity region in Costa Rica, and its drinking water was rich in Ca and Mg [
40]. The high Ca in the drinking might contribute to strong bones and reduce other associated diseases among the elderly population; and this was regarded as one of the most important contributors for the longevity phenomenon [
40]. In addition, in Taiwan, Yang et al. have also confirmed that Ca in drinking water could reduce the risk of death from acute myocardial infarction and the risk cardiovascular diseases [
41].
Nonetheless, the relationship between longevity indicators and Se in drinking water was weak compared to that of the other indicators, as shown in
Table 5. Previous studies have confirmed that trace elements such as Se can play decisive roles in maintaining human health and preventing many age-related diseases [
42,
43]. For example, the trace element Se functions as a potent anti-oxidant that protects the body from oxygen free radicals and slows the aging process in humans [
17]. In addition, Se and its associated compounds could also help to reduce the incidence of Keshan disease. Thus, appropriate amounts of Se in food and in drinking water may help to promote human lifespan and prevent many diseases. Our results suggest Se in drinking water might not contribute as much as other indicators such as H
2SiO
3, Ca, Fe, and Na, in Hechi under the karst landscape.
In addition, Li is known to exert life-prolonging effects to human [
44,
45]. In the study, lithium in drinking water in Bama was relatively higher compared with other areas in Hechi and this might be one of the reasons that contribute to its excellent longevity. Other studies have confirmed that Li in drinking water may have positive impacts on regional longevity and human health, consistent with the conclusions of this study. For example, Zarse et al. found that the suitable amount of lithium concentration in the food and drinking water has played the function of anti-aging; reducing mortality and promoting longevity among the selected Japanese municipalities [
44]. In addition, Fajardo et al. concluded that the Li in the tap water could play a role of life prolonging effect of human based on the study in Texas [
45].
In contrast, heavy metals, such Pb and Cd, in drinking water are known to be toxic pollutants that can negatively impact human health. The present study confirmed the two longevity indicators to be negatively associated with Pb and Cd in drinking water. Pb accumulation in the human body can affect the cognitive functions and contribute to symptoms such as memory loss, insomnia, and blurred vision [
46,
47], and indirectly reduce the number of centenarians. In addition, Cd can contribute to heart failure, cerebrovascular infarction, osteoporosis and other conditions. Studies have confirmed that heavy metals in drinking water may have negative impacts on regional longevity, consistent with the conclusions of this study. For example, based on the studies of 17 cities, Hao et al. confirmed that Pb may have had negative impacts on longevity in Hainan Province in 2016 [
14].
4.4. Potential Factors That Impact Trace Element and Mineral Levels in Drinking Water in Hechi
The karst landscape plays a significant role in the formation of drinking water in Hechi, contributing beneficial trace elements, in Hechi. The counties with the rockiest areas (greater than 90%) are Fengshan, Bama, and Donglan, with relatively high longevity indicators in the mentioned researches. Many researches have been conducted on the chemical characteristics of the karst landscape. Studies have concluded that rock weathering in the basin areas is the most important contributor to the local water chemistry and carbonate weathering and that the river systems are rich in H
2SiO
3, Ca, Na, Fe, Mg, HCO
−3 and other beneficial trace elements. The rivers in the karst landscape are slightly alkaline. In addition, the landforms of the karst landscape are prone to developing a series of underground caverns due to the strong weathering. For example, the drinking water in Bama is mostly derived from the depths of local caves, absorbing beneficial trace elements in large quantities during the formation processes [
48].
As many centenarians live in rocky areas [
49], most of the members of the longevity population in Hechi use the underground water source as their primary water supply. These areas do not have much surface water but do have abundant quantities of underground water from underground caverns. For example, in 1982, 1990, and 2000, approximately 75% of the centenarians in Bama County lived in rocky and semi-rocky areas and used underground water as their main water source, and approximately 30% of the underground water areas have sustained approximately 70% of the centenarians in Bama over decades [
48]. It can be inferred that the distinctive tectonic settings of Hechi might have long-term impacts on longevity in this region.