4.2. Neural Networks
We find the results obtained through ANN calculations more representative (compared to linear and Spearman correlations) because they eliminate the inhomogeneity of the datasets. Based on the obtained results we documented, Ca + Mg, Ca, Mg, TDS, HCO3,
as the most influential elements and parameters of chemical composition of groundwater for individual HI (Table 4
and Table 5
). These six parameters are found in the group of the first 10 most influential EI in all evaluated HI. Other EI − Cl, NO3
, Na, and K were ranked among the 10 most influential parameters only in the case of some evaluated HI. Their averaged influence (xP) on HI is relatively low (7.85–16) and their mean levels of sensitivity coefficient are low (sr
< 1.01). Three groups of chemical elements and parameters among the most influential EI on HI can be clearly identified. The first group contains Ca, Mg, and Ca + Mg. We attribute the highest influence on HI to these three EI. They show the highest levels of sr
. The second group of EI (TDS and HCO3
) is considered to have only a statistical relationship with HI (i.e., the second group has only a statistical influence on HI). This is demonstrated by the fact that the chemical composition of groundwater in the Slovak Republic is mainly of a Ca-Mg-HCO3
character. TDS and HCO3
can be generally seen as indicators of Ca and Mg groundwater content. HCO3
is the most common anion in groundwater in the Slovak Republic, and its concentrations are mainly due to Ca and Mg cations (mineralization of water that occurs due to the dissolution of carbonates). Similarly, values of groundwater mineralization (TDS) depend mainly on the Ca and Mg contents (the most common cations) and HCO3
(the most common anion) content in the groundwater of the Slovak Republic [32
]. The third group of influential elements consists of SO4
, Cl, and NO3
. These three parameters are typical of anthropogenic groundwater contamination in the Slovak Republic. Their influence is, based on levels of sensitivity coefficients sr
, markedly lower compared to the influence of Ca, Mg, and Ca + Mg. The important fact regarding these three parameters is that their increased contents in the groundwater of the Slovak Republic is due to anthropogenic contamination, and are accompanied mainly by increased contents of Ca and Mg, which were documented in this study as the most influential parameters in relation to HI. Therefore, the mentioned anions do not have significant influence on the health of the Slovak population. This statement does not deny any potential negative effects of nitrates, chlorides, and sulphates on human health at all. All of these chemicals can have significantly adverse health effects at contents locally increased in particular groundwater sources. Such highly contaminated groundwater sources are not used for drinking purposes and therefore we do not consider the influence of these three parameters in groundwater as a significant determinant on the health of the population within the Slovak Republic. A very important fact is that all potentially toxic elements, such as As, Pb, Hg, Zn, Sb, and others have a very low influence on HI, or are characterized as not influential on HI. In the majority of cases, their sensitivity coefficients are lower than, 1 or they are very low (sr
< 1.01). This finding is fully aligned with our current knowledge on the low impact of potentially toxic elements on the health of the population in contaminated abandoned mining areas present in the Slovak Republic [51
Based on the results of ANN calculations, Ca, Mg and water hardness (Ca + Mg) were clearly identified as the most influential EI for evaluated HI. Other evaluated EI are found to be less influential, or to have a stochastic relationship to HI. Therefore, we will not discuss them further.
Ca and Mg are important intracellular cations, which are significantly involved in many enzymatic systems. They are essential for several biological processes, including hematopoiesis, the proper functioning of the heart, and in the prevention of oncological diseases [9
]. The significance of both elements in drinking water for cardiovascular diseases has been documented many times in academic literature, mainly for the association of Ca and Mg deficits and increased incidence or mortality rates for CVD [21
]. However, there are very few works linking the incidence or mortality rates of OD with Ca and Mg deficiency in drinking water [25
]. Nriagu et al. link increased carcinogenity of desalinated water with electrolyte disturbance and hypomagnesemia [54
]. Generally, the epidemiologic studies show that the influence of Ca and Mg on the increased occurrence of oncological diseases is ambiguous. Some of the studies attribute an increased incidence of these diseases (cancers of the breast, prostate, stomach, and digestive tract) to raised Ca or Mg contents in human tissues and fluids, while some report exactly the opposite results [55
]. However, these studies did not deal with water intake, but with artificial supplementation of calcium consumed from diet and supplements.
We were not able to find any reference in any academic literature worldwide dealing with an increased incidence or mortality rate for diseases of the gastrointestinal or respiratory systems and deficits of Ca and Mg in drinking water. Only one Russian ecological study describes a significantly higher incidence of stomach and duodenal ulcer related to soft water with water hardness less than 1.5 mmol·L−1
(Lutai, 1992 in Kožíšek, 2003 [60
]). In our study, observations are of the highest differences in mortality for only diseases of the gastrointestinal and respiratory systems.
Silicate geological environment with low mineralized groundwater/drinking water is generally less favorable in relation to the increased mortality from DGT and DRS compared to carbonate geological environment associated with higher Ca and Mg groundwater contents. Currently, we are not able to find clear and relevant explanation. It is definitely recommended that more specific studies should be done in the future in other countries to confirm or confute this finding.
Mortality for CD, OD, DGT, and DRA represent about 80%–85% of the causes of death in Slovakia [61
]. Increased mortality of Slovak population for these diseases is documented in silicate geological environment at national as well as regional level (the Krupina district). It is strongly reflected in demographic indicators represented in this study by life expectancy (LE) and potential years of lost life (PYLL100). This difference can most markedly be seen in the comparison of the two discussed districts. The difference in life expectancy is more than four years, and in potential years of lost life the difference is more than 100% to the detriment of the Krupina district. It is therefore evident that the deficit of Ca and Mg or water hardness are significantly reflected in all the main causes of death in Slovakia, including cardiovascular and oncological diseases, as well as diseases of the gastrointestinal and respiratory systems. Moreover, with increased concentrations of these chemical elements, life expectancy is higher (i.e., life expectancy is higher).
4.4. Proposed Limit Values
The most important output of our work is the definition of limit values for evaluated EI, for which we document the lowest mortality rates or for maximum life expectancy. We review here the limit values for evaluated HI together with recommended (not obligatory) values defined by the Slovak guidelines for drinking water for comparison in Table 8
. If we have to take into account the importance of a particular HI, we will characterize life expectancy and potential years of lost life as the most significant. They reflect all other HI. Moreover, these are followed by other significant HI, such as the mortality rate for cardiovascular diseases (about 48% of all causes of death) and that of oncological diseases (about 25% of all causes of deaths). The mortality rate for diseases of the gastrointestinal system (about 6% of all causes of deaths) and that of the respiratory system (about 5% of all causes of death) have a lower level of significance. We cannot define the levels only on a mathematical basis. For about half of the HI, the limit value does not exist or cannot be defined (Table 8
). The absence of limit values means that increasing or decreasing content of chemical elements does not have an influence on the HI. In defining limit values, we also have to take into account the potential adverse health effects of very hard water. One of the potential health effects of hard water that should be mentioned is the formation of urinary stones. However, some epidemiological studies did not confirm this relationship [62
]. Currently there is no direct evidence that increased water hardness can cause adverse health effects [60
], except for extremely high Mg water content (hundreds of mg·L−1
) that cause diarrheal diseases. Among the other adverse effects of hard water are sensory properties, such as unfavorable tastes, formation of coatings on the surface of coffee or tea glasses, and the loss of aromatic substances from food and beverages caused by binding with Ca carbonate. From the technological point of view even hard water is not favorable because of scale formation, but it is also the case that soft water has corrosive abilities.
Optimal water hardness from the point of view of human health is hard to determine. Most authors recommend the most favorable values for Mg at a minimum of 20–30 mg·L−1
, for Ca 40–80 mg·L−1
and for water hardness 2–4 mmol·L−1
After taking into account our calculations and all other known facts we propose the following limit values for Ca, Mg, and water hardness used for public supply in the Slovak Republic. For water used for drinking we propose values of Ca > 50 mg·L−1
, Mg > 25 mg·L−1
, and water hardness > 2 mmol·L−1
). In the case of Ca, we propose a lower limit value compared to that derived by ANN calculations in this study, due to the generally reviewed fact that the potential health effects are more likely attributed to Mg in drinking water than to Ca [64
]. Both elements almost always occur together in groundwater in the Slovak Republic, mainly in the Ca/Mg ratio 3:1 (calculated from mg·L−1
) in the case of both low mineralized and high mineralized water. Therefore, we are not able to evaluate the health effects of Ca and Mg separately. The proposed limit values for Ca, Mg and water hardness are about two times higher compared to the recommended values defined in the Slovak guideline for drinking water (Table 8
). Following this fact, we recommend increasing existing limits of these parameters to reach a lower level of mortality for the most common causes of death and a longer lifetime.
In Slovakia, about 25% of groundwater sources are characterized by lower Ca, Mg, and Ca + Mg concentration levels than the proposed limit values.
Our proposed lower limit values are aligned with recommended limit values reported by other authors [9
]. However, levels of water hardness that are too high could have unfavorable effects on consumers. Water with increased hardness (>5 mmol·L−1
) or with increased Ca contents (>180 mg·L−1
) and Mg contents (>50 mg·L−1
) do not naturally occur in the territory of the Slovak Republic and they are not used for drinking purposes. Therefore, we do not find it relevant to define upper limit values.
4.5. Other Impacts Besides Environmental Factors
In conclusion, we mention some other factors that can be characterized as confounding factors for our results. In addition to the chemical composition of the groundwater already discussed, the health of inhabitants depends on a series of other factors, such as eating habits, lifestyle, quality and access to health care, air pollution, socio-economic conditions, etc. Such data is not available for particular Slovak municipalities but is available in other selected areas and districts.
Therefore, below we provide a review of available information concerning the Krupina district (with the most unfavorable geological environment) and the Bardejov district (with the most favorable geological environment), where we document the highest differences in the levels of mortality for the discussed causes of death, as well as that of life expectancy.
Both Krupina and Bardejov represent typical rural districts situated in mountain areas of the Slovak Republic. The population in both districts lives mainly in family houses. Most residents grow vegetables and fruit in their gardens for their own consumption. Regarding air quality, we can state that neither in the two districts, nor in their surroundings, is there any significant source of air pollution (e.g., industrial chemicals, coal power plant). The level of air pollution in both districts is low and that is why no local station for monitoring of air quality is situated there [65
]. We also document very low levels of soil contamination in both evaluated districts [66
Probably a very important confounding factor is rate of Gypsy population. The Gypsy population is characterized by a significantly worse socio-economic level; it has a lower health status and also lower life expectancy in comparison with other Slovak population. In the Bardejov district with documented more favorable health status of resident population a total number of inhabitants of Gypsy population is approximately two times higher than that in the Krupina district (Table 10
Other important socio-economic factors that could have some impact on HI include registered levels of unemployment, as well as the average salary of local residents. Both factors show more unfavorable levels in the Bardejov district compared to the Krupina district.
There are other health determinants regarding lifestyle and health-care that influence human health both positively and negatively, and the available data on them are reviewed in Table 10
. Based on the comparison of the reviewed data, we can conclude that there are no significant differences between the listed health determinants in either of the discussed districts. On the other hand, slightly but not significantly better values for these factors can be observed in the Krupina district, where significantly worse health and shorter life expectancy were reported.