The objective of this part is to assess the quality of groundwater in Preal commune mainly based on the secondary data source. Based on the result from the previous part, 100% of households in Preal commune mainly rely on groundwater wells for their domestic activities, and more than half of households directly consumed raw groundwater as drinking water without any treatments. Therefore, this groundwater quality assessment is very important in order to clarify the potential effect of contaminants in groundwater as well as on human health. The result of this part was divided into three parts including the water quality collection of groundwater well, determining the water quality index, and spatial variation of main associated contaminants.
3.2.1. Statistical Analysis of Groundwater Quality
In this section, the nine groundwater quality parameters including pH, iron, arsenic, fluoride, hardness, chloride, manganese, nitrate, and turbidity, were collected from secondary data sources and determined their statistical analysis such as minimum, mean, maximum, and standard deviation. The statistical analysis results were presented in
Table 12 and value distributions in
Figure 9.
In this study area, pH values varied between 5.30 and 7.42 with a mean value of 6.80 ± 0.47 (see
Table 12). From the pH values distributed across Preal commune (see
Figure 9), four groundwater wells, i.e., WDB ID 5395, 5397, 5401, and 5402, exceeds the permissible range (<6.5). For iron in water, its presence results in an unpleasantness of taste in the mouth and anesthetic red or brown stains on clothes and sanitary facilities or even foods. This parameter varied between 0.0 and 9.0 mg/L with mean value of 2.7 mg/L. From the investigated groundwater wells (see
Figure 9), all samples are desirable limit (<0.3 mg/L), except for WDB ID 5391, 5393, and 5397. The mean value of 6.0 mg/L of arsenic in groundwater of Preal commune was assessed between the range of 0 and 25 mg/L, whereas almost haft of groundwater wells exceeds standard level (0.01 mg/L) (see
Figure 9).
For fluoride in groundwater sample, its concentration varied between 0.35 and 5.33 mg/L with a mean value of 2.46 mg/L and almost half of samples (see
Figure 9) presented in higher amount compared to standard limitation (1.5 mg/L). Water hardness can be divided in to various conditions including soft water (<150 mg/L), moderately hard water (150–300 mg/L), hard water (300–450 mg/L), and extremely hard water (>450 mg/L). Total hardness varied between 18 and 198 mg/L as CaCO
3 with a mean value of 131 mg/L (see
Figure 9). More than half of groundwater samples can be considered as soft water while the other remains are slightly hard water (<200 mg/L). Compared to the WHO standard, about 70% of groundwater wells in the study area is higher than standard level while all wells can pass the drinking water standard of Cambodia (<300 mg/L). For chlorine and nitrate levels in groundwater of Preal commune, these parameters are 0.29–14.00 mg/L (4.32 mg/L in mean) and 0.00–10.54 mg/L (1.38 mg/L in mean), respectively (see
Figure 9). Compared to the drinking water standard, these two parameters are in allowable condition. For manganese, it distributed between 0.01 and 0.15 mg/L with a mean value of 0.10 ± 0.03 mg/L. Based on the distribution of its concentration across the map of Preal commune (see
Figure 9), 95% of groundwater wells demonstrated a higher level than the standard condition (>0.05 mg/L). Last, turbidity varied between 0.42 and 106 NTU with a mean value of 10.82 NTU. If the standard limitation set 1 NTU, about 75% of groundwater wells are over permission condition, however, only 20% exceeded if 5 NTU was considered as the standard condition of turbidity in drinking water (see
Figure 9).
In addition, the correlation between any parameters is another important investigation in order to understand their relationships. It can be conducted through various methods, e.g., genetic programming (GP) [
32], linear regression [
33], correlation coefficient matrix [
19,
20], etc. In this study, the correlation between various parameters of Preal commune aquifer was calculated which is presented in
Table 13. The correlation coefficient between water quality variables and regression analysis indicates indirect means for water quality [
34]. A greater value of coefficient demonstrated the better and more useful of variables [
35]. From
Table 13, it revealed the highest correlations between hardness and pH (R~0.789), iron and turbidity (R~0.732), arsenic and fluoride (R~0.638), and chloride and pH (R~−0.590). The linear regression analysis has been carried out for groundwater quality parameters in a high level of significance in their correlation (R > 0.500) [
36]. In terms of statistical analysis, four pars of water quality parameters were found as very significant (
p-value < 0.01) while other three were significant at the 0.05 level. Four correlations between very significant variables including hardness vs. pH, iron vs. turbidity, arsenic vs. fluoride, and chloride vs. pH, were constructed as expressed with R squared in Equations (5)–(8), respectively. Based on the results, it indicated that groundwater in Preal commune is mainly contaminated by iron, arsenic, fluoride, and manganese, which are mainly associated with human health effects from daily consumption. Additionally, the water quality index should be estimated in order to understand the water condition of each groundwater wells in the study area as well as to find out the main associated contaminants on the water quality index.
3.2.3. Spatial Variation of Main Associated Contaminants
Even the most associated contaminants on the water quality index were defined in the previous part, it was counted from each groundwater well in percentage of sub-index compared to water quality index value. Another multiple correlation analysis between groundwater quality parameters and a score of water quality index was determined in terms of Pearson correlation coefficient [
34] and
p-value. The result indicates that two contaminants, i.e., arsenic and fluoride, are statically significant (
p-value < 0.01) in contribution to water water quality index (data not shown). However, the top five contaminants including arsenic, fluoride, turbidity, chloride, and iron were found as the main associated contaminants in terms of Pearson correlation coefficient (|R| > 0.18).
Considering both results, it can be concluded that arsenic, fluoride, and iron are the main associated contaminants in groundwater on the water quality index. These three contaminants, arsenic, fluoride, and iron, in groundwater were mapped their spatial variation in Preal commune using spatial analyst tool of ArcMap of ArcGIS computer software. Spatial variation of arsenic, iron, and fluoride concentration in Preal commune were mapped in
Figure 10. For arsenic, it can be observed that the contribution of its concentration was high at the west and lower contributions to the east of the study area. In the opposite pattern, iron concentration highly distributes at the east and lower to the west side. In a similar pattern to arsenic, fluoride is highly presented at the west and lower to the east of the study area.