Baseline geochemical characteristics of groundwater in the mountainous area of Jeju Island, South Korea: Implications for degree of mineralization and nitrate contamination

SummaryHydrogeochemical characteristics of groundwater were investigated to elucidate baseline quality for basaltic aquifers of a mountainous area dominated by natural land cover in the volcanic island of Jeju, South Korea. Principal component analysis (PCA) resulted in four principal components (PC) of which PC1, PC3 and PC4 represented natural mineralization by water–rock interaction, while PC2 corresponded to anthropogenic contamination from nitrate sources. Hierarchical cluster analysis (HCA) was performed to assess the effect of the geochemical and hydrologic processes on each sample. Six sample groups, distinguished by total dissolved solids (TDS), nitrate concentration, and water type, resulted from the HCA: high-altitude springs, low-mineral water, nitrate-contaminated water, intermediate-mineral water, and two groups of high-mineral water. Water types were transformed from Na(Mg, Ca)–HCO3 for low-mineral water to Na–HCO3 for high-mineral water. Nitrate-contaminated water occurred near the boundary of natural and agricultural land uses of the western area. High-mineral water had higher calcite saturation states than other groups. The high-mineral water also had tritium (3H) values lower than 1.5 TU, indicating a significant contribution of old groundwater. In contrast, low-mineral water had tritium values of about 3 TU, which is close to those of rainwater. Geographically, the high-mineral water was concentrated in the southwestern area, where intermediate-mineral water was also found. This distribution was likely related to the presence of extensive trachytic rocks, which could have formed locally isolated aquifers, enabling prolonged water–rock interactions; the higher location of low-permeability hydrovolcanic tuffs in the subsurface of the southern area would also increase chances of tapping aquifers below the hydrovolcanic tuffs. The cumulative probability of TDS showed two breakpoints of 50 and 150 mg/L that distinguished high-altitude springs, low-mineral water, and high-mineral water. The cumulative probability of nitrate provided possible threshold values for anthropogenic contamination of 2.5 and 5.5 mg/L. That 32% of samples had nitrate concentrations higher than 2.5 mg/L indicated high vulnerability to surface contamination sources in this mountainous area. HCA seems to be more effective for interpreting the baseline chemistry of groundwater than other threshold-calculating statistics in that HCA revealed geochemical processes including natural mineralization and anthropogenic contamination. Our study suggests that the cumulative probability and HCA are useful as complementary methods for determining threshold values.