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In total, 353 water samples were collected from April 2012 to May 2015. Sampling was performed at seven resident wells (RW), 21 public water supply wells (W), two observation wells (O), five springs (S), and one river (R) (Figure 1). To ensure spatially representative groundwater conditions, sampling sites covering the whole area of Shimabara except for forest and other land use (Figures 1 and 2) were used. Sampling was done four times annually with 2–4 month intervals to ensure temporally varying groundwater conditions. Sampling at specific locations (RW-14, b, W-21, O-2, S-2, 3, 5, and R-2) was done with less frequency. The hydrogeochemical data used in this study consist of major dissolved ion concentrations for Cl, NO3, SO42–, HCO3, Na+, K+, Mg2+, and Ca2+. Mean and standard deviation of 36 sampling sites using averaged temporal ion concentrations for each of the sampling sites are summarized in Table 1. It is necessary to normalize the data prior to application of SOM to ensure that all parameters are given the same importance. SOM results are highly sensitive to data pre-processing method due to the fact that the Euclidean distance between input data is used (e.g., Jin et al. 2011). To solve this problem, the range between minimum and maximum ion concentrations was standardized into [0, 1] (Nishiyama et al. 2007; Jin et al. 2011) as preprocessing in this study.

Table 1

Mean and standard deviations of 36 sampling sites using averaged temporal ion concentrations for each sampling site used in the SOM

Major ion (mg L−1)MeanSD
Cl 12.4 1.4 
NO3 38.4 5.0 
SO42– 21.9 3.2 
HCO3 55.7 6.6 
Na+ 12.1 2.4 
K+ 6.4 1.2 
Mg2+ 8.7 1.1 
Ca2+ 22.4 2.9 
Major ion (mg L−1)MeanSD
Cl 12.4 1.4 
NO3 38.4 5.0 
SO42– 21.9 3.2 
HCO3 55.7 6.6 
Na+ 12.1 2.4 
K+ 6.4 1.2 
Mg2+ 8.7 1.1 
Ca2+ 22.4 2.9 

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