Groundwater Quality and Potential Pollution in the Southern Shimabara Peninsula, Japan

Abstract

Nitrate pollution in groundwater is a severe problem in Shimabara Peninsula, Nagasaki Prefecture, Japan. Previous studies have investigated water quality characteristics in the northern part of the peninsula and shown serious effects of nitrate pollution in the groundwater. The present study aimed to investigate the groundwater quality in the southern areas of the peninsula for improved understanding of the water quality status for the entire peninsula. Groundwater samples were collected at 56 locations in Minami-Shimabara City from 28 July to 4 August 2021. The spatial distribution of water quality constituents was assessed by Piper-trilinear and Stiff diagrams for major ion concentrations. One agricultural area in the western parts exceeded Japanese recommended standards for water. According to the Piper-trilinear diagram, 44 sampling sites (78.6%) were classified as alkaline earth carbonate type, nine sites (16.1%) as alkaline earth non-carbonate type, and three sites (5.3%) as alkaline carbonate type. Stiff diagrams displayed Ca-HCO₃ water type for most of the sites. Na-HCO₃ and Mg-HCO₃ types were found in coastal areas. Principal component analyses showed that the first component corresponded to dissolved constituents in groundwater and denitrification, the second effects of ion exchange and low nitrate pollution, and the third effects of severe nitrate pollution. Hierarchical cluster analysis was used to classify the groundwater into five groups. The first group included sites with relatively high nitrate concentration. The second group had relatively low ion concentration, distributed from center to eastern parts. The third group included intermediate ion concentration, distributed at lower altitudes along the coastal line. The fourth and fifth groups had a higher ion concentration, especially characterized by high sodium and bicarbonate concentration.

1. Introduction

Groundwater is a globally important resource used for domestic, industrial, and agricultural purposes. In 2018, groundwater dependency for these uses in Japan was 20.1%, 27.3%, and 5.4%, respectively [1]. In some areas, the dependence on groundwater for domestic use is 100%. Therefore, groundwater quality assessment is essential to protect residents’ health and preserve their living environment. In Japan, assessments have continuously been carried out for 28 water quality parameters (e.g., cadmium, lead, arsenic, dichloromethane, carbon tetrachloride, benzene, nitrate+nitrite-nitrogen (NO₃+NO₂-N), fluoride, and boron) for which environmental standards of groundwater pollution are established under the Basic Environment Law. In 2020, an investigation was conducted for 3103 wells to assess the overall groundwater quality. In total, 10 parameters were found to exceed environmental standards [2]. NO₃+NO₂-N had the highest exceedance at 3.3%. This was followed by arsenic at 2.1% and fluoride at 0.8%. NO₃+NO₂-N has continued to present the highest exceedance rates since it was added to the criteria in 1999, and this is typified throughout Japan. Nitrate pollution in groundwater is the most prevalent type of anthropogenic pollution [3]. In many cases, nitrate pollution in groundwater is caused by fertilizer and livestock wastewater in agricultural areas [3]. Other pollutants, such as arsenic and fluoride stem from natural sources originally contained in rocks [3]. The release and fate of these elements in groundwater are controlled by water-rock interaction processes in hydrogeological paths of the water [4,5].

The Shimabara Peninsula in Nagasaki Prefecture, Japan, depends on groundwater for most of its water supply due to the small amounts of surface water [6]. However, the peninsula is experiencing severe nitrate pollution in groundwater. Water quality monitoring has shown that the number of tap water source (groundwater) wells above environmental standards for NO₃+NO₂-N ranged from 5.1% to 10.0% during 2005 to 2020 [7]. Although the general pollution level is slowly decreasing, some wells are still showing increasing concentrations and require continuous monitoring. The Shimabara Peninsula consists of three cities: Shimabara, Unzen, and Minami-Shimabara. In 2011, we started an investigation on groundwater quality in Shimabara City, where pollution is particularly severe. We found concentrations up to 26.6 mg/L NO₃-N related to intensive agricultural activities (fertilizer and livestock production) [8]. Further investigation displayed pollution spread with depth in groundwater [9] and that there is a strong relationship between pollution in soil and groundwater [10]. In 2018, the study area was expanded to Unzen City. We found maximum NO₃-N concentrations of 19.9 mg/L and that the pollution is related to agriculture as in Shimabara City [11]. Although the assessment of groundwater pollution has been established for Shimabara and Unzen Cities; groundwater chemistry including nitrate pollution has not been studied in the Minami-Shimabara City. For this reason, we evaluated spatial characteristics of groundwater chemistry in the Minami-Shimabara City for improved understanding of the status of nitrate pollution in this area and the hydrochemical characteristics of groundwater in vulnerable aquifers compared to the hydrochemistry of the Shimabara and Unzen Cities.

2. Materials and Methods

2.1. Study Area

Minami-Shimabara City is located in the south of Shimabara Peninsula, Nagasaki Prefecture (Figure 1) with an area of approximately 17,000 ha. In 2017, the population was 44,200 with a water supply coverage of 92.8% [12]. There are 5 surface reservoirs, a main river, 60 groundwater sources, and 2 springs supplying water to the city [12]. Groundwater has the highest ratio of water withdrawal, reaching 86.1% [12]. The land use map is shown in Figure 1a [13]. Forests cover 35.6% of the total area [14]. The agricultural land area is 4730 ha (paddy fields 1650 ha; upland fields 3070 ha) [14], equivalent to 27.8% of the city area. This ratio is higher than that of Nagasaki Prefecture as a whole and is characterized by agriculture use in upland fields. Mainly cultivated crops are rice, potatoes, fodder crops, and leaf tobacco. Nitrogen application by chemical fertilizer and manure for the 10 major crops is estimated at 1715 kg/day [7]. Livestock production is thriving, with 223 livestock facilities (49 dairies, 138 beef cattle, 14 pork, 5 egg, and 17 broiler production units) in 2019 [7]. This corresponds to 35,950 cattle, 79,000 pigs, and 2,900,000 chicken [7]. The resulting nitrogen generation from livestock waste is estimated at 6625 kg/day [7].

Figure 2b shows the geology of the study area [15]. In the southern part of the area, the Kuchinotsu formation, which consists mainly of freshwater sediment and shallow-marine sediment of middle Pliocene to early Pleistocene age [16], is exposed. Pre-Unzen volcanic rocks (4 Ma–500 ka) are partially distributed above the Kuchinotu formation. Pre-Unzen volcanic rocks and Kuchinotsu formation are overlain by the Unzen volcanic rocks (500 ka–present), which are widely distributed in the Shimabara Peninsula [16]. Unzen volcanic rocks are observed near the southern border and in the center of the peninsula. Alluvial deposits are found at lower elevation areas in the north. Pre-unzen volcanic rocks are mainly composed of olivine basalt and amphibole andesite, while Unzen volcanic rocks contain hornblende andesite to dacite rich in plagioclase, hornblende, and biotite mottling [16]. Figure 2c shows the hydrogeological map of the study area [17]. Groundwater levels are only evident in a part of the study area. There are many faults in this area, and the groundwater levels differ greatly due to these. Especially, in the south and east, groundwater levels are complex due to faults.

The climate is humid subtropical. Mean temperature and mean annual precipitation are 17.3 °C and 1836 mm, respectively (1992 to 2021; data from Kuchinotsu observatory, 32°36.7′ N, 130°11.6′ E) [18].

2.2. Sampling and Analyses

Groundwater samples were taken from 56 municipal wells (Figure 1) during July to August 2021. All samples were stored in pre-washed bottles. Stagnant water in the well pipes was removed before water sampling. pH, electrical conductivity (EC), oxidant redox potential (ORP), and dissolved oxygen (DO) were measured on-site by using hand-held instruments (HORIBA D-51 and D-54, and HACH HQ30d). Bicarbonate ion (HCO₃⁻) was determined by the titration method with 0.1 N HCl on-site. All water samples were filtered by 0.45 µm membrane filter on-site. Major anions (Cl⁻, NO₃⁻, SO₄²⁻) and cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) were measured by ion-chromatography (DKK-TOA, IA-300) in the laboratory. The validity of the analytical results was confirmed by converting ion concentrations from mg/L to mmol_c/L and then calculating the charge balance errors (CBE) using:

$$\mathrm{CBE}=\left(\frac{{\displaystyle \sum \mathrm{cations}-{\displaystyle \sum \mathrm{anions}}}}{{\displaystyle \sum \mathrm{cations}+{\displaystyle \sum \mathrm{anions}}}}\right)\times 100,$$

(1)

According to Rahman et al. [19], CBE < ±5 is good, and CBE < ±10 is acceptable. In this study, the CBEs of the samples ranged from −3.6 to 7.3, with <±5 in 41 samples and <±10 in 15 samples.

Principal component analysis (PCA) and hierarchical cluster analysis (HCA) for the hydrochemical parameters were used to interpret processes controlling water chemistry (mineral dissolution, anthropogenic input, sea water intrusion, ion exchange, evapotranspiration, etc.) and grouping the water samples e.g., [20,21,22,23,24]. These methods were also used previously for the Shimabara and Unzen groundwater [8,11]. PCA and HAC were performed based on major ion concentrations (Cl⁻, NO₃⁻, SO₄²⁻, HCO₃⁻, Na⁺, K⁺, Mg²⁺, and Ca²⁺). The principal components were extracted based on the Kaiser criterion to only retain components with eigenvalues greater than 1. The HCA was performed based on Ward’s method. All analyses were performed by using the statistical software JMP Pro 13 (SAS Institute Inc., 100 SAS Campus Drive, Cary, North Carolina 27513-2414, USA).

3. Results and Discussion

3.1. Water Chemistry

The analysis results of hydrochemical parameters are shown in

Table 1. Descriptive statistics of hydrochemical components of groundwater samples.

	Cl−	NO3−	SO42−	HCO3−	Na+	K+	Mg2+	Ca2+	pH	EC	ORP	DO
	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L		µS/cm	mV	mg/L
Min	3.3	0.1	1.4	11.4	6.3	3.1	1.6	3.9	6.3	74	−21	1.7
Max	49.2	55.7	43.5	299.7	134.3	7.5	18.2	37.0	8.7	563	738	10.2
Mean	10.0	7.9	12.7	81.5	17.9	4.9	6.3	14.6	7.4	210	235	6.8
S.D. 1	7.7	11.2	11.2	60.3	23.8	1.0	4.0	8.7	0.5	126	106	2.5

Note: ¹ S.D. = standard deviation.

and Figure 3. pH ranged from 6.3 to 8.7, indicating that groundwater in the study is weakly acidic to weakly alkaline. EC ranged from 74 to 563 μS/cm. The range of pH and EC was generally close to the Shimabara and Unzen groundwater [8,11]. ORPs ranged from −21 to 738 mV, meaning that groundwater is oxic or anoxic. The latter was observed at two sampling points (MW51 and MW55). The redox conditions are strong indicators of contaminants that might be present at elevated concentrations [25]. The nitrate concentration is more likely to exceed the recommended limits for oxic conditions [25]. On the other hand, microbially driven reduction of nitrate to nitrogen gas occurs only under anoxic conditions [25]. Gillham and Cherry [26] reported that denitrification processes can occur when the DO in groundwater is less than 2 mg/L. In our study, DO ranged from 1.7 to 10.2 mg/L, with two sampling points (MW44 and MW50) showing less than 2.0 mg/L. Thus, at sampling points (MW44, MW50, MW51, and MW55) with low DO or ORP, NO₃⁻ concentrations between 0.1 to 1.9 mg/L indicate that denitrification is likely to occur. These conditions were not observed in Shimabara and Unzen [8,11]. The total dissolved solids (TDS) were estimated from EC by the following equation [27].

$$\mathrm{TDS}=640\times \mathrm{EC},$$

(2)

where TDS and EC are in mg/L and dS/m, respectively. TDS ranged from 47.4 to 360.3 mg/L with a mean of 134.4 mg/L. The samples were characterized by HCO₃⁻ followed by SO₄²⁻, Cl⁻, and NO₃⁻ for anions (Figure 3). Cations were characterized by Ca²⁺ and Na⁺ followed by Mg²⁺ and K⁺.

Groundwater samples were plotted using trilinear Piper diagram for easy visual understanding the characteristics of water chemistry (Figure 4). Forty-four sampling points (78.6%) were classified as alkaline earth carbonate type, which is common in shallow groundwater in Japan. Nine points (16.1%) showed alkaline earth non-carbonate type, which is classified as groundwater with high nitrate concentration in Shimabara and Unzen [8,11]. Three samples (5.3%) were classified as alkaline carbonate type. This type of water is commonly found in deep groundwater with long residence time and was not identified in Shimabara and Unzen [8,11]. To consider salinity, Total Ionic Salinity (TIS) is shown in Figure 5 [28]. According to the figure groundwater in Minami-Shimabara has low TIS (<10 mmol_c/L).

The concentration of major dissolved ions is visualized using Stiff diagram in Figure 6. Most of the samples represent the Ca-HCO₃ type. The Ca-(SO₄+NO₃), Ca-SO₄, Na-HCO₃, and Mg-HCO₃ types were observed in a small number of wells. Na-HCO₃ and Mg-HCO₃ were found in the coastal area and thus, have larger arrows than other samples. To decipher mechanisms controlling groundwater chemistry, Gibbs diagram [29,30,31] is shown in Figure 7. Gibbs diagram is described by the ratio of cation and anion endmembers (Na⁺/(Na⁺ + Ca²⁺)) and (Cl⁻/(Cl⁻ + HCO₃⁻)), respectively, as a function of TDS. This indicates the functional sources of dissolved chemical constituents, such as precipitation, rock weathering, or evaporation dominance. The diagram shows that most groundwater samples are affected by rock weathering processes, suggesting that longer residence time, large contribution of mineral dissolution, and the influence of ion exchange form water chemistry in the study area. A few samples were influenced by precipitation. The difference of these factors appears as the difference of ion concentrations and water type. The water samples in precipitation dominance have lower concentration. Higher concentrations and water types of Na-HCO₃ and Mg-HCO₃ are dominated by rock weathering processes. Influence of seawater was mainly found in the south part of Minami-Shimabara City [17], but no Na-Cl type samples were identified. Figure 8 shows the distribution of groundwater samples in the Hydrochemical Facies Evolution (HFE) diagram [32] by using Excel Macro [33]. HFE-diagram reveals that 70% of the groundwater samples correspond to freshening phase and 30% correspond to intrusion phase. Since the study area faces to the sea, it is assumed that groundwater samples, which is characterized by high concentration and ratio of Na⁺, such as MW42, MW43, and MW44 in the left upper corner of freshening phase, are affected by seawater intrusion.

The southern part of the city has little surface water and relies on groundwater for irrigation. However, as mentioned above, groundwater with high Na⁺ concentrations were observed. Therefore, the suitability of groundwater for irrigation was evaluated by plotting a USSL diagram (Figure 9). The USSL diagram is drawn based on the salinity (EC) and sodium hazard (Sodium Adsorption Ratio: SAR) [34]. The SAR is calculated by [34]:

$$\mathrm{SAR}=\frac{{\mathrm{Na}}^{+}}{\sqrt{\left({\mathrm{Ca}}^{2+}{+\mathrm{Mg}}^{2+}\right)/2}},$$

(3)

where the concentrations of Na⁺, Ca²⁺, and Mg²⁺ are all expressed in mmol_c/L. Most of the groundwater samples belong to C1-S1 (low salinity hazard and low sodium hazard) class or C2-S1 (medium salinity hazard and low sodium hazard). Only two samples (MW42 and MW43) were classified in the C2-S2 (medium salinity hazard and medium sodium hazard) class. The following is a summary of each class [34]. Low salinity water (C1) can be used for irrigation with most crops on most soils with little likelihood that soil salinity will develop. Some leaching is required, but this occurs under normal irrigation practices except in soils of extremely low permeability. Medium salinity water (C2) can be used if a moderate amount of leaching occurs. Plants with moderate salt tolerance can be grown in most cases without species practices for salinity control. Low salinity water (S1) can be used for irrigation on almost all soils with little danger of the development of harmful levels of exchangeable sodium. However, sodium sensitive crops, such as stone-fruit trees and avocados may accumulate harmful concentrations of sodium. Medium sodium water (S2) will present an appreciable sodium hazard in fine textured soils having high cation exchange capacity, especially under low leaching conditions, unless gypsum is present in the soil. Consequently, Figure 9 shows that groundwater in the study area can be used for irrigation, but attention must be paid to the use of groundwater belonging to the C2-S2 class. Further evaluation of water quality for irrigation was conducted based on Food and Agriculture Organization (FAO) guidelines [35] (

Table 2. Guidelines for interpretations of water quality for irrigation.

	Unit	Degree of Restriction on Use
		None	Slight to Moderate	Severe
EC	dS/m	<0.7	0.7–3.0	>3.0
TDS	mg/L	<450	450–2000	>2000
Na (surface irrigation)	SAR	<3	3–9	>9
Na (sprinkler irrigation)	meq/L	<3	>3
Cl (surface irrigation)	meq/L	<4	4–10	>10
Cl (sprinkler irrigation)	meq/L	<3	>3
NO3-N	mg/L	<5	5–30	>30
HCO3	meq/L	<1.5	1.5–8.5	>8.5
pH		Normal range 6.5–8.4

). According to the FAO guidelines, although most of the groundwater samples has no potential irrigation problem, some samples have elevated EC, SAR, Na, NO₃-N, and HCO₃ or even potentially severe problem in view of SAR. In addition, some of the samples have a pH somewhat out of range. Therefore, caution should be exercised in some places when using groundwater. Since most tree crops and woody plants are sensitive to sodium [35], groundwater having high SAR should be avoided. These results are generally consistent with the caution given from the USSL diagram.

3.2. Nitrate Pollution

The nitrate (NO₃-N) concentration ranged between 0.02 and 12.6 mg/L, with an average of 1.8 mg/L. The maximum concentration was found at MW39, the only point where NO₃-N concentration exceeded the Japanese drinking water standard (10 mg/L). The drinking water standard of World Health Organization [36] for NO₃⁻ concentration (50 mg/L) was exceeded in one point. The maximum nitrate concentration in Shimabara City was 26.6 mg/L [8] and that in Unzen City 19.9 mg/L [11]. Compared to these values, the pollution level in Minami-Shimabara City is lower. Only one sampling point exceeded the standard limits for NO₃-N concentration. This is lower than the 38% in Shimabara City [8] and the 12% in Unzen City [11]. Even if the standard limits were not exceeded, relatively high NO₃-N concentrations were observed, e.g., 8.4 mg/L at MW49, 7.3 mg/L at MW53, and 6.5 mg/L at MW15. Nitrate can occur naturally, but nitrate concentrations greater than 1 mg/L are likely to indicate effects of human activities [37]. Under this criterion, 21 sampling points (38%: except for the smallest blue circle in Figure 10) can be considered polluted by nitrate. As mentioned above, denitrification may occur in the groundwater in this area, and it is considered that at some sampling points the nitrate concentration has decreased below 1 mg/L due to denitrification. Sampling points with nitrate concentration exceeding the criteria of 1.0 mg/L [37] were observed in the southwestern and eastern parts of the study area (Figure 10), and the land use here is upland agriculture (Figure 2). This suggests that nitrate pollution of groundwater in this area is related to agricultural activities.

Correlation among the eight major dissolved ion components is shown in

Table 3. Correlation matrix for eight dissolved ions (n = 56).

	Cl−	NO3−	SO42−	HCO3−	Na+	K+	Mg2+	Ca2+
Cl−	1.00	0.15	0.50	0.65	0.74	−0.10	0.52	0.39
NO3−	-	1.00	−0.05	−0.26	−0.13	0.24	0.14	0.10
SO42−	-	-	1.00	0.57	0.38	0.12	0.71	0.72
HCO3−	-	-	-	1.00	0.84	0.07	0.56	0.52
Na+	-	-	-	-	1.00	−0.10	0.19	0.10
K+	-	-	-	-	-	1.00	0.20	0.32
Mg2+	-	-	-	-	-	-	1.00	0.83
Ca2+	-	-	-	-	-	-	-	1.00

. In the investigation of Shimabara and Unzen [8,11], NO₃⁻ showed a high positive correlation with Cl⁻, SO₄²⁻, and K⁺, suggesting that nitrate is derived from chemical fertilizers ((NH₄)₂SO₄), manure, and livestock waste. However, in Minami-Shimabara, NO₃⁻ is not correlated with Cl⁻, SO₄²⁻, and K⁺. Focusing on Cl⁻, SO₄²⁻, and K⁺ concentrations in MW39, where NO₃-N concentration exceeded the Japanese drinking water standard, the relationship is as follows: Cl⁻ 16.9 mg/L > K⁺ 5.1 mg/L > SO₄²⁻ 4.2 mg/L. For MW49 with NO₃-N of 8.4 mg/L, which has the next highest NO₃-N concentration, a relationship of Cl⁻ 12.3 mg/L > K⁺ 6.4 mg/L > SO₄²⁻ 2.7 mg/L was observed. The SO₄²⁻ concentration at these points is similar to or lower than those at other points with low NO₃-N concentration (Figure 6). At the sampling points where such a relationship between ion concentrations is found, SO₄²⁻ concentration is low, and manure and/or livestock waste are supposed to be a dominant nitrate source. Analysis of stable nitrogen oxygen isotopes NO₃⁻ is required for a more valid assessment of the nitrate source.

3.3. Multivariate Analysis

A principal component analysis summarized the 12 hydrochemical parameters into three principal components based on the Kaiser index (

Table 4. Relationship between extracted principal components (PCs) and ions.

	PC 1	PC 2	PC 3
Cl−	0.770	−0.213	0.337
NO3−	−0.083	0.421	0.775
SO42−	0.793	0.278	−0.093
HCO3−	0.903	−0.267	0.006
Na+	0.708	−0.599	0.235
K+	0.104	0.543	0.286
Mg2+	0.716	0.548	−0.002
Ca2+	0.773	0.508	−0.029
pH	0.430	−0.560	0.227
EC	0.969	−0.082	0.182
ORP	−0.518	−0.113	0.435
DO	−0.755	−0.084	0.447
Eigenvalue	5.64	1.92	1.33
Explained variance %	46.9	16.0	11.1
Cumulative % of variance	46.9	62.9	74.0

). The eigenvalues of principal component 1 (PC1), 2 (PC2), and 3 (PC3) were 5.64, 1.92, and 1.33, respectively. These explained 46.9%, 16.0%, and 11.1%, respectively, of the total variance. PC1 had positive loadings for EC and all ions except NO₃⁻ and K⁺, indicating that it is related to dissolved constituents controlled by rock weathering and precipitation. The negative loadings of ORP and DO in PC1 are related to denitrification. PC2 had positive loadings for Ca²⁺ and negative loadings for Na⁺, implying ion exchange. The positive loading of NO₃⁻ in PC2 represents lower nitrate pollution exceeding the criteria of 1.0 mg/L indicating effects of human activities. PC3 had positive loadings for Cl⁻ and NO₃⁻, indicating relatively severe nitrate pollution including the exceedance of standard limits.

Results of the HCA are shown in

Table 5. Hydrochemical component depending on each group.

	Samples		Cl−	NO3−	SO42−	HCO3−	Na+	K+	Mg2+	Ca2+	NO3-N	pH	EC	ORP	DO
			mg/L	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L		µS/cm	mV	mg/L
Group 1	26	Min	3.3	0.1	1.4	23.2	6.3	3.1	1.6	4.2	0.0	6.8	74	242	7.2
		Max	12.1	26.1	17.9	97.5	15.6	6.1	6.4	14.7	5.9	7.9	190	738	10.1
		Mean	5.9	7.4	4.3	44.4	8.7	4.7	3.5	8.2	1.7	7.3	116	287	8.8
		S.D.	1.9	7.4	3.2	13.7	2.0	0.8	1.2	2.6	1.7	0.2	29	93	0.7
Group 2	3	Min	10.9	28.6	2.2	41.3	11.6	5.1	5.7	14.2	6.5	7.0	187	201	7.3
		Max	16.9	55.7	4.2	65.7	11.7	6.4	7.4	19.9	12.6	7.4	241	287	10.2
		Mean	13.4	40.5	3.0	52.5	11.7	5.8	6.8	17.6	9.2	7.3	222	230	8.5
		S.D.	3.1	13.8	1.0	12.3	0.1	0.7	1.0	3.0	3.1	0.2	30	49	1.5
Group 3	20	Min	5.0	0.1	6.5	11.4	7.7	3.3	2.5	9.9	0.0	6.3	136	−21	1.9
		Max	16.1	28.9	37.3	171.8	34.7	7.5	11.0	33.3	6.5	8.0	383	302	9.0
		Mean	9.6	3.8	19.3	90.4	14.1	5.3	7.6	19.3	0.9	7.3	234	204	5.1
		S.D.	3.4	6.7	8.0	33.6	5.8	1.3	2.2	6.5	1.5	0.5	52	101	2.2
Group 4	3	Min	13.8	1.9	17.3	231.3	76.3	3.4	2.6	3.9	0.4	8.1	466	74	1.7
		Max	49.2	2.5	26.6	299.7	134.3	4.8	8.2	16.5	0.6	8.7	563	231	6.2
		Mean	26.2	2.1	21.9	254.3	108.8	4.3	5.5	10.3	0.5	8.4	529	177	3.7
		S.D.	19.9	0.3	4.7	39.3	29.6	0.8	2.8	6.3	0.1	0.3	55	89	2.3
Group 5	4	Min	17.5	0.7	22.0	111.2	23.6	3.9	16.6	24.3	0.2	6.9	407	32	2.3
		Max	28.9	32.1	43.5	224.2	43.7	5.2	18.2	37.0	7.3	8.3	507	207	6.2
		Mean	24.2	11.2	34.5	170.6	33.8	4.6	17.5	33.2	2.5	7.7	453	104	3.9
		S.D.	5.4	14.7	9.0	55.5	9.2	0.6	0.7	6.0	3.3	0.6	44	81	1.7

and Figure 11. The 56 groundwater samples were classified into five groups, with the number of samples in each group ranging from 3 to 26. Figure 12 shows the scatter plot of the principal components related to groups. PC1 effectively separated groups, meaning that dissolved constituents are different for each group. Group 1 shows relatively low scores for PC1, indicating lower dissolved constituents. Mainly the sampling points indicated by smaller Stiff diagrams in Figure 6 are contained in this group. Group 1 is distributed from the center to east in the study area. Group 2 and 3 has similar scores of PC1 and PC2, representing intermediate ion concentrations. However these groups are be distinguished by PC3, meaning that nitrate pollution level are different for these groups. As shown in Table 5, water samples having higher NO₃⁻ concentrations were classified into Group 2. The difference between Group 2 and 3 is characterized by difference in locality (Figure 13). Group 2 is located in the western part of study area, while Group 3 is mainly located at lower altitude along the coastal line. PC1 and PC3 of Group 4 is similar to that of Group 5, but these groups are clearly distinguished by PC2. In other words, Group 4 has relatively high negative scores for PC2, indicating negative effects of ion exchange, and anthropogenic activities. Group 4 is associated with deep groundwater with high concentrations of sodium and bicarbonate ions (Table 5). The five groups can be generally summarized in three categories.PC1 and Figure 11 show that Group 4 and 5, which are characterized by high ion concentration especially Na⁺ and HCO₃⁻, are different from the other groups. Based on overall ion concentration including nitrate, the remaining groups can be categorized by either intermediate ion concentration (Group 3) or lower ion concentration (Group 1 and 2).

4. Conclusions

To improve the understanding of cause and effects of nitrate pollution and hydrochemical characteristics of groundwater in the Shimabara Peninsula, groundwater samples were collected from 56 municipal wells in Minami-Shimabara City. Major dissolved ions and pH, EC, ORP, and DO were analyzed. The ORP and DO values suggested that denitrification may be responsible for decreasing nitrate concentrations. The major groundwater composition was Ca-HCO₃ type. In addition, Na-HCO₃, Mg-HCO₃, and Ca-(SO₄+NO₃) types were also observed at a few locations. This water chemistry is formed by rock weathering, precipitation, and mixing with saltwater from seawater intrusion. NO₃-N concentrations exceeded Japanese drinking water standards (10 mg/L) at one location. The pollution is related to agricultural land use. The high Cl⁻ and low SO₄²⁺ concentrations at this point suggest that the pollution source is manure and/or livestock waste. PCA showed that processes controlling water chemistry are explained by three principal components: PC1 corresponds to dissolved constituents in groundwater and denitrification, PC2 represents ion exchange and low nitrate pollution, and PC3 represents severe nitrate pollution. HCA classified the 56 water samples 5 five groups. These can be broadly divided into 3 categories: the first characterized by high ion concentration especially in Na⁺ and HCO₃⁻ (Group 4 and 5); the second representing the intermediate ion concentration group (Group 3), and the third with low ion concentration (Group 1 and 2).

The study revealed that the extent of nitrate pollution in Minami-Shimabara City is small. In other words, NO₃-N concentrations are lower than in Shimabara and Unzen Cities, and the percentage of NO₃-N exceeding the standard limits is also small. The sampling campaign was between July and August when rainfall amount was large. In the investigations conducted during this period, a decrease in nitrate concentration was observed in Shimabara City due to dilution caused by rainfall [8,10]. Therefore, future surveys are needed at different seasons. It is also necessary to introduce analysis of nitrogen oxygen stable isotopes of NO₃⁻ to clarify the nitrate source.

Since this study found high Na⁺ concentrations in some wells, we additionally evaluated the suitability of groundwater for agricultural use. Groundwater was evaluated using the USSL diagram and FAO guideline. In some places, due to high EC, SAR, Na, NO₃-N, HCO₃, and pH, caution is necessary when using groundwater for irrigation. In addition, groundwater with high SAR should not be used for some types of crop species. In Minami-Shimabara City, agricultural wells have been installed in addition to the municipal wells that were investigated in this study. Since groundwater is affected by seawater in some areas [17], it is necessary to collect groundwater samples for agricultural wells to evaluate their suitability for agricultural production.