Trends and Sustainability of Groundwater in Highly Stressed Aquifers (Proc. of Symposium JS.2 at the Joint IAHS & IAH Convention, Hyderabad, India, September 2009). IAHS Publ. 329, 2009. 201 Cause of groundwater contamination in Jakarta alluvium volcanic fan deduced by sulfate and strontium isotope ratios TAKAHIRO HOSONO 1, ROBERT DELINOM 2, SHIN-ICHI ONODERA 3, YU UMEZAWA 4, TAKANORI NAKANO 5 & MAKOTO TANIGUCHI 5 1 Priority Organization for Innovation and Excellence, Kumamoto University, 39-1 Kurokami, Kumamoto 860-8555, Japan hosono@kumamoto-u.ac.jp 2 Research Centre for Geotechnology, Indonesia Institute of Science, Jln. Cisitu Sangkuriang, Bandung 40135, Indonesia 3 Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8521, Japan 4 Faculty of Fisheries, Nagasaki University, 1-14 Bunkyo-machi Nagasaki 858521, Japan 5 Research Institute for Humanity and Nature, 457-4 Motoyama Kamigamo, Kita-ku, Kyoto Abstract Sulfate and strontium isotope ratios were analysed in groundwater, river water, and spring water samples in order to examine the groundwater quality under the Jakarta volcanic fan watershed. On the basis of sulfate isotopic comparison with possible source materials, it is suggested that anthropogenic materials such as chemical fertilizers are responsible for the pollution of groundwater. Moreover, strontium isotopic signatures with ions concentrations data clearly suggest that the groundwater nitrate pollution is occurring in an area where aquifer sediments originated from volcanic rocks other than the sedimentary rocks. This study demonstrates the usefulness of sulfate and strontium isotopes in understanding the cause of groundwater deterioration under the Jakarta volcanic fan watershed. Detailed pollution status and deterioration mechanisms with sufficient water samples need to be investigated in future research. Key words sulfate isotope; strontium isotope; groundwater; environment; Jakarta INTRODUCTION The stable isotope ratios of nitrate (δ 15 N-NO - 3 and δ 18 O-NO - 3 ), sulfate (δ 34 S-SO 4 and δ 18 O-SO 4 ), and strontium ( 87 Sr/ 86 Sr) have been developed in the field of earth science as powerful chemical tracers (Faure, 1986; Hoefs, 2004), and have recently been applied in the field of groundwater environmental study (Kendall & McDonnell, 1998; Cook & Herczeg, 2000). Nitrate pollution is one of the serious environmental problems affecting groundwater. Because of its importance, nitrate isotope ratios have firstly been applied to the groundwater pollution study. Similarly, sulfate isotope ratios are useful in discriminating the sources of dissolved sulfate among natural and/or anthropogenic origins and are also good indicators for judging the occurrence of sulfate reduction and its degree (Moncaster et al., 2000; Dogramaci et al., 2001; Spence et al., 2001). Moreover, since the mineral elements such as strontium (Sr) dissolved in water are commonly derived from geological materials, strontium isotope ratio can be used to identify the type of geological materials in aquifers (Singleton et al., 2006; Hosono et al., 2009a). However, even though sulfate and strontium isotope tracers are important for better understanding the groundwater environmental system, their applications have not been sufficiently established, probably because sulfate and strontium themselves are not harmful components for human life. Jakarta city is located in the northwest part of Java Islands, Indonesia. The city is developed in the northern part of the volcanic fan. The geology of this area is composed mainly of sedimentary basement rocks of Tertiary age, overlying Quaternary volcanic rocks, and alluvium sediments originating from both volcanic and sedimentary rocks. The Jakarta metropolitan area, including the suburb urbanized area, is one of the largest megalopolis in Asian counties, with a population of around 13.6 million. The highly populated urbanized area (including industrial and commercial areas) is distributed in the northern plain and coast, whereas the agricultural area, such as fruit farms and orchards is distributed at the middle of the volcanic alluvium fan (Fig. 1). Recently, the presence of groundwater with characteristically high proportions of SO 4 and Cl - ions was reported in the agricultural area (Fig. 1) and it is suggested that it is caused by contamination due to agricultural activity (Hosono et al., 2009b). This paper attempts to control the source of sulfate Copyright 2009 IAHS Press
202 Takahiro Hosono et al. Fig. 1 Sample locality on the land-use map after Hosono et al. (2009b). The letters on the map indicate the water quality type base on the classification of the Piper diagram. A, Ca(CO 3 ) 2 type; B, NaHCO 3 type; C, CaSO 4 -CaCl 2 type; D, Na 2 SO 4 -NaCl type; and e and f, intermediate type. dissolved in the groundwater in the agricultural area by using sulfate isotope ratios. The relationship between the groundwater pollution and the geology of this area is also discussed. SAMPLING AND ANALYTICAL METHODS Seventy eight groundwater, five spring water, and six river water samples were collected from the whole watershed area of the Jakarta alluvium fan during 3 14 September 2006 (Fig. 1). From the collected samples we selected representative samples for isotopic analysis. Details of sampling localities and division of aquifers have already been reported (Hosono et al., 2009b; Umezawa et al., 2009). We divided the groundwater samples into two types: (a) shallow groundwater of unconfined aquifer with depth of 2 14 m (from ground level) and (b) deep groundwater of confined aquifer with depth up to 350 m. The sulfate isotope ratios (δ 34 S-SO 4 and δ 18 O-SO 4 ) were determined by continuous-flow gas-ratio mass spectrometer (Thermo Scientific Delta PlusXL) at the University of Arizona. Analytical precisions of δ 34 S-SO 4 and δ 18 O-SO 4 were ±0.2 and ±0.3, respectively. The 87 Sr/ 86 Sr values were determined using a thermal ionization mass spectrometer (Thermo electron Triton) at RIHN. The 87 Sr/ 86 Sr values were normalized to 86 Sr/ 88 Sr = 0.1194, and the 87 Sr/ 86 Sr of the NIST-SRM987 throughout this study were 0.710240 ± 0.000007 (2σ, n = 15). All 87 Sr/ 86 Sr data were normalized to a NIST-SRM987 value of 0.710250. Details of sample procedures for sulfate and strontium isotopes are after Hosono et al. (2007). GROUNDWATER CONTAMINATION: CONSTRAINT FROM ISOTOPE RATIOS Almost all of analysed water samples display sulfate isotopic ranges between 0 and 30 for δ 34 S- SO 4 and 3 and 17 for δ 18 O-SO 4, respectively (Fig. 2). On the δ 34 S-SO 4 vs δ 18 O-SO 4
Cause of groundwater contamination in Jakarta alluvium volcanic fan 203 Fig. 2 δ 34 S-SO 4 vs δ 18 O-SO 4 diagram for water samples. Data for sea water (Krouse & Mayer, 2000), geological constituents (Krouse & Mayer, 2000), and chemical fertilizers (Hosono et al., unpublished data) are also shown for comparison. The solid line indicates the simple mixing line between water affected by pollution and saline water which has increased sulfate reduction (dotted arrow). Fig. 3 δ 34 S-SO 4 vs. (NO 3 - ) diagram for water samples. diagram (Fig. 2), water samples of Jakarta alluvial fan watershed are isotopically divided into two types based on their plots distribution: type-i and type-ii. The δ 18 O-SO 4 values of type-i water vary between 3 and 13 with relatively constant δ 34 S-SO 4 values. By contrast, type-ii groundwater shows relatively constant δ 18 O-SO 4 values with δ 34 S-SO 4 values of wide range between 8 and 30. All spring and river water samples are plotted on the compositional field of type-i water (Fig. 2(a)), whereas the shallow and deep groundwater samples are plotted on the compositional fields of both type-i and -II. Of the type-i water samples, samples of spring water in the upstream area show characteristically low δ 18 O-SO 4 compositions, which is similar to those of sulfate in geological constituents (Fig. 2(a)). On the contrary, most of the samples of river water and groundwater have intermediate compositions between sulfate in geological constituents and sulfate originated from
204 Takahiro Hosono et al. Fig. 4 Relationship between 87 Sr/ 86 Sr values of water samples and latitude of sampling points. chemical fertilizers. This isotopic feature indicates the contamination by fertilizer-derived sulfate in the river water and groundwater. The groundwater in the suburb agricultural fields with high proportions of SO 4 and Cl - ions (C and e type in Fig. 1; Hosono et al., 2009b) mostly belongs to type-i water (Fig. 2(b)). Moreover, it is remarkable from the relationship between δ 34 S-SO 4 values and nitrate concentrations (Fig. 3) that the high nitrate groundwater has a narrow δ 34 S-SO 4 range between 3 and 8, which is identical to the compositional field of type-i water (Fig. 2). It is therefore summarized from the above that the dissolved sulfate in spring water in the upstream watershed area is mostly of geological origin. However, sulfate in groundwater in the agricultural fields could be contaminated by applied chemical fertilizers. On the other hand, increasing δ 34 S-SO 4 values in type-ii water (Fig. 3; 8 30 ) suggests the occurrence of sulfate reduction under the reducing subsurface environment (Grassi & Cortecci, 2005; Yamanaka & Kumagai, 2006). However, if this was the case, the δ 18 O-SO 4 value of the water sample should also be increased with increasing δ 34 S-SO 4 value (Moncaster et al., 2000; Dogramaci et al., 2001; Spence et al., 2001). Since observed plots do not perfectly reflect this effect (Fig. 2), it might not be the case that type-ii groundwater was created by simple sulfate reduction from type-i groundwater. For the other possibility, the type-ii groundwater might be created by the mixing of two kinds of waters (mixing line in Fig. 2) between type-i groundwater and source water; saline groundwater with high sulfate concentration has promoted sulfate reduction (dotted arrow in Fig. 2). This possibility is supported by the fact that type-ii groundwater is found in the coastal area at tidal flats and by its high salinity (D type in Fig. 1). However, many more samples need to be analysed to answer this question. The strontium isotope ratios ( 87 Sr/ 86 Sr) for analysed water samples show a wide range, between 0.7045 and 0.7090 (Fig. 4). The spring water collected at the foot of the volcanic mountain (latitude between 6.7 and 6.6 ) shows a typical 87 Sr/ 86 Sr compositional range of Quaternary subduction-related island arc volcanic rocks between 0.7045 and 0.7055, indicating that dissolved strontium in it was largely derived from rock-constituting minerals. However, the 87 Sr/ 86 Sr value of groundwater near the coastal area (latitude between 6.2 and 6.1 ) tends to increase from 0.7065 to 0.7090, toward the present sea water value (0.70916; Banner, 2004) as groundwater gets close to the coastal line. This clearly confirms the presence of saline water intrusion (Hosono et al., 2009b; Fig. 1). In the middle of the alluvial fan watershed area 87 Sr/ 86 Sr values of water samples display an intermediate value range between those at mountainous and coastal areas (0.7055 to 0.7065). This may suggest that strontium dissolved in such water was derived not only from volcanic rocks, but also partly from basement sedimentary rocks exposed at the middle part of the watershed area, which should have much higher 87 Sr/ 86 Sr compositions than younger Quaternary volcanic rocks.
Cause of groundwater contamination in Jakarta alluvium volcanic fan 205 Fig. 5 87 Sr/ 86 Sr vs (NO 3 - ) diagram for water samples. From the 87 Sr/ 86 Sr vs nitrate concentration diagram (Fig. 5), it can be concluded that nitrate pollution occurs on the groundwater that has limited 87 Sr/ 86 Sr composition between 0.7050 and 0.7055. This implies that nitrate pollution occurs at aquifers where groundwater interacts with rocks or sediments of volcanic origin. This also implies that the volcanic aquifer supplies more preferable conditions for nitrate to be accumulated and/or to be preserved in the groundwater than the sedimentary aquifer does. For example, pollutant intrusion from surface into aquifer might be achieved much easier in the field of volcanic deposits than in the sedimentary deposits with relatively low permeability. Moreover, aerobic conditions in the volcanic field might decrease the progress of the denitrification reaction. Likewise, the occurrence of groundwater pollution and the type of geology is clearly correlated with each other, and the Sr isotope ratio is useful to examine such a relationship. More detailed investigation must be undertaken in order to elucidate the accumulation and preservation mechanism for nitrate and other pollutants. However, our preliminary study demonstrates that future research with increasing numbers of samples and considering groundwater flow will be able to clarify these problems at Jakarta volcanic alluvium fan watershed. CONCUDING REMARKS The sulfate and strontium isotope data revealed that the shallow groundwaters are contaminated by the application of chemical fertilizers at the agricultural zone in the central part of the Jakarta volcanic watershed areas. In the coastal areas the occurrence of groundwater salinization was observed. The sulfate isotope signature of these groundwaters suggests that the groundwater salinization occurred by the mixing of saline water, which has caused reducing conditions, but not due to the direct intrusion of the present seawater. Further investigations have to be conducted to elucidate the detailed mechanism of groundwater contamination as well as salinization with additional water samples for chemical and isotope analysis. Acknowledgements We thank the staff of the Division of Hydrology, Indonesia Institute of Science for supporting our water sampling and supplying GIS data. We also thank the staff of
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