Potential Sources of Ammonium-Nitrogen in the Coastal Groundwater Determined from a Combined Analysis of Nitrogen Isotope, Biological and Geological Parameters, and Land Use

: The origin of ammonium-nitrogen in Indonesian coastal groundwater has not been inten-sively examined, meanwhile the elevated concentration remains a concern. This research aims at tracing the potential sources of ammonium-nitrogen in the groundwater of Indramayu, Indonesia where groundwater is vital for livelihood. From results, a combined examination of nitrogen isotope, coliform bacteria, land-use, and geology conﬁrmed the natural and anthropogenic origins of ammonium-nitrogen in the groundwater. In the brackish-water aquaculture region, groundwater has δ 15 N NH4 values from +1.8 to +4.8‰ signifying that ammonium-nitrogen is derived from mineralization of organic nitrogen to ammonium. Furthermore, ammonium has a signiﬁcantly positive relationship with sodium indicating the exchangeable ammonium is mobilized to groundwater via cation exchange. Meanwhile ammonium-nitrogen from anthropogenic waste was detected in agricultural and residential region. The groundwater has more varied δ 15 N NH4 values, from − 2.9 to +16.1‰, which implies attenuation of ammonium-nitrogen from several sources namely manure, mineral fertilizer, sewage, and pit latrines. Also, the presence of E. coli conﬁrms the indication of human and animal waste contamination. However, since ammonium has no relationship with sodium, cation exchange is not feasible and ammonium-nitrogen ﬂows into the groundwater from anthropogenic sources along with liquid wastes.


Introduction
Groundwater as one of the clean water sources is presently experiencing nitrogen contamination because of human activities [1][2][3]. Ammonium is one form of dissolved inorganic nitrogen that is naturally occurring in low concentrations within groundwater. Generally, under aerobic condition, ammonium-nitrogen (NH 4 + -N) concentration in groundwater is <0.2 mg/L, and it may increase more than ten times under anaerobic environment [4]. The considerably high ammonium groundwater concentrations are indications of anthropogenic contamination from sources such as fertilizer, manure, septic tanks, and sewage [4][5][6]; therefore, these undesirable concentrations need to be removed [7,8]. Additionally, previous research found that high concentration of ammonium in groundwater is significantly correlated with high contents of more dangerous contaminants, which are dissolved iron and manganese [9].
Several studies in Indonesian coastal alluvial region have reported the groundwater quality issues with emphasis laid on metals and nitrate contaminations [10][11][12]. Equally, NH 4 + -N was also detected in shallow and deep groundwater with concentrations approximately 7.8 and 12 mg/L, respectively [13]. However, the evaluation of the ammonium source in this study was limited, which was mainly derived from nitrate denitrification. An in-depth study to trace other potential sources of ammonium in Indonesia is yet to be investigated. Additionally, Indonesian coastal groundwater is also vulnerable to salinization [14]. At the same time, salinization itself is known to have a positive relationship with the elevated concentrations of other contaminants, including ammonium [9,[15][16][17].
Many methods have been devised to understand the origin of ammonium-nitrogen. Among them, nitrogen isotope (δ 15 N) is one of the most reliable and widely used ammonium tracer [6,18,19]. However, some issues need to be overcome for interpreting the δ 15 N value. For example, the different sources of ammonium may have similar δ 15 N signatures, and fractionation may occur during ammonium transport from the subsurface to the groundwater [20]. Consequently, multi-tracer parameters are required to eliminate these problems [20]. Land-use evaluation can help to identify contamination sources. This type of evaluation has been successfully used to appraise nitrate groundwater contamination [21]. The combination of nitrogen isotope and land-use to trace ammonium source can still be enhanced by analyzing the geological settling and bacterial content in the water. Previous findings revealed that soils and sediments are related to elevated concentrations of ammonium in groundwater [6,22]. On the other hand, coliform and E. coli parameters are useful tracers to confirm human waste contamination which is also a potential source of ammonium contamination [4]. In this research, we trace the potential sources of ammonium in coastal alluvial groundwater using the following combined tracers; nitrogen isotope (δ 15 N), bacterial coliform analysis (total coliform and E. coli), land use, and geological features. In addition, the effect of saline water on ammonium was evaluated with respect to major cations contents. Finally, from the combined tracing parameters, we expect to find the anthropogenic and salinization impact on NH 4 + -N contents in groundwater.

Study Site
This study was conducted in the coastal alluvial aquifer of Indramayu, located in the northern part of West Java Island, Indonesia (107 • 52 -108 • 36 E and 6 • 15 -6 • 40 W). Indramayu is vital as a rice-and fish-production area with increasing gross domestic regional product (GRDP) particularly in the farming and fishery sector [23]. Groundwater is essential to support livelihood activities in this region because only less than 60% of the population can access the potable water provided by the government [23]. However, comprehensive information on the groundwater quality is limited. Hence, understanding groundwater ammonium sources in this area is valuable to provide a strong scientific base for the sustainable management of groundwater.

Land Uses of Indramayu
Indramayu is a lowland area with a total area greater than 240,000 ha. The main land-use categories are agriculture, brackish-water aquaculture, and residential ( Figure 1a). Agricultural land that mainly consists of paddy fields is the primary land-use category, covering over 70% of the total area. The second-largest land-use category is brackish-water aquaculture, accounting for about 13% of the land use-aquaculture is mainly adopted in the northernmost part. The last major land-use category is the settlement that covers less than 11% and is mostly located along Cimanuk River. From 2002 to 2017, the agricultural and settlement areas slightly expanded, while the area under brackish-water aquaculture decreased. Since then, there was no significant change in the three main land-use categories (Figure 1b). tural and settlement areas slightly expanded, while the area under brackish-water aquaculture decreased. Since then, there was no significant change in the three main land-use categories (Figure 1b).  During the field campaigns, we observed that agricultural areas and settlements are not strictly defined, this is explained by the fact that the agricultural area is extensive and surrounds the entire residential area. The collection of groundwater samples from agriculture and settlement areas were from houses that have dug or bore wells. Groundwater in agriculture and settlement areas were expected to have similar characteristics because of their undefined boundaries. On the other hand, the boundary between the brackish-water aquaculture and agriculture was quite clear. The groundwater samples from brackish-water aquaculture were collected from bore wells which are used to fill the ponds. From the aforementioned land-uses, we classified them into two groups: lower coastal zone (LC) comprising brackish-water aquaculture area and the upper coastal zone (UC) consisting of agricultural and settlement areas ( Figure 2).
During the field campaigns, we observed that agricultural areas and settlements are not strictly defined, this is explained by the fact that the agricultural area is extensive and surrounds the entire residential area. The collection of groundwater samples from agriculture and settlement areas were from houses that have dug or bore wells. Groundwater in agriculture and settlement areas were expected to have similar characteristics because of their undefined boundaries. On the other hand, the boundary between the brackishwater aquaculture and agriculture was quite clear. The groundwater samples from brackish-water aquaculture were collected from bore wells which are used to fill the ponds. From the aforementioned land-uses, we classified them into two groups: lower coastal zone (LC) comprising brackish-water aquaculture area and the upper coastal zone (UC) consisting of agricultural and settlement areas ( Figure 2).

Geology and Aquifer System of Indramayu
The Indramayu Coast is a delta [24][25][26] formed by sedimentary materials supplied by the Cimanuk River [27]. Based on the Geological Map of Indramayu Quadrangle in Java [28], the study area generally consists of old to young lithologies, including tuffaceous sandstone and conglomerate (Qav), flood plain deposits (Qaf), beach ridge deposits (Qbr), coastal deposits (Qac), deltaic deposits (Qad), and young river deposits (Qa) (Figure 3a). The Indramayu region is essentially composed of quaternary rocks. The oldest Pleistocene quaternary rock units in the region are tuffaceous sandstone and conglomerate (Qav), conglomeratic sandstone, tuffaceous sandstone, and tuff. Holocene sediments are deposited on the upper part. They include flood plain deposits (Qaf) in the form of sandy-humic clay, clayey sand, and partly tuff; beach ridge deposits (Qbr) in the form of coarse to finegrained sand and clay with abundant mollusks; coastal deposits (Qac) in the form of silt, clay, sand, and fragments of mollusks; deltaic deposits (Qad) in the form of silt, brown clay, and some mollusks; and young river deposits (Qa) in the form of sand, silt, and brown clay.

Geology and Aquifer System of Indramayu
The Indramayu Coast is a delta [24][25][26] formed by sedimentary materials supplied by the Cimanuk River [27]. Based on the Geological Map of Indramayu Quadrangle in Java [28], the study area generally consists of old to young lithologies, including tuffaceous sandstone and conglomerate (Qav), flood plain deposits (Qaf), beach ridge deposits (Qbr), coastal deposits (Qac), deltaic deposits (Qad), and young river deposits (Qa) (Figure 3a). The Indramayu region is essentially composed of quaternary rocks. The oldest Pleistocene quaternary rock units in the region are tuffaceous sandstone and conglomerate (Qav), conglomeratic sandstone, tuffaceous sandstone, and tuff. Holocene sediments are deposited on the upper part. They include flood plain deposits (Qaf) in the form of sandy-humic clay, clayey sand, and partly tuff; beach ridge deposits (Qbr) in the form of coarse to fine-grained sand and clay with abundant mollusks; coastal deposits (Qac) in the form of silt, clay, sand, and fragments of mollusks; deltaic deposits (Qad) in the form of silt, brown clay, and some mollusks; and young river deposits (Qa) in the form of sand, silt, and brown clay.  The vertical geological profile from south to north shows a large aquifer reservoir interspersed with clay and silt in the area (Figure 3b). In the north, there is the thickest clay layer derived from marine deposits. According to previous studies, the groundwater here has high chloride concentrations, ranging from 1500 to 12,000 mg/L, which is categorized as brackish-salt and salt water [30].

Sediment Samples
Sediment analysis was performed to evaluate the general geological conditions that possibly promote the natural attenuation of NH4 + -N in the groundwater. The sediments samples were collected at different depths from two boreholes in UC area (DH01) and LC area (DH05) (Figure 3a). We selected the depth based on the textures-clay, silt, and sand ( Figure 3b). The analysis was conducted for ratio of exchangeable ammonium (NH4 + ) to total cations (NH4 + , Na + , K + , Mg 2+ , and Ca 2+ ), stable isotopes of nitrogen (δ 15 N), nitrogen content (%), and the cation exchange capacity (CEC). Method of analysis for each param- The vertical geological profile from south to north shows a large aquifer reservoir interspersed with clay and silt in the area (Figure 3b). In the north, there is the thickest clay layer derived from marine deposits. According to previous studies, the groundwater here has high chloride concentrations, ranging from 1500 to 12,000 mg/L, which is categorized as brackish-salt and salt water [30].

Sediment Samples
Sediment analysis was performed to evaluate the general geological conditions that possibly promote the natural attenuation of NH 4 + -N in the groundwater. The sediments samples were collected at different depths from two boreholes in UC area (DH01) and LC area (DH05) (Figure 3a). We selected the depth based on the textures-clay, silt, and sand ( Figure 3b). The analysis was conducted for ratio of exchangeable ammonium (NH 4 + ) to total cations (NH 4 + , Na + , K + , Mg 2+ , and Ca 2+ ), stable isotopes of nitrogen (δ 15 N), nitrogen content (%), and the cation exchange capacity (CEC). Method of analysis for each parameter is as follows: Composition of δ 15 N (‰) was determined by Thermo Finnigan DELTAplus Advantage+FLASH2000; N content (%) was measured by high temperature combustion; exchangeable NH 4 + was measured by soil extraction with NH 4 O ac at pH 7.0 and follow by atomic absorption spectrophotometry (AAS); and CEC value was measured by summing cations that are attracted to the negative surface charges in sediments samples. Analysis of δ 15 N and N content were conducted at the Chikyu Kagaku Kenkyusho (Geo-Science Laboratory, Nagoya, Japan) while ratio of exchangeable ammonium to total cations and CEC were measured at the Soil Fertility and Plant Nutrition Laboratory, University of Padjajaran, Bandung, Indonesia.

Groundwater Samples
In all, 28 groundwater samples were collected during dry season in November 2019 from LC area (n = 10) and UC area (n = 18) ( Figure 2). The concentrations of NH 4 + -N, nitrate-nitrogen (NO 3 − -N), and nitrite-nitrogen (NO 2 − -N) were determined using a continuous-flow automated nutrient analyzer (SwAAt, BLTEC, Tokyo, Japan) at the Biogeochemistry Laboratory, Hiroshima University, Japan. The analysis of NH 4 + -N (δ 15 N NH4 ) was conducted by using a continuous-flow type stable isotope ratio mass spectrometer (DELTA plus Advantage + FLASH2000; Thermo Finnigan, San Jose, CA, USA) with a precision of ±0.2‰ at the Chikyu Kagaku Kenkyusho (Geo-Science Laboratory, Dubai, UAE), Japan. Glycine, L-Alanine, and L-Histidine were used as working standards to calibrate the isotope ratio with the international standard. Sample preparation for δ 15 N NH4 analysis was performed at the Laboratory of Biogeochemistry, Hiroshima University, Japan. The NH 4 + -N in the sample was first diffused following the procedure and materials and chemicals adopted by Koba et al. (2010) [31]. However, some adjustments were made for the analysis in this study. The incubation period here was 24 h, which is sufficient for capturing all NH 4 + -N from samples to the glass filter and the maximum volume of sample was 40 mL. Besides, the laboratory analysis is limited for groundwater samples with NH 4 + -N content greater than 20 µg. First, 10 samples and 14 samples from the LC and UC groundwater, respectively, meeting the requirements for δ 15 N NH4 measurement were used. Furthermore, coliform bacteria analysis (total coliform and E. coli) was completed in the health department facility in Indramayu by using the most probable number method on the same day of sample collection.
Furthermore, the contents of major cations, comprising Na + , K + , Mg 2+ , and Ca 2+ , were analyzed by ion chromatography with conductivity detection on an ICS-2100 (Thermo Fisher Scientific Inc., Waltham, MA, USA) with analytical precisions of <1% for Na + , Mg 2+ , and Ca 2+ , and <2% for K + . For dissolved organic carbon (DOC), analysis was performed using a total organic carbon analyzer (TOC-V CPH , Shimadzu, Kyoto City, Japan). The analysis of major ions and DOC were carried out at the Laboratory of Geothermal Engineering, Hirosaki University, Japan.
Samples for the nitrogen isotope and chemical analysis were first filtered with 0.2 µm polytetrafluoroethylene or DISMIC-25HP (for DOC) membranes and then placed into polyethylene bottles and glass vials (for DOC). The samples bottles were pretreated before collecting the samples: they had been soaked in 10% HCl solution for 24 h, washed, and then rinsed thrice with distilled water; further, they were rinsed thrice in the field with well water before collecting the sample. For biological analysis, samples were kept in sterilized glass bottles prepared by the health department in Indramayu, Indonesia.
The statistical analysis was carried out to find the NH 4 + -N relationship with DOC and major cations. The effect of DOC and major cations on NH 4 + -N concentrations is demonstrated by coefficient determination (R 2 ). Furthermore, the significance of the relationship is shown by the p value with 95% confidence level.

Chemical Properties of Sediments
Sediments analysis results show that the ratio of exchangeable NH 4 + to total cations (NH 4 + and major cations) is high in both UC and LC deposits and does not vary significantly with depth (Figure 4a,b). Similarly, the δ 15 N value does not change considerably with sediment depth and texture in either UC or LC deposits (Figure 4c of UC and LC deposits varies from +3.1 to +3.9‰ and +2.9 to +3.5‰ and, respectively. According to Evans [32], the δ 15 N value between 0 and +5‰ may indicate nitrogen conversion from organic matter to ammonium by mineralization. Furthermore, the nitrogen content in the first layer of clay and silt in UC deposits is relatively lower than that in LC deposits (Figure 4e,f). Therefore, the mineralization of soil nitrogen is more likely to occur in LC deposits. Subsequently, ammonium as a mineralization product is then mobilized by the cation exchange process and transported to sampling wells. This assumption is supported by the CEC value of LC deposits, which is relatively higher than UC deposits (Figure 4g,h). However, the properties of sediments can differ spatially. Therefore, the available sediment data only indicates the general condition of the study area.

Chemical Properties of Sediments
Sediments analysis results show that the ratio of exchangeable NH4 + to total cations (NH4 + and major cations) is high in both UC and LC deposits and does not vary significantly with depth (Figure 4a,b). Similarly, the δ 15 N value does not change considerably with sediment depth and texture in either UC or LC deposits (Figure 4c,d). The δ 15 N ratio of UC and LC deposits varies from +3.1 to +3.9‰ and +2.9 to +3.5‰ and, respectively. According to Evans [32], the δ 15 N value between 0 and +5‰ may indicate nitrogen conversion from organic matter to ammonium by mineralization. Furthermore, the nitrogen content in the first layer of clay and silt in UC deposits is relatively lower than that in LC deposits (Figure 4e,f). Therefore, the mineralization of soil nitrogen is more likely to occur in LC deposits. Subsequently, ammonium as a mineralization product is then mobilized by the cation exchange process and transported to sampling wells. This assumption is supported by the CEC value of LC deposits, which is relatively higher than UC deposits (Figure 4g,h). However, the properties of sediments can differ spatially. Therefore, the available sediment data only indicates the general condition of the study area.

Potential Sources of Ammonium-Nitrogen in Groundwater Systems
Previous findings have shown that the concentration of NH4 + -N in the samples is predominant compared to NO3 − -N and NO2 − -N, particularly in the LC region [30]. NH4 + -N concentrations in LC and UC groundwater from previous studies ranged from 2.3 to 9.0 mg/L and 0.04 to 7.0 mg/L, respectively. On the other hand, the NO3 − -N concentration

Potential Sources of Ammonium-Nitrogen in Groundwater Systems
Previous findings have shown that the concentration of NH 4 + -N in the samples is predominant compared to NO 3 − -N and NO 2 − -N, particularly in the LC region [30]. NH 4 + -N concentrations in LC and UC groundwater from previous studies ranged from 2.3 to 9.0 mg/L and 0.04 to 7.0 mg/L, respectively. On the other hand, the NO 3 − -N concentration was mostly <0.2 mg/L, and NO 2 − -N content was <0.02 mg/L. Similar to a previous study, the current study also found a high content of NH 4 + -N ( Figure 5). Only two samples from UC groundwater (DH07 and DH13) had a high concentration of NO 3 − -N. However, this value was still below the standard drinking water threshold [4]. The NO 2 − -N content was very low (≤0.02 mg/L). The content of NH 4 + -N was generally higher in LC than in UC groundwater. The concentration of NH 4 + -N in LC and UC groundwater varied from 2.35 to 12.97 mg/L and 0.07 to 3.19 mg/L, respectively. Significant amounts of NH 4 + -N compared to NO 3 − -N or NO 2 − -N may be an indicator of a reducing environment that can cause the dissolution of metals such as iron, manganese, and arsenic [22,33]. was mostly <0.2 mg/L, and NO2 − -N content was <0.02 mg/L. Similar to a previous study, the current study also found a high content of NH4 + -N ( Figure 5). Only two samples from UC groundwater (DH07 and DH13) had a high concentration of NO3 − -N. However, this value was still below the standard drinking water threshold [4]. The NO2 − -N content was very low (≤0.02 mg/L). The content of NH4 + -N was generally higher in LC than in UC groundwater. The concentration of NH4 + -N in LC and UC groundwater varied from 2.35 to 12.97 mg/L and 0.07 to 3.19 mg/L, respectively. Significant amounts of NH4 + -N compared to NO3 − -N or NO2 − -N may be an indicator of a reducing environment that can cause the dissolution of metals such as iron, manganese, and arsenic [22,33]. Nitrate may be reduced in the dissimilatory nitrate reduction to ammonium (DNRA) as a result of microbial activities facilitated by organic carbon or nitrate [33]. This process is likely to occur in the LC groundwater since DOC has a positive and significant relationship with both NO3 − -N and NH4 + -N (Figure 6a,b, respectively). However, further investigation is required to confirm this origin. The relationship of δ 15 NNH4 and NH4 + -N in Figure 7 shows that δ 15 NNH4 in the LC groundwater has a narrow range of values, from 1.8 to 4.8‰ (Table 1), and it is relatively stable over the wide range of NH4 + -N concentrations, from 2.35 to 14.6 mg/L (Table 1). Still in Figure 7, this also presents the isotopic composition of δ 15 NNH4 from several sources which have been discovered from previous studies [6,19]. By using these references, we propose that the ammonium in the LC groundwater is unlikely to be of human and animal Nitrate may be reduced in the dissimilatory nitrate reduction to ammonium (DNRA) as a result of microbial activities facilitated by organic carbon or nitrate [33]. This process is likely to occur in the LC groundwater since DOC has a positive and significant relationship with both NO 3 − -N and NH 4 + -N (Figure 6a,b, respectively). However, further investigation is required to confirm this origin. was mostly <0.2 mg/L, and NO2 − -N content was <0.02 mg/L. Similar to a previous study, the current study also found a high content of NH4 + -N ( Figure 5). Only two samples from UC groundwater (DH07 and DH13) had a high concentration of NO3 − -N. However, this value was still below the standard drinking water threshold [4]. The NO2 − -N content was very low (≤0.02 mg/L). The content of NH4 + -N was generally higher in LC than in UC groundwater. The concentration of NH4 + -N in LC and UC groundwater varied from 2.35 to 12.97 mg/L and 0.07 to 3.19 mg/L, respectively. Significant amounts of NH4 + -N compared to NO3 − -N or NO2 − -N may be an indicator of a reducing environment that can cause the dissolution of metals such as iron, manganese, and arsenic [22,33]. Nitrate may be reduced in the dissimilatory nitrate reduction to ammonium (DNRA) as a result of microbial activities facilitated by organic carbon or nitrate [33]. This process is likely to occur in the LC groundwater since DOC has a positive and significant relationship with both NO3 − -N and NH4 + -N (Figure 6a,b, respectively). However, further investigation is required to confirm this origin. The relationship of δ 15 NNH4 and NH4 + -N in Figure 7 shows that δ 15 NNH4 in the LC groundwater has a narrow range of values, from 1.8 to 4.8‰ (Table 1), and it is relatively stable over the wide range of NH4 + -N concentrations, from 2.35 to 14.6 mg/L (Table 1). Still in Figure 7, this also presents the isotopic composition of δ 15 NNH4 from several sources which have been discovered from previous studies [6,19]. By using these references, we propose that the ammonium in the LC groundwater is unlikely to be of human and animal The relationship of δ 15 N NH4 and NH 4 + -N in Figure 7 shows that δ 15 N NH4 in the LC groundwater has a narrow range of values, from 1.8 to 4.8‰ (Table 1), and it is relatively stable over the wide range of NH 4 + -N concentrations, from 2.35 to 14.6 mg/L (Table 1). Still in Figure 7, this also presents the isotopic composition of δ 15 N NH4 from several sources which have been discovered from previous studies [6,19]. By using these references, we propose that the ammonium in the LC groundwater is unlikely to be of human and animal waste origin; it probably is mainly derived from the mineralization of organic and inorganic nitrogen in sediment. Meanwhile, the groundwater at LC reveals the presence of coliform bacteria, but none of it originated from E. coli (Table 1). This finding indicates the absence of contamination by human and animal feces [4] which further supports the interpretation based on δ 15 N NH4 . Additionally, the LC area has a thick clay layer (Figure 3b) with potentially high organic matter contents [34]. The organic matter is not only a natural source of ammonium, but is also a facilitator of microbial activities that, in turn, generate a reducing environment. This condition was confirmed earlier in several studies [6,22]. potentially derived from animal manure and household waste [6,19]. This observation is consistent with the presence of E. coli in the samples (Table 1). Furthermore, the source of ammonium for δ 15 NNH4 values ≤ 5‰ may be from the application of mineral fertilizers in paddy fields area and sewage infiltration [19]. Since most of the UC groundwater samples were collected from a shallow aquifer (≤8 m), the results indicate the possibility that poor wastewater treatment and utilization of pit latrine less than 2 m from the well led to the transport of leaked wastewater into the aquifer [35,36].  [6]; and for organic matter and household waste are adapted from Nikolenko et al. 2018 [19]).
The comparison of NH4 + -N concentrations and δ 15 NNH4 compositions between LC and UC groundwater showed that the LC groundwater was potentially more vulnerable to ammonium contamination. Interestingly, the relative high ammonium-nitrogen in LC groundwater was from natural source. Previous findings in more developing Indonesian coastal area suggest that anthropogenic waste is the primary source of ammonium in groundwater [13]. The presence of ammonium in both areas shows the heterogeneity of groundwater conditions in Indonesia coastal areas. On the other hand, Figure 7 also shows that the δ 15 N NH4 values in the UC groundwater are varied, from −2.9 to 16.1‰ (Table 1), within a relatively narrow concentration range of NH 4 + -N, from 0.42 to 3.19 mg/L ( Table 1), suggesting that ammonium is derived from diverse sources. The compositions of δ 15 N NH4 , >5‰, suggest that ammonium-nitrate are potentially derived from animal manure and household waste [6,19]. This observation is consistent with the presence of E. coli in the samples (Table 1). Furthermore, the source of ammonium for δ 15 N NH4 values ≤ 5‰ may be from the application of mineral fertilizers in paddy fields area and sewage infiltration [19]. Since most of the UC groundwater samples were collected from a shallow aquifer (≤8 m), the results indicate the possibility that poor wastewater treatment and utilization of pit latrine less than 2 m from the well led to the transport of leaked wastewater into the aquifer [35,36].
The comparison of NH 4 + -N concentrations and δ 15 N NH4 compositions between LC and UC groundwater showed that the LC groundwater was potentially more vulnerable to ammonium contamination. Interestingly, the relative high ammonium-nitrogen in LC groundwater was from natural source. Previous findings in more developing Indonesian coastal area suggest that anthropogenic waste is the primary source of ammonium in groundwater [13]. The presence of ammonium in both areas shows the heterogeneity of groundwater conditions in Indonesia coastal areas.

Relationship between Ammonium-Nitrogen and Water Chemical Properties
The relatively high concentration of NH 4 + -N in LC groundwater is consistent with the significantly elevated concentrations of major cations, mostly Na + (Figure 8). High cation content is typical of high salinity water [37]. The source of high salinity is possibly associated with infiltration of brackish-water from the pond or marine clay sediment. Figure 8a-d show a strong positive and significant relationship between NH 4 + -N and cations. Hence, exchangeable ammonium from sediments can be released into groundwater by exchanging cations, particularly Na + , as the dominant cation [15,16,38]. It is also important to consider that the lithology of LC site sediments is composed of thick clay (Figure 3b), which generally impedes exchangeable ammonium by the sorption process [16]. Therefore, more exchangeable ammonium can be deposited in sediments [16] and released into groundwater through the cation exchange process under high salinity conditions [15,38].
associated with infiltration of brackish-water from the pond or marine clay sediment. Figure 8a-d show a strong positive and significant relationship between NH4 + -N and cations. Hence, exchangeable ammonium from sediments can be released into groundwater by exchanging cations, particularly Na + , as the dominant cation [15,16,38]. It is also important to consider that the lithology of LC site sediments is composed of thick clay (Figure 3b), which generally impedes exchangeable ammonium by the sorption process [16]. Therefore, more exchangeable ammonium can be deposited in sediments [16] and released into groundwater through the cation exchange process under high salinity conditions [15,38].
On the other hand, the content of major cations in UC groundwater is relatively low and has no rational relationship with NH4 + -N ( Figure 8). Moreover, the p-value for the relationship between NH4 + -and all cations is not considerable. This implies that the transfer of ammonium from sediments to groundwater by the cation exchange mechanism is unlikely.  On the other hand, the content of major cations in UC groundwater is relatively low and has no rational relationship with NH 4 + -N ( Figure 8). Moreover, the p-value for the relationship between NH 4 + -and all cations is not considerable. This implies that the transfer of ammonium from sediments to groundwater by the cation exchange mechanism is unlikely. Figure 9 shows the average concentrations of total major cations, NH 4 + , and δ 15 N NH4, in three different classifications of groundwater depth: shallow (≤5 m), medium (≥5-20 m), and deep (≥25 m). The total content of major cations in UC groundwater is relatively low and similar at all depths, in the range of 23 to 48 meq/L (Figure 9a). LC groundwater, on the other hand, has a very high content of major cations, which implies high salinity groundwater, especially at shallow (377 meq/L) and medium (352 meq/L) depths (Figure 9b). As a compromise, UC groundwater (Figure 9c) has significantly lower ammonium ions than LC groundwater, respectively from 0.06 to 0.11 meq/L and 0.43 to 0.57 meq/L (Figure 9d). Moreover, the concentration of NH 4 + in groundwater decreases as groundwater depth increases, according to the pattern of the total content of major cations relative to depth. Overall, the cations and ammonium ion contents at these two sites indicate that ammonium is likely to be mobilized because of the cation exchange at the LC site. Figure 9 shows the average concentrations of total major cations, NH4 + , and δ 15 NNH4, in three different classifications of groundwater depth: shallow (≤5 m), medium (≥5-20 m), and deep (≥25 m). The total content of major cations in UC groundwater is relatively low and similar at all depths, in the range of 23 to 48 meq/L (Figure 9a). LC groundwater, on the other hand, has a very high content of major cations, which implies high salinity groundwater, especially at shallow (377 meq/L) and medium (352 meq/L) depths ( Figure  9b). As a compromise, UC groundwater (Figure 9c) has significantly lower ammonium ions than LC groundwater, respectively from 0.06 to 0.11 meq/L and 0.43 to 0.57 meq/L (Figure 9d). Moreover, the concentration of NH4 + in groundwater decreases as groundwater depth increases, according to the pattern of the total content of major cations relative to depth. Overall, the cations and ammonium ion contents at these two sites indicate that ammonium is likely to be mobilized because of the cation exchange at the LC site. In addition, shallow UC groundwater has a relatively high value of δ 15 NNH4, ±7.6‰ on average (Figure 9e), which is typical of the δ 15 NNH4 ratio of household waste and animal manure [6,19]. This finding is consistent with previous assumptions about the leakage of wastewater and manure into shallow groundwater. Nevertheless, still in Figure 9e, UC groundwater at medium depths has a low δ 15 NNH4 value (±0.8‰ on average) in the range of ammonium from mineral fertilizers. At the same time, the δ 15 NNH4 value was below the detection limit in deeper UC groundwater. Finally, the δ 15 NNH4 ratio in LC groundwater was more uniform (±3‰ on average) (Figure 9f), suggesting that ammonium at all depths was potentially derived from the mineralization of nitrogen in sediments [6,19]. In addition, shallow UC groundwater has a relatively high value of δ 15 N NH4 , ±7.6‰ on average (Figure 9e), which is typical of the δ 15 N NH4 ratio of household waste and animal manure [6,19]. This finding is consistent with previous assumptions about the leakage of wastewater and manure into shallow groundwater. Nevertheless, still in Figure 9e, UC groundwater at medium depths has a low δ 15 N NH4 value (±0.8‰ on average) in the range of ammonium from mineral fertilizers. At the same time, the δ 15 N NH4 value was below the detection limit in deeper UC groundwater. Finally, the δ 15 N NH4 ratio in LC groundwater was more uniform (±3‰ on average) (Figure 9f), suggesting that ammonium at all depths was potentially derived from the mineralization of nitrogen in sediments [6,19].

Conclusions
The combined parameters of δ 15 N NH4 , coliform bacteria, land use, and geology in this research can trace well the potential sources of NH 4 + -N in the coastal alluvial groundwater in Indramayu, Indonesia. The origins of NH 4 + -N are natural and anthropogenic sources located in the lower and upper coastal areas, respectively. The mobilization of NH 4 + -N to the groundwater differ depending on the source. From the results, the following conclusions can be drawn: • NH 4 + -N in the lower coastal region, occupied by brackish-water aquaculture, potentially originated from the mineralization of organic nitrogen in sediments to ammonium. In agreement with this origin, the ratios of δ 15 N in the sediments indicate the mineralization of nitrogen. However, contamination by anthropogenic activity is possible considering the high values of total coliform bacteria. The strongly positive and significant relationship of NH 4 + -N and Na + suggests that under high salinity, the exchangeable NH 4 + is mobilized from sediments to the groundwater through cation exchange. Additionally, the high salinity of groundwater possibly arises from the brackish-water pond and marine clay. • Further, attenuation of ammonium-nitrogen from manure, sewage, and pit latrines occurs in the groundwater in the upper coastal region, where land is used mainly for agricultural and residential purposes. Both total coliform and E. coli values confirm this condition. The ratios of δ 15 N in several layers of sediments suggest the possibility of nitrogen mineralization to ammonium; nevertheless, the nitrogen contents suggest that this process is more likely in the sediments of lower coastal region. The significantly lower salinity followed by weak and not significant relationships of NH 4 + -N and all major cations indicate less possibility of NH 4 + -N mobilize to groundwater through cation exchange.