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

Anna Fadliah Rusydi

^1,4

Shin-Ichi Onodera

^2,*,

Mitsuyo Saito

³,

Fujio Hyodo

³,

Morihiro Maeda

³,

Khori Sugianti

⁴ and

Sunarya Wibawa

⁴

Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashihiroshima 7398521, Japan

Graduate School of Advanced Science and Engineering, Hiroshima University, Higashihiroshima 7398521, Japan

Graduate School of Environmental and Life Science, Okayama University, Okamaya 7008530, Japan

⁴

Research Center for Geotechnology, Indonesian Institute of Sciences, Bandung 40135, Indonesia

Author to whom correspondence should be addressed.

Water 2021, 13(1), 25; https://doi.org/10.3390/w13010025

Submission received: 25 November 2020 / Revised: 16 December 2020 / Accepted: 22 December 2020 / Published: 25 December 2020

(This article belongs to the Section Hydrology)

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Graphical abstract
"> Figure 1
Land-use categories (a) and land-uses change from 2002 to 2019 (b) in Indramayu, Indonesia. "> Figure 2
Sampling points in the study site. "> Figure 3
Geology (a) and vertical profile in the B–A cross section (b) of Indramayu, Indonesia (Completely redrawn and modified after Maria et al., 2018 [<a href="#B29-water-13-00025" class="html-bibr">29</a>]). "> Figure 4
Exchangeable NH4+ ratio (a,b), δ15N (c,d), %N (e,f), and cation exchange capacity (CEC) (g,h), in the sediments of the upper and lower coastal area. "> Figure 5
Concentrations of NO3−–N and NH4+–N (mg/L) in the groundwater samples. "> Figure 6
Relationship of dissolved organic carbon (DOC) with NO3−–N (a) and NH4+–N (b) in the groundwater samples. "> Figure 7
NH4+–N concentrations (mg/L) and δ15NNH4 values (‰) in the groundwater samples, and potential sources of NH4+–N based on δ15NNH4 values of several sources (ranges values of δ15NNH4 compositions for soil nitrogen, animal manure, and mineral fertilizer are adapted from Norrman et al., 2015 [<a href="#B6-water-13-00025" class="html-bibr">6</a>]; and for organic matter and household waste are adapted from Nikolenko et al., 2018 [<a href="#B19-water-13-00025" class="html-bibr">19</a>]). "> Figure 8
Relationship of NH4+–N and Na+ (a), K+ (b), Mg2+ (c), Ca2+ (d) in groundwater samples. "> Figure 9
Average total major cations (a,b) (meq/L), NH4+ contents (meq/L) (c,d), and δ15NNH4 values (‰) (e,f) in three depths of groundwater in upper and lower coastal areas. ">

Versions Notes

Abstract

The origin of ammonium-nitrogen in Indonesian coastal groundwater has not been intensively 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 δ¹⁵N_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 δ¹⁵N_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.

Keywords:

ammonium-nitrogen; nitrogen isotope; coliform; sodium; alluvial groundwater; Indramayu

Graphical Abstract

1. 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 (NH₄⁺–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, NH₄⁺–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 (δ¹⁵N) is one of the most reliable and widely used ammonium tracer [6,18,19]. However, some issues need to be overcome for interpreting the δ¹⁵N value. For example, the different sources of ammonium may have similar δ¹⁵N 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 (δ¹⁵N), 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 NH₄⁺–N contents in groundwater.

2. 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.

2.1. 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).

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).

2.2. 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 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].

3. Samples Collection and Analysis

3.1. 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⁺, Mg²⁺, and Ca²⁺), stable isotopes of nitrogen (δ¹⁵N), nitrogen content (%), and the cation exchange capacity (CEC). Method of analysis for each parameter is as follows: Composition of δ¹⁵N (‰) was determined by Thermo Finnigan DELTAplus Advantage+FLASH2000; N content (%) was measured by high temperature combustion; exchangeable NH₄⁺ was measured by soil extraction with NH₄O_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 δ¹⁵N 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.

3.2. 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 NH₄⁺–N, nitrate-nitrogen (NO₃⁻–N), and nitrite-nitrogen (NO₂⁻–N) were determined using a continuous-flow automated nutrient analyzer (SwAAt, BLTEC, Tokyo, Japan) at the Biogeochemistry Laboratory, Hiroshima University, Japan. The analysis of NH₄⁺–N (δ¹⁵N_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 δ¹⁵N_NH4 analysis was performed at the Laboratory of Biogeochemistry, Hiroshima University, Japan. The NH₄⁺–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 NH₄⁺–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 NH₄⁺–N content greater than 20 μg. First, 10 samples and 14 samples from the LC and UC groundwater, respectively, meeting the requirements for δ¹⁵N_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⁺, Mg²⁺, and Ca²⁺, 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⁺, Mg²⁺, and Ca²⁺, 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 NH₄⁺–N relationship with DOC and major cations. The effect of DOC and major cations on NH₄⁺–N concentrations is demonstrated by coefficient determination (R²). Furthermore, the significance of the relationship is shown by the p value with 95% confidence level.

4. Results and Discussion

4.1. 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 δ¹⁵N value does not change considerably with sediment depth and texture in either UC or LC deposits (Figure 4c,d). The δ¹⁵N 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 δ¹⁵N 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.

4.2. 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 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 δ¹⁵N_NH4 and NH₄⁺–N in Figure 7 shows that δ¹⁵N_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 NH₄⁺–N concentrations, from 2.35 to 14.6 mg/L (Table 1). Still in Figure 7, this also presents the isotopic composition of δ¹⁵N_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 δ¹⁵N_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].

On the other hand, Figure 7 also shows that the δ¹⁵N_NH4 values in the UC groundwater are varied, from −2.9 to 16.1‰ (Table 1), within a relatively narrow concentration range of NH₄⁺–N, from 0.42 to 3.19 mg/L (Table 1), suggesting that ammonium is derived from diverse sources. The compositions of δ¹⁵N_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 δ¹⁵N_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 NH₄⁺–N concentrations and δ¹⁵N_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.

4.3. Relationship between Ammonium-Nitrogen and Water Chemical Properties

The relatively high concentration of NH₄⁺–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 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.

Figure 9 shows the average concentrations of total major cations, NH₄⁺, and δ¹⁵N_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 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 δ¹⁵N_NH4, ±7.6‰ on average (Figure 9e), which is typical of the δ¹⁵N_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 δ¹⁵N_NH4 value (±0.8‰ on average) in the range of ammonium from mineral fertilizers. At the same time, the δ¹⁵N_NH4 value was below the detection limit in deeper UC groundwater. Finally, the δ¹⁵N_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].

5. Conclusions

The combined parameters of δ¹⁵N_NH4, coliform bacteria, land use, and geology in this research can trace well the potential sources of NH₄⁺–N in the coastal alluvial groundwater in Indramayu, Indonesia. The origins of NH₄⁺–N are natural and anthropogenic sources located in the lower and upper coastal areas, respectively. The mobilization of NH₄⁺–N to the groundwater differ depending on the source. From the results, the following conclusions can be drawn:

NH₄⁺–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 δ¹⁵N 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 NH₄⁺–N and Na⁺ suggests that under high salinity, the exchangeable NH₄⁺ 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 δ¹⁵N 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 NH₄⁺–N and all major cations indicate less possibility of NH₄⁺–N mobilize to groundwater through cation exchange.

Author Contributions

Conceptualization, A.F.R. and S.-I.O.; formal analysis, A.F.R., M.S., M.M., F.H., K.S., and S.W.; funding acquisition, S.-I.O. and M.S.; investigation, A.F.R., S.-I.O., M.S., K.S., and S.W.; methodology, A.F.R., S.-I.O., and M.S.; project administration, S.-I.O. and M.S.; supervision, S.-I.O., M.M., and F.H.; writing—original draft, A.F.R.; writing—review and editing, A.F.R., S.-I.O., and M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This material is based on work supported by: 1. The Asia-Pacific Network for Global Change Research (APN) under Grant No. CRRP2019-09 MY-Onodera (funder ID: http://dx.doi.org/10.13039/100005536). 2. All opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of APN. While the information and advice in this publication are believed to be true and accurate at the date of publication, neither the editors nor APN accept any legal responsibility for any errors or omissions that may be made. APN and its member countries make no warranty, expressed or implied, with respect to the material contained herein. 3. Japanese Society for the Promotion of Sciences Grant-in-Aid for Scientific Research B (JSPS KAKENHI B) with Grant Number 17H04494. 4. Priority Research Program of Deputy of Earth Science, the Indonesian Institute of Sciences (LIPI), entitled “Degradation of Groundwater Resources and The Potential of Land Subsidence due to Climate Changes and Anthropogenic Activities in The Coastal of Cimanuk Watershed”, which was funded by LIPI in 2016–2017 fiscal year.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing not applicable.

Acknowledgments

Authors wish to thank Seiichiro Ioka from Hirosaki University and Robert M. Delinom for providing valuable discussion and highly supported analysis of some water quality parameters. Many thanks to Rizka Maria, Wahyu Purwoko, and Dady Sukmayadi from Research Center for Geotechnology Indonesian Institute of Sciences for field research support; and Kimbi Sharon Bih and Yusuke Tomozawa from Laboratory of Biogeochemistry for supporting laboratory analysis.

Conflicts of Interest

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Figure 1. Land-use categories (a) and land-uses change from 2002 to 2019 (b) in Indramayu, Indonesia.

Figure 2. Sampling points in the study site.

Figure 3. Geology (a) and vertical profile in the B–A cross section (b) of Indramayu, Indonesia (Completely redrawn and modified after Maria et al., 2018 [29]).

Figure 4. Exchangeable NH₄⁺ ratio (a,b), δ¹⁵N (c,d), %N (e,f), and cation exchange capacity (CEC) (g,h), in the sediments of the upper and lower coastal area.

Figure 5. Concentrations of NO₃⁻–N and NH₄⁺–N (mg/L) in the groundwater samples.

Figure 6. Relationship of dissolved organic carbon (DOC) with NO₃⁻–N (a) and NH₄⁺–N (b) in the groundwater samples.

Figure 7. NH₄⁺–N concentrations (mg/L) and δ¹⁵N_NH4 values (‰) in the groundwater samples, and potential sources of NH₄⁺–N based on δ¹⁵N_NH4 values of several sources (ranges values of δ¹⁵N_NH4 compositions for soil nitrogen, animal manure, and mineral fertilizer are adapted from Norrman et al., 2015 [6]; and for organic matter and household waste are adapted from Nikolenko et al., 2018 [19]).

Figure 8. Relationship of NH₄⁺–N and Na⁺ (a), K⁺ (b), Mg²⁺ (c), Ca²⁺ (d) in groundwater samples.

Figure 9. Average total major cations (a,b) (meq/L), NH₄⁺ contents (meq/L) (c,d), and δ¹⁵N_NH4 values (‰) (e,f) in three depths of groundwater in upper and lower coastal areas.

Table 1. NH₄⁺−N concentrations, δ¹⁵N_NH4 values, and coliform bacteria.

Sample	Aquifer	Location	NH₄⁺−N	δ¹⁵N_NH4	Total Coliform	E. coli
Sample	m	Location	mg/L	‰	(No./100 mL)
DH01	25	Upper coastal aquifer	0.2	UD	NM	NM
DH02	16		3.15	9.2	1100	150
DH03	2		0.71	4.7	1100	210
DH07	3		2.48	16.1	2400	2400
DH08	4		1.87	3.7	2400	210
DH09	8		1.58	3.6	210	210
DH12	12		0.32	UD	NM	NM
DH13	16		0.18	UD	NM	NM
DH14	5		2.33	9.5	NM	NM
DH16	5		3.19	9.5	2400	1100
DH17	10		0.07	UD	NM	NM
DH18	5		0.53	UD	NM	NM
DH19	6		0.42	7.5	2400	2400
DH23	15		1.64	1.6	1100	0
DH26	5		0.77	8.7	2400	3
DH27	10		1.62	−2.9	2400	210
DH29	5		0.97	3.5	2400	1100
DH30	5		1.69	5	2400	2400
Minimum			0.42	−2.9	210	-
Maximum			3.19	16.1	2400	2400
Average			1.64	6.1	1892	866
DH05	30	Lower coastal aquifer	12.97	2.9	2400	0
DH06	20		2.35	4.0	NM	NM
DH10	20		10.88	3.8	NM	NM
DH11	100		4.21	1.8	20	0
DH15	80		2.79	1.9	7	0
DH22	20		3.19	2.4	1100	0
DH24	25		5.66	3.7	0	0
DH25	20		8.26	2.3	2400	0
DH28	20		11.81	2.2	NM	NM
DH31	5		7.95	4.8	2400	0
Minimum			2.35	1.8	0	0
Maximum			14.60	4.8	2400	0
Average			7.7	3.0	1190	0

NM = not measured; UD = under the detection limit.

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Rusydi, A.F.; Onodera, S.-I.; Saito, M.; Hyodo, F.; Maeda, M.; Sugianti, K.; Wibawa, S. Potential Sources of Ammonium-Nitrogen in the Coastal Groundwater Determined from a Combined Analysis of Nitrogen Isotope, Biological and Geological Parameters, and Land Use. Water 2021, 13, 25. https://doi.org/10.3390/w13010025

AMA Style

Rusydi AF, Onodera S-I, Saito M, Hyodo F, Maeda M, Sugianti K, Wibawa S. Potential Sources of Ammonium-Nitrogen in the Coastal Groundwater Determined from a Combined Analysis of Nitrogen Isotope, Biological and Geological Parameters, and Land Use. Water. 2021; 13(1):25. https://doi.org/10.3390/w13010025

Chicago/Turabian Style

Rusydi, Anna Fadliah, Shin-Ichi Onodera, Mitsuyo Saito, Fujio Hyodo, Morihiro Maeda, Khori Sugianti, and Sunarya Wibawa. 2021. "Potential Sources of Ammonium-Nitrogen in the Coastal Groundwater Determined from a Combined Analysis of Nitrogen Isotope, Biological and Geological Parameters, and Land Use" Water 13, no. 1: 25. https://doi.org/10.3390/w13010025

APA Style

Rusydi, A. F., Onodera, S.-I., Saito, M., Hyodo, F., Maeda, M., Sugianti, K., & Wibawa, S. (2021). Potential Sources of Ammonium-Nitrogen in the Coastal Groundwater Determined from a Combined Analysis of Nitrogen Isotope, Biological and Geological Parameters, and Land Use. Water, 13(1), 25. https://doi.org/10.3390/w13010025

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