N-budget system and applications of nitrogen/oxygen isotope in nitrate

Although NO3− is the most common N compound in oxygenated groundwater, NH4+

can be the dominant form because groundwater is in a strongly reductive state

(Lindenbaum, 2012). The transformation processes among main N compounds (NO3−,

NO2−, N2O, NH4+, N2) include the cycling processes of nitrification, denitrification, N

fixation, assimilation, mineralization, anammox, and feammox (Fig. 2). Researchers

often use the N isotope to identify the causes of depletion of and increase in each N

compound and to distinguish between the sources of NO3− and NH4+ (Norrman et al.,

2015; Scheiber et al., 2016; Otero et al., 2008; Hosono et al., 2011).

Due to its relatively stable characteristics, stable isotopes such as ²H, ¹⁸O, ³⁴S and

15N have been useful in tracing the origins of water, contaminants and the source of

dissolved constituents. Stable isotopes could be the fingerprints of the environment,

regardless of temporal and geographic scales. For more than a decade, researchers in

Taiwan have demonstrated considerable cases in evaluating the source of contaminants

in groundwater. With information from local spatial distribution, researchers can provide

strong and conclusive evidence about the source of contaminants, which is immediate

concern of the affected populace. As of, stable isotopes have been utilized to understand

the biogeochemical processes in groundwater systems as well (Kendall, 1998; Cook and

Herczeg, 2000; Kao et al., 2011).

Many investigators have successfully applied several isotopes in groundwater

environment studies, including δD and δ¹⁸O in H2O, δ³⁴S and δ¹⁸O in SO42-, δ¹⁵N and

δ¹⁸O in NO3-. Briefly, δD and δ¹⁸O have commonly been used to understand the origin

and mixing of H2O in groundwater (Clark and Fritz, 1997; IAEA 1983). In Taiwan, the

stable isotopes δD and δ¹⁸O are regarded as applicable tracers for investigating hydrologic

relations between different water bodies (Liu 1984; Wang and Peng 2001; Peng et al.,

2007). δ³⁴SSO4 and δ¹⁸OSO4 have been useful in determining the sulfur cycling that occurs

in the coastal aquifer. The origin of SO42- in groundwater is various, which may be derived

naturally during dissolution of gypsum or oxidation of S^2-. δ³⁴SSO4 and δ¹⁸OSO4 have been

used as tracers of (1) in different natural sources of SO42- (modern seawater, dissolution

of sulfate minerals, and soil sulfates) (Clark and Fritz, 1997; Krouse and Mauer, 2000);

(2) man-made SO42- (sewage, agrochemicals, detergents and SO42- of industrial origin)

(Torssander et al., 2006; Brenot et al., 2007; Otero et al., 2008); (3) S redox processes

(oxidation of S^2- and reduction of SO42-) (Seiler et al., 2011; Kao et al., 2011). In addition,

many studies have been carried out using δ¹⁵NNO3 and δ¹⁸ONO3 in order to discriminate

between organic (e.g., human or animal manure) and inorganic (e.g., chemical fertilizers)

N contaminants in waters (Robinson and Bottrell, 1997; Otero et al., 2008; Hosono et al.,

2011).

Tracing of NO3- and SO42- sources/sinks by N and S isotope compositions,

respectively, is depending on kinetic and thermodynamic fractionation processes. The

NO3- contamination in shallow groundwater may suffer by the combined impact of

fertilizer and septic tank effluent. The jointed data of ¹⁵N and ¹⁸O provide an effective

tool to distinguish between NO3- of different origin and to evaluate the N-budget of a

soil-water system (Fig. 3). The organic N, which is originated from manure or fertilizers, can

be transformed back to NH4+ for recycling in the N-budget system, and that the

commercial urea fertilizers (NH2CONH2) decompose in groundwater to NH4+ may also

be subsequently oxidized by nitrification to NO3- (Fig. 2; Eq. (2)).

In proximal fan of Choushui river alluvial fan, NO3- is the most stable species in

oxidizing condition in groundwater, and the various sources of NO3- can be distinguished

by analyzing ¹⁵N and ¹⁸O. Next, a variable biological reaction requires anoxic conditions

and accessible organic substrates such as dissolved organic carbon (DOC) (Eq. (3)).

These principal N-transforming reactions are regarded as a crucial factor for the

distribution of isotopes in NO3- and NH4+ in groundwater. Kendall and Aravena (2000)

reported that the negative correlation of δ¹⁵NNO3 versus NO3- concentration showed that

the residual NO3- was enriched in ¹⁵N exponentially as NO3- concentration decreased,

which might be caused by denitrification process. The denitrification of a NO3- fertilizer

with an original δ¹⁵NNO3 value of +0 ‰ can yield residual δ¹⁵NNO3 value of +15 to +30

‰. However, it may be an obstacle in differentiating N sources, since the range is similar

with that being expected from manure or septic waste (Clark and Fritz, 1997).

The NO3− contamination of shallow groundwater may result from the combined

impact of fertilizer application and septic tank effluent leakage. The combination of

δ¹⁵NNO3 and δ¹⁸ONO3 data can be effectively used for distinguishing between NO3− from

different origins and for evaluating the N-budget of a soil-water system (Fig. 3).

Nitrification NH4++2O2→NO3-+2H⁺+H2O (2)

Denitrification NO3-+5/4CH2O→1/2N2+5/4HCO3-+1/4H⁺+1/2H2O (3)

Fig. 2. Schematic of major N transformation pathways (modified from Canfield, 2010).

Fig. 3. Typical ranges of δ¹⁵NNO3 and δ¹⁸ONO3 values for various nitrate sources (modified from Kendall et al., 2007).

The purpose of the study is to identify of the possible multiple redox processes of N

associated with As mobilization in groundwater. As we know, a few number of researches

focused on the impact of N cycling on As migration. It has been reported that NO3- limited

the reduction of iron oxides by consuming available electron donors, and consequently

the release of As (Pauwels et al., 2000). Previous N isotope study showed that

denitrification occurred in the aquifer (Li et al., 2010), yet the influence of denitrification

on As mobilization remained to be understood. The negative linear relationship between

NO3- and SO42- during denitrification processes is governed by Eq. (4). In this process,

the Rayleigh function can be applied to both ¹⁵N and ¹⁸O to determine the enrichment

factors that dominate in groundwater. The oxidation of As-bearing pyrite results in

subsequent As release, which can be identified by the correlation between δ³⁴SSO4and

δ¹⁸OSO4 (Van Stempvoort and Krouse, 1994). The adsorption of As by newly precipitated

hydrous ferric oxides may be occurred by Eq. (4), then reductive dissolution of

As-Fe(OH)3 may result in enrichment of As in groundwater of reducing conditions. However

under typical aquifer conditions, iron (and sometimes manganese) sulfide (pyrite) is

typically expected to be the electron donor (Korom, 1992; Ottley et al., 1997) (Eq. (5))

FeS2 +6NO3-+4H2O→2Fe(OH)3+4SO42-+3N2+2H+ (4)

5FeS2 +14NO3-+4H⁺→N2+10SO42-+5Fe²⁺+2H2O (5)

0.5N2+5FeOOH+9H⁺↔NO3-+5Fe²⁺+7H2O (6)

The reaction (Eq. (5)) is favorable in soils as adsorption of dissolved Fe²⁺ and NH4+

onto sediment particles, and the situation is described as a reversible, linear equilibrium

reaction. Molecular oxygen was indirectly incorporated into the SO42- via the

sequestration by nitrification, however high salinity might inhibit denitrification (Rivett

et al., 2008). Geochemical studies of S and N isotopes found that pyrite oxidation

accounted for approximately 70% of the SO42- present in the zone of denitrification

(Zhang et al., 2014). Isotopic analysis suggested that denitrification might be fueled by

Eq. (6). As NO3- was consumed, more goethite might dissolve, accompanying by the

release of absorbed As. Hence, N isotope analysis can elucidate the presence of

denitrification in groundwater; it may also be an important influencer of As mobilization.

With the aid of multiple isotopes of H, O, S, and H, together with hydrogeochemical

investigation, the biogeochemical processes occurring in groundwater system of

Choushui river alluvial fan can be elucidated, and the geochemical behavior of As

affected by the biologically uptakes of essential nutrients elements for metabolisms

should be explained as well.

The results of isotopic compositions (δD and δ¹⁸OH2O, δ¹⁵NNO3 and δ¹⁸ONO3,) will be

used to reveal the potential biogeochemical processes related to the biogeochemical

cycling of N, also, to provide useful information about the sources of NO3- as well as the

impact of N cycling on As mobilization in groundwater systems.