As mobility in the N-budget system

Fig. 13 indicates that high concentrations of As appeared in relatively high δ¹⁵NNO3

values in the distal fan and relatively low δ¹⁵NNO3 values in the mid-fan, which may be

governed by denitrification and feammox processes, respectively. Denitrification elevates

the values of δ¹⁵NNO3 in NO3−, whereas feammox lowers them. The released As to

groundwater is associated with reactions of Fe oxyhydroxides in both the high and the

low values of δ¹⁵NNO3 environment.

proximal fan mid fan distal fan

Smedley et al. (2002) suggested that a sequence of reduction reactions occurs when

conditions in aquifers change from aerobic conditions to anaerobic conditions. The

change begins from the microbial decomposition of organic matter, and simultaneously,

O2 (DO) is consumed and dissolved CO2 in the groundwater increases. Subsequently,

NO3− decreases because of its reduction to NO2−,N2O, and N2 (denitrification). Insoluble

manganic oxides dissolve and are reduced to soluble Mn²⁺, and hydrous ferric oxides

(often Fe³⁺ compounds) are reduced to Fe²⁺. This reduction to Fe²⁺ is followed by the

reduction of SO42− to S²⁻, fermentation, and methanogenesis, resulting in the formation

of CH4−. Finally, N2 is reduced to NH4+. The reduction of As⁵⁺ to As³⁺ often occurs after

the reduction of Fe³⁺ and before the reduction of SO42−. Because the most possible

hypothesis of As release mechanisms is the reduction of Fe oxyhydroxides (Nickson et

al., 2000; Harvey et al., 2002; Lu et al., 2010), the processes influencing Fe redox

reactions are crucial.

The enrichment of both As and NH4+ in groundwater may be attributed to the high

concentration of N and the consumption of O2 by microorganisms. The reductive

environment further enhances the reductive dissolution of As-bearing Fe oxyhydroxides

and the desorption of adsorbed As, resulting in the release of As into groundwater (Xiong

et al., 2015).

Furthermore, the presence of NO3− has a deciding influence on the redox

environment, which directly affects the mobility of As (Harvey et al., 2002; Mayorga et

al., 2013). High NO3−

concentrations in the groundwater of the proximal fan may not be

favorable for the dissolution of As-containing Fe oxyhydroxides. However, after the

reduction of NO3− and the dissolution of As-containing Fe oxyhydroxides, As⁵⁺ is

sequentially reduced to As³⁺ in the groundwater of the mid-fan and the distal fan. The

distinct denitrification process lowers the ORP, creating an anaerobic environment and

promoting the reductive dissolution of As-containing Fe oxyhydroxides; the reductive

dissolution leads to the release of As into the groundwater.

In the mid-fan and the distal fan, the reductive dissolution of Fe oxyhydroxides and

the auxiliary denitrification process are suggested to be the main processes responsible

for As release into the groundwater. Hsu et al. (2010) used factor analysis to identify

factors that govern the chemistry of As-affected groundwater, and the results suggested

that the reductive dissolution of Fe oxyhydroxides occurred in the high NH4+

concentration area in the distal fan of the Choushui River alluvial fan. According to the

statistical analysis results, Hsu et al. (2010) estimated that the denitrification processes

might lower the redox potential, creating an anaerobic environment, and also causing a

reductive release of As to groundwater. In this study, the occurrence of denitrification in

the distal fan supports their results.

Furthermore, feammox process may influence δ¹⁵N values in NO3−

. An anaerobic

reaction termed feammox is the reduction of Fe³⁺ being coupled to NH4+ oxidation

through Eq. (10) (Yang et. al, 2012; Zhang et. al, 2014).

3Fe(OH)₃+5H⁺+NH4+→3Fe²⁺ +9H2O+0.5N2 (10)

Yang et al. (2012) used labeled Fe3+ and NH4+ to assess the presence of feammox.

However, the importance of feammox remains unknown because microcosm experiments

have not been executed. Given the complexity of N cycling, feammox remains a potential

reaction that needs to be further studied (Tekin, 2012). Feammox to N2 is energetically

favorable over a wide range of conditions including pH range. Table 3 and 4 show the

concentrations of As, NH₄⁺ and Fe (especially Fe2+) were relatively high in the mid-fan

and the distal fan, and Table 5 shows the relations between As, NH4+ and Fe were

statistically significant, reaching moderate-to-high positive correlations, both interpreting

a likely environment and enough concentrations for the occurrence of anaerobic reaction

of feammox. According to Eq. (10), when Fe oxyhydroxides are reduced by NH4+, the

desorption of adsorbed As from Fe oxyhydroxides may occur, resulting in an enriched As

and Fe²⁺ environment.

In conclusion, the denitrification and the feammox are two main processes

responsible for the release of As into the groundwater in the Choushui River alluvial fan.

However, the contribution of denitrification and feammox to As release needs to be

further quantified because some chemical reactions without presence of N compound,

such as anaerobic organic matter degradation and anaerobic methane oxidation, may also

contribute to Fe³⁺ reduction, and indirectly influence the release of As.

Table 6 shows the summary of dominant N sources, N compounds and N redox

reactions in the Choushui River alluvial fan. Fig. 14 presents a site conceptual model

(SCM) of the sources and transformation of N-containing contaminants in the

As-contaminated groundwater of the Choushui River alluvial fan.

Fig. 13. Plot of As concentration on δ¹⁵NNO3 versus δ¹⁸ONO3 diagram for the 46 groundwater samples.

Table 6. Dominant N sources, N compounds and N redox reactions in the Choushui River alluvial fan.

Fan region distal fan mid-fan proximal fan

Redox status more anaerobic facultative more aerobic

Dominant

Fig. 14. The SCM of the sources and transformation of N-containing contaminants in the arsenic contaminated groundwater of the Choushui River alluvial fan.