Nitrate leaching and its implication for Fe and As mobility in a Southeast Asian aquifer

The drinking water quality of millions of people in South and Southeast Asia is at risk due to arsenic (As) contamination of groundwater and insufficient access to water treatment facilities. Intensive use of nitrogen (N) fertilizer increases the possibility of nitrate (NO3-) leaching into aquifers, yet very little is known about how the N cycle will interact with and affect the iron (Fe) and As mobility in aquifers. We hypothesized that input of NO3- into highly methanogenic aquifers can stimulate nitrate-dependent anaerobic methane oxidation (N-DAMO) and subsequently help to remove NO3- and decrease CH4 emission. We, therefore, investigated the effects of N input into aquifers and its effect on Fe and As mobility, by running a set of microcosm experiments using aquifer sediment from Van Phuc, Vietnam supplemented with 15NO3- and 13CH4. Additionally, we assessed the effect of N-DAMO by inoculating the sediment with two different N-DAMO enrichment cultures (N-DAMO(O) and N-DAMO(V)). We found that native microbial communities and both N-DAMO enrichments could efficiently consume nearly 5 mM NO3- in 5 days. In an uninoculated setup, NO3- was preferentially used over Fe(III) as electron acceptor and consequently inhibited Fe(III) reduction and As mobilization. The addition of N-DAMO(O) and N-DAMO(V) enrichment cultures led to substantial Fe(III) reduction followed by the release of Fe2+ (0.190{+/-}0.002 mM and 0.350{+/-}0.007 mM, respectively) and buildup of sedimentary Fe(II) (11.20{+/-}0.20 mM and 10.91{+/-}0.47 mM, respectively) at the end of the experiment (day 64). Only in the N-DAMO(O) inoculated setup, As was mobilized (27.1{+/-}10.8 g/L), while in the setup inoculated with N-DAMO(V) a significant amount of Mn (24.15{+/-}0.41 mg/L) was released to the water. Methane oxidation and 13CO2 formation were observed only in the inoculated setups, suggesting that the native microbial community did not have sufficient potential for N-DAMO. An increase of NH4+ implied that dissimilatory nitrate reduction to ammonium (DNRA) took place in both inoculated setups. The archaeal community in all treatments was dominated by Ca. Methanoperedens while the bacterial community consisted largely of various denitrifiers. Overall, our results suggest that input of N fertilizers to the aquifer decreases As mobility and that CH4 cannot serve as an electron donor for the native NO3- reducing community. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=141 SRC="FIGDIR/small/511258v1_ufig1.gif" ALT="Figure 1"> View larger version (31K): org.highwire.dtl.DTLVardef@1ac9adborg.highwire.dtl.DTLVardef@eaa3ccorg.highwire.dtl.DTLVardef@a7d1a1org.highwire.dtl.DTLVardef@1b3dee9_HPS_FORMAT_FIGEXP M_FIG C_FIG