Tracing sources of ammonium in reducing groundwater in a well field in Hanoi (Vietnam) by means of stable nitrogen isotope (δ15N) values

In the Southern part of Hanoi, high ammonium (NH4+) concentrations in reducing groundwater have been an issue over the last 25 years. Elevated NH4+ concentrations in groundwater, in general, are an indicator of influences from anthropogenic sources, but the buried peat layers in the Red River delta formation are also hypothesized to contribute to the high NH4+ levels (up to 100 mg/l). We traced the sources of NH4+ at the Nam Du well field of the Hanoi water works by means of isotope ratios (N-15/N-14). The delta N-15 values were determined for total sedimentary N and exchangeable NH4+ of the peat material, and for NH4+ dissolved in deep and shallow groundwater, sewage, and surface water. Groundwater NH4+ of the upper (Holocene) and the lower (Pleistocene) aquifers had higher delta N-15 values than did total N and NH4+ of the sediments, and were somewhat higher than the delta N-15 values of NH4+ in sewage and surface water. We conclude that the present conditions of temperature and pH tend to promote deprotonation of NH4+ to ammonia (NH3), which eventually degasses from the groundwater table to the unsaturated pore space. This can cause an enrichment of N-15 in the remaining NH4+, as the lighter N-14 in NH3 is volatilized at a slightly faster rate. The intermediate delta N-15 values within the Pleistocene aquifer can be explained by the recharge thereto, which is a mixture of the high delta N-15 values of the Holocene aquifer and the low delta N-15 values of water infiltrating from the Red River into the Pleistocene aquifer. Some part of the increased groundwater NH4+ is likely to arise from anthropogenic activities, as supported by several indications: a large drawdown in the Pleistocene aquifer caused by Hanoi's extensive water abstraction and subsequent downward gradient from the upper Holocene aquifer; the presence of coliforms in groundwater; and a positive correlation between ammonium and DOC, Cl, Br and Ni, but a lack of correlation with As. However, the much higher concentrations of NH4+ in the groundwater compared to the potential surface sources, the positive correlation between NH4+ and DOC, the abundance of natural organic matter (OM), the amount of exchangeable NH4+ in the sediments, and the highly reducing conditions in the aquifers indicate that N-mineralization of organic N from the peat contribute substantially to the high NH4+ levels in groundwater of the Nam Du well field. (C) 2015 Elsevier Ltd. All rights reserved.