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

Artikel i vetenskaplig tidskrift, 2015

In the Southern part of Hanoi, high ammonium (NH 4 +) concentrations in reducing groundwater have been an issue over the last 25years. Elevated NH 4 + 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 NH 4 + levels (up to 100mg/l). We traced the sources of NH 4 + at the Nam Du well field of the Hanoi water works by means of isotope ratios (15N/14N). The δ15N values were determined for total sedimentary N and exchangeable NH 4 + of the peat material, and for NH 4 + dissolved in deep and shallow groundwater, sewage, and surface water. Groundwater NH 4 + of the upper (Holocene) and the lower (Pleistocene) aquifers had higher δ15N values than did total N and NH 4 + of the sediments, and were somewhat higher than the δ15N values of NH 4 + in sewage and surface water. We conclude that the present conditions of temperature and pH tend to promote deprotonation of NH 4 + to ammonia (NH 3 ), which eventually degasses from the groundwater table to the unsaturated pore space. This can cause an enrichment of 15N in the remaining NH 4 +, as the lighter 14N in NH 3 is volatilized at a slightly faster rate. The intermediate δ15N values within the Pleistocene aquifer can be explained by the recharge thereto, which is a mixture of the high δ15N values of the Holocene aquifer and the low δ15N values of water infiltrating from the Red River into the Pleistocene aquifer. Some part of the increased groundwater NH 4 + 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 NH 4 + in the groundwater compared to the potential surface sources, the positive correlation between NH 4 + and DOC, the abundance of natural organic matter (OM), the amount of exchangeable NH 4 + 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 NH 4 + levels in groundwater of the Nam Du well field.