Major ions, trace elements and evidence of groundwater contamination in Hanoi, Vietnam

Groundwater contamination is a critical issue in Hanoi because it affects the water supply and treatment of a crowded city. This study aimed to explore the groundwater quality across the whole area of Hanoi city and identify potential contamination sources. 206 groundwater samples were collected in the Holocene and Pleistocene aquifers during two consecutive seasons (rainy 2015 and dry 2016) to assess the groundwater contamination and understand which elements can be used as tracers for future monitoring. As a result, the spatial distributions of major ions (Na, K, Ca, Mg, Cl-, and SO42-) and some selected elements (As, Li, Sr, Ba, Mn, Fe, Al, B, Br, total nitrogen (TN), and P) were obtained. Distributions of elements and contaminants varied not only on depth and regions (aquifers) but also season-wise. In the upper aquifer, SO42-, B, As, Br-, TN, and P concentrations were significantly higher than the common range. In the south of Hanoi city, we found elevated Li, Na, K, Mg, Cl- and Br concentrations in the Pleistocene aquifer. Additionally, the concentrations of such as As, Sr, Ba, Mn, Fe did not follow a clear pattern. Their concentrations seem to be predominantly influenced by geological processes controlling the study area's hydro-geochemical environment. However, high concentrations of SO42-, B, Br-, TN, and P, particularly in central Hanoi, supported the idea of contamination from the surface. This study's results might help managers foster sustainable groundwater resource strategies.