Excessive exploitation of groundwater causes groundwater level decline, but statistical significance and spatio-temporal patterns of groundwater level trends in Hanoi, Vietnam still remain poorly understood. A fairly wide groundwater monitoring network was not set up until 1995, and we have created and maintained a groundwater monitoring database (GMD). Using the longest monthly records to date (1995-2009) at a dense network of 21 observation wells available in Hanoi obtained from our GMD, this paper explored trends and their slopes in groundwater levels of Holocene unconfined aquifers (HUA) by utilizing the non-parametric Mann-Kendall trend test and Sen’s slope estimator. At each well, 17 other time series encompassing important components (e.g. seasonal mean, annual mean, and so on) were analyzed to provide further insights into the trends. A brief interpretation of possible causes and impacts of the identified trends, and comparisons to other Asian cities were also done. Analyses for monthly data revealed that half the points showed downward trends while, about 10% of the points showed upward trends. The spatial patterns of different trend and their slopes were also identified. The results of trends and trend slopes in the other time series were quite similar to those in monthly time series. While annual cycles in groundwater levels were primarily caused by rainfall and river water levels, downward trends could be explained by increasing groundwater pumping. The findings are indispensable for further groundwater analyses required to ensure sustainable groundwater development.
