The plains of north Bihar, drained by numerous rivers originating in the Himalayas also experience a reasonably high rainfall of ~1200 mm per year. Still, more than 80% of the irrigation demand in this region is met by groundwater resources. Also, the increasing population and industrialization are likely to lead to overexploitation of groundwater as in several other states of northwest India over the last 4–5 decades. This article aims to assess the groundwater dynamics in the plains of north Bihar using 30 years (1983– 2013) of groundwater level data to understand the spatial and temporal, pre- and post-monsoon characteristics using Geographical Information System (GIS) and ordinary kriging (interpolation technique) method. Groundwater storage change was estimated using the water table fluctuation method. Our analysis shows 2–3 m decline in groundwater level in several districts such as Begusarai, Bhagalpur, Samastipur, Katihar and Purnea in both pre- and post-monsoon periods in the last decade (2004–2013). Similar trends were observed in groundwater storage for Samastipur and Purnea districts; the maximum reductions in groundwater storage for the pre-monsoon period are computed as 636 MCM and 631 MCM respectively, and the values for the post-monsoon period are 289 MCM and 216 MCM respectively. Such large scale depletion in groundwater storage in such a short time span is alarming. If this trend continues unabated, it may lead to serious scarcity of water resources in this region, negatively impacting agricultural productivity and food security.