The study described here is based on the measurements of soil gas radon–thoron concentrations performed at Dharamsala region of north-west (NW) Himalayas, India. The study area is tectonically and environmentally significant and shows the features of ductile shear zone due to the presence of distinct thrust planes. Solid state nuclear track detectors (LR-115 films) have been used for the soil gas radon–thoron monitoring. Twenty five radon–thoron discriminators with LR-115 films were installed in the borehole of about 50 cm in the study areas. The recorded radon concentration varies from 1593 to 13570 Bq/m³ with an average value of 5292 Bq/m³. The recorded thoron concentration varies from 223 to 2920 Bq/m³ with an average value of 901 Bq/m³. The anomalous value of radon–thoron has been observed near to the faults like main boundary thrust (MBT and MBT2) as well as neotectonic lineaments in the region.

This study integrates the time series of satellite data, i.e. (terrestrial water storage (TWS) extracted from Gravity Recovery and Climate Experiment (GRACE), soil moisture (SM) from Global Land Data Assimilation System (GLDAS), rainfall from Tropical Rainfall Measuring Mission (TRMM)) with in-situ data, i.e. (groundwater (GW) observatory well data and surface water (SW) as reservoir level from Tamil Nadu Public Work Department (TN PWD)) to estimate the water storage of the study area for the period from 2002 March to 2016 December. The study area encompasses three districts of Tamil Nadu State – Chennai, Kancheepuram and Tiruvallur. It was previously observed that the groundwater level in this study area is decreasing at an alarming rate. Trends and residuals of the times series of all the previously stated components were analyzed. Negative trends were observed in the case of $\Delta$SW, $\Delta$SM, $\Delta$GW, $\Delta$TWS-GRACE, but rainfall shows a slight positive trend. Rainfall, being the input for water storage, a marginal increase in rainfall cannot affect the variability of $\Delta$SM, $\Delta$SW, $\Delta$GW and $\Delta$TWS-GRACE. $\Delta$TWS decreases approximately 0.12 cm yr$^{-1}$ averaged water level equal to a total volume −7.5 km$^{3}$. The result of this study shows a decrease in the total storage of groundwater in the study area during the study period from 2002 March to 2016 December. Seawater intrusion due to an increase in sea surface height also shows an impact on $\Delta$TWS derived from grace, in the form of anomalously elevated $\Delta$TWS. A good agreement between $\Delta$TWS-GRACE and $\Delta$TWS- in-situ is found except where sea-water intrusion occurs.

$\bf{Highlights}$

$\bullet$ Areal extent of the study area is less than the optimum suggested for GRACE related investigation. The amplitude of the GRACE signal is 300 km and it is recommended that the study area to be larger than that. Our study is perhaps the first such attempt where the study area is much less than recommended.

$\bullet$ We were able to get a good correlation between $\Delta$GWGRACE and $\Delta$GWINSITU despite the challenge of spatial resolution.

$\bullet$ A significant finding of the investigation is that distinction in estimates of groundwater changes between GRACE estimates and in-situ perception happens in grids which mostly have sea-area or are intensely impact by urbanization related LULC changes.

$\bullet$ The impact of various precipitation mechanisms in the examination area was contemplated.