Indo-Gangetic Fluvial System

Geomorphology

Our geomorphic and stratigraphic investigations in this region are aimed at documenting how aquifer properties vary across the region, and relating this to the geomorphic setting of each part of the system. This will give us a conceptual or predictive model for aquifer body dimensions and how they vary across the region.

The aquifer system consists of large sedimentary fans deposited by the Sutlej and Yamuna rivers. These fans were built by sandy river channel deposits which radiate outward from the fan heads at the Himalayan mountain front and which form the individual aquifer bodies within this system. These aquifer bodies are limited in both

thickness (1 to ~80 m) and width (1 to 5 km), and cannot be correlated across the entire study area. The channel deposits are separated, laterally and vertically, by silt and clay deposits. Lateral shifts in channel position, termed avulsions, have built up the conical fans over time. While the present-day river channels and the most recent paleochannels are visible at the surface, it is important to realise that similar channel deposits are found everywhere below the fan surfaces, albeit at different depths in different locations.

The fans are separated by an interfan area that has not received sediment from the Sutlej or Yamuna rivers, but has been built up instead by sediments transported from smaller, foothills-fed rivers such as the Ghaggar River. At present, the Ghaggar flows along the boundary between the Sutlej and Yamuna fans, and has re-occupied themost recently abandoned paleochannel of the Sutlej River (termed the Ghaggar-Hakra paleochannel) along part of its length.

Sedimentology

We have compiled aquifer thickness data from ~250 CGWB wells that extend to at least 200 m depth. These data show that the overall percentage of aquifer material – that is, the percentage of subsurface material that consists of sand or gravel – is high in the Sutlej and Yamuna fans, but much lower in the interfan area, even close to the Himalayan mountain front, and along the Ghaggar-Hakra palaeochannel. Aquifer bodies are also thicker, on average, in the fans compared to the interfan area and Ghaggar-Hakra palaeochannel.

The low aquifer percentage and thin aquifer bodies in this palaeochannel coincide with the area of greatest water-level decline. Aquifers make up a smaller proportion of the subsurface in the distal fan areas, but their average thickness remains uniform across the study area. These observations provide a way of predicting the proportion and thickness of aquifer bodies, even in areas without existing borehole data.

In summary, the Sutlej and Yamuna fans form the major aquifer systems, and control the thickness and stacking of individual aquifer bodies. These individual bodies are limited in both thickness and width, and are not laterally extensive. This high spatial heterogeneity appears to be related to patterns of groundwater decline, and must be considered in any future aquifer management scheme. We are now developing probabilistic maps of likely palaeochannel position, using existing aquifer thickness data and simple geomorphic rules that govern channel geometry. Such maps can be refined as additional wells are drilled, but provide an initial estimate of the likelihood of finding suitable aquifer material at a given position and depth.

Hydrogeology

To document the spatial pattern of groundwater loss, we have integrated historical water level data from state groundwater departments of Haryana, Punjab and Rajasthan (2915 wells for the period 1974-2010) together with Central Groundwater Board data from an additional 997 wells (2002-2011).

The greatest decline in groundwater levels occurred within the Ghaggar River basin along the Punjab-Haryana border, with broader areas of loss across central Punjab and Haryana. Many areas show near-stable or gradually declining groundwater levels between 1974 and 1998, but increasing rates of decline beginning in 1998-2000. Preliminary analysis suggests that district-wide patterns of rainfall, abstraction, and tubewell densities do not match these patterns, and so cannot fully explain the observed decline.

To probe further, we are estimating groundwater volume loss during 1974-2011, and carrying out detailed time series analysis using a 10x10 km grid to identify areas that show similar changes in water levels over time, to help understand the causes of groundwater level change.