Intensive Groundwater Exploitation in the Punjab – an Evaluation of Resource and Quality Trends

This report summarises initial findings from a case study investigating the response of groundwater resources in Punjab State, India, to irrigated agriculture. Punjab was central to India’s green revolution, and with fertile soils, abundant surface water and groundwater resources, Indian’s farmers soon transformed the State to be the “bread basket” of India. Currently approximately 20% and 11% respectively of India’s wheat and rice production, 10% of cotton production comes from Punjab. The aim of the case study is to examine the response of groundwater in a representative area within Punjab to current pressures from sustained intensive abstraction and pollution, investigate groundwater recharge, and forecast likely future trajectories. The Bist-Doab area was chosen as for the case study: the geology and hydrogeology is typical of the Punjab, situated on the thick and extensive multi-layered alluvial Indo-Gangetic aquifer and has an annual average rainfall of 700 mm. The Doab is one of the most productive agricultural regions in the Punjab and has experienced intense groundwater pumping from shallow aquifers for at least the last four decades. The hydrogeology of this region is best understood as an aquifer system comprising a series of thick high permeability horizons (>10 m thick) divided by thick low permeability horizons with highly variable lateral extent. Locally these are referred to as the first (shallow), second and third etc. (deep) aquifers, although the aquifers are not laterally continuous over long distrances. The following work was undertaken from 2013 to 2014: An analysis of groundwater level monitoring data (1970-2012) from the Indian Central Ground Water Board (CGWB) was carried out. New hydrochemical observations and residence time indicators (CFC and SF6) taken from 19 locations were obtained from paired shallow ( 100 mbgl) sites across the Bist-Doab under pre and post monsoon conditions. Stable isotope observations were collected and assessed within the context of an ongoing study by NIH investigating spatial and temporal changes in stable isotope chemistry in groundwater and surface water across Bist-Doab. Long-term groundwater monitoring undertaken by the CGWB since the 1970s shows declining shallow pre-monsoon groundwater levels (up to 0.8 m/y in places) across 20-25% of the Bist-Doab. Hydrographs responses imply that for some areas this has led to enhanced recharge during the monsoon. However, for the most affected region of the Bist-Doab, declining post monsoon water levels suggest that abstraction for irrigation is now outstripping the enhanced recharge potential. In the long-term this will lead to a continued decline in shallow groundwater levels pre-monsoon, currently commonly found to be >20 mbgl, with future implications for irrigation. For most sites there is a significant difference between stable isotope values for the paired deep and the shallow groundwater, with deeper sites showing isotopically depleted signatures relative to the shallow samples. This is consistent with different recharge areas and pathways for the paired sites at any given location, with the deeper sites have a greater component of water that was recharged some distance up-gradient (i.e. towards the recharge zone at the foot of the Shiwalik range). This source has a depleted isotope signature compared to the shallow aquifer due to Raleigh distillation processes as monsoon moisture tracks from the Bay of Bengal. Based on the distinct depleted stable isotope values of the Sutlej canal system, there is no evidence of significant component of regional groundwater recharge in either the shallow or deeper aquifer from this source. However, it is likely that this is important at locations in close proximity to the canal network. Results obtained using chlorofluorocarbon (CFC-12) groundwater age tracers show that average shallow groundwater mean residence times (MRTs) are 29 years and 30 years under post-monsoon and pre-monsoon conditions. Deep groundwater (>100 mbgl) had median MRTs of 45 vii years. There is no obvious relationship between deep groundwater MRTs and distance from the recharge zone at the foot of the Shiwalik hills. However, deep groundwater MRTs are much younger than would be expected under natural groundwater flow regimes, where groundwater residence times of the order of ca.102-103 years or more might be expected based on the aquifer properties and the distance from the recharge zone, some 50- 100 km down-gradient in many cases. Areas with fastest long-term declining groundwater levels show evidence of enhanced modern recharge in both shallow and deep groundwater, suggesting that there is a significant component of vertical leakage to deeper aquifers induced by long-term intensive pumping. This corroborates findings from modelling studies undertaken in analogous multi-layered alluvial systems in Gujerat, India (Rushton 1986). There is evidence of nitrate breakthrough from the shallow groundwater to depth and this is likely to be enhanced in the future if the current increases in pumping from the shallow and deep aquifers continue. This has implications for future contamination of deep sources of drinking water from other anthropogenic contaminants such as pesticides. The naturally occurring contaminants arsenic and fluoride were present at concentrations below WHO guideline drinking water limits for all sites and median concentrations were below 2 g/L and 0.4 mg/L respectively. Uranium concentrations in deep groundwater are significantly higher compared to shallow groundwater (p 15 g/L, the provisional WHO guideline concentration for drinking water is currently 30 g/L (WHO, 2012). This is a result of water-rock interactions and mineral dissolution and longer residence times. The findings from this case study have broad relevance across a large geographical area as similar groundwater typologies extend within the Indus basin, to the west across Indian Punjab, Rajasthan and Pakisan Punjab as well as in the Ganga basin to the east in the Indian states of Haryana and Delhi. While the broad findings from this study are relevant across a large geographical area, local anthropogenic and geogenic factors, as well as heterogeneity, will of course influence the recharge, hydraulic flow processes and geochemistry, and need to be considered in a consistent way.