Unplanned urbanisation and population growth are increasing the vulnerability of groundwater resources, both in terms of quality and quantity in India leading to over-abstraction of groundwater.
Groundwater management in urban areas
Urbanisation can alter hydrological cycles and groundwater recharge by increasing impervious surfaces, decreasing infiltration processes and increasing surface runoff. Expansion of urban areas lead to significant changes in the surface runoff index and urban percolation index and can increase the intensity and frequency of flooding.
Thus, groundwater management in urban areas requires an understanding of aquifers and the interactions between soil and groundwater systems. Thus estimating groundwater recharge is crucial for the management and sustainability of the resource.
Groundwater recharge in the city of Guwahati
Groundwater recharge in urban areas can occur through natural processes and also through leakages from water supply mains, sewerage networks, septic tanks, and soak ways. Such leakages can contribute significantly to groundwater recharge.
This study titled 'Quantification of urban groundwater recharge: A case study of rapidly urbanising Guwahati city, India' published in Urban Science aimed at quantifying groundwater recharge in the rapidly urbanising city of Guwahati in India.
Both natural and urban recharge processes were looked into to estimate groundwater recharge. Natural recharge sources such as rainfall, evapotranspiration, runoff, and surface water bodies were included in the study while leakages from water supply, domestic wastewater, and industrial wastewater were considered as urban recharge components.
The city receives 66.5 million litres per day (MLD) of water supply, that caters to the needs of 42.61 percent of the population. This includes water supply from the Guwahati Municipal Committee (GMC), Guwahati Jal Board (GJB), Jawaharlal Nehru National Urban Renewal Mission (JNNURM), and Japan International Cooperation Agency (JICA). The remaining population has to rely on groundwater to meet their water needs. The city has no sewerage networks or sewage treatment plants (STPs) and the generated wastewater often flows into open drains. Majority of the industries discharge their effluents into open drainage channels with the exception of a select few who have their own effluent treatment plants (ETPs).
Total natural groundwater recharge
Total natural groundwater recharge for the year 2022 was calculated by subtracting the effects of evapotranspiration and runoff from the total annual rainfall. The remaining portion of rainfall, after accounting for these losses, was taken as the amount available for groundwater recharge. Since the rate of recharge depends on the presence of impervious surfaces and the rock structure, the area available for recharge was calculated separately for alluvial plains and hilly areas. Alluvial plains, more conducive to recharge, covered an area of 98.66 sq. km, while hilly regions with a less permeable surface covered an area of 46.34 sq. km.
Different infiltration factors were used to factor in differences in infiltration capacity between plains and hills, with the value being 0.22 and 0.11 for the alluvial plains and residual hills respectively. The total volume of groundwater recharge from rainfall was calculated to be 7.99 MCM/yr, based on these area-specific infiltration rates.
Guwahati city also has many large perennial surface water bodies, which play a significant role in the groundwater recharge. Recharge from these perennial surface water bodies was estimated to be 3.11 MCM/yr.
Adding recharge from both rainfall and perennial surface water bodies, the total natural groundwater recharge for the year 2022 was calculated as 11.1 MCM/yr.
Total urban groundwater recharge
The net amount of water distributed to end-users was estimated to be 56.53 MLD (20.63 MCM/yr) after accounting for a leakage of 9.97 MLD (3.64 MCM/yr). The potential domestic wastewater production was estimated at 46.61 MLD (16.97 MCM/yr).
According to the Central Public Health and Environmental Engineering Organization (CPHEEO), Government of India, the recommended domestic water requirement in urban areas is 135 LPCD. The public water supply is unable to meet this gap and this is taken care of by extracting groundwater amounting to 91.11 MLD (33.27 MCM/yr).
After accounting for consumptive use, the potential domestic wastewater production was estimated to be 77.61MLD (28.34MCM/yr). The estimated recharge from domestic wastewater was calculated to be 111.68MLD (40.78MCM/yr) assuming that 90 percent of the wastewater would be utilised for recharge. By applying the same 90 percent recharge contribution, the recharge from industrial wastewater was estimated to be 0.32 MCM/yr.
By combining the contributions from water supply leakage, domestic wastewater, and industrial wastewater, the total urban groundwater recharge was estimated to be 44.74MCM/yr.
Urban infrastructure contributes to a major chunk of recharge
The estimated recharge of the city worked out to be 55.84 MCM, with urban infrastructure contributing 44.74 MCM, approximately four times greater than the 11.1 MCM from natural processes in 2022. This urban recharge poses a serious risk of groundwater contamination due to presence of high nitrate loads and the absence of wastewater treatment facilities. Additionally, sewer leakages could further threaten groundwater quality in the city.
Groundwater recharge in urban areas can vary due to changes in water supply, the presence of sewage treatment facilities, variations in population and land use and land cover changes. Hydrogeological conditions, rainfall patterns, and irrigation practices can also impact recharge.
There has been a steady decline in groundwater recharge in Guwahati city over the years due to rapid urbanisation and an increase in built-up areas and impervious surfaces that limit natural recharge, adding stress on the groundwater resources in the city.
This study shows that urban infrastructure plays an important role in groundwater recharge, with leakages from water supply networks and domestic and industrial wastewater contributing to the recharge. Absence of a sewage treatment facilities can lead to direct discharge of wastewater into the groundwater, affecting groundwater quality.
Construction of operational and efficient sewage treatment facilities is crucial to prevent risk to groundwater quality in the city. Additionally, reducing potable water losses by addressing leakages in the water supply network are equally important.
Thus careful planning of cities by integration of green infrastructure is extremely essential to preserve natural recharge processes. While urban infrastructure can significantly enhance recharge rates, it can also introduce contamination in the absence of effective wastewater treatment systems. As cities grow and their water demands increase, the challenge of balancing urban recharge with groundwater quality and sustainable management becomes increasingly urgent.