Unending traffic jams, road closures and fatal accidents are common in India’s megacities during the monsoon.
In June, 14 people were killed in Delhi during rainfall-related flooding incidents when the Safdarjang area recorded a single-day record 228mm of rain.
The rapid pace of urbanisation, along with climate change and other factors, has created unprecedented environmental challenges for cities.
Two such challenges are groundwater depletion and urban waterlogging.
But by using urban runoff more wisely, cities could solve the twin problems of urban waterlogging and depleted urban groundwater resources.
Urbanisation and its discontents
With less natural land cover and vegetation and more concrete, bricks, metal and asphalt, urban areas experience higher temperatures than their rural surroundings. This is commonly referred to as the urban heat island effect.
It’s not only hotter in these places, but several other micro-level changes such as the creation of hot spots in urban environments can also cause wide variations in rainfall at city level such as uneven rainfall distribution within short distances.
Haryana’s Faridabad has received more than 100mm per day of rainfall for at least two days during the monsoon in the last three consecutive years. This was not the case until a few years back.
Urban waterlogging is considered a human-made disaster that can severely affect people’s day-to-day life.
City roads can be submerged for a few hours to a few days. The resulting traffic congestion causes irreversible loss of productive hours; contamination of land, soil and water increases; the excess water creates a breeding ground for mosquitoes; and puts especially women and children under unwanted house arrest.
Even after the water subsides, mud, dirt and garbage deposits create an unhealthy atmosphere, and parks and gardens remain unavailable for children to play in.
Apart from extreme rainfall events, congested drains, urban density, altering the natural slope of the land and blocking streams or gullies can also promote urban waterlogging.
Similarly, depleted groundwater resources in the aquifers cities rely on are a form of environmental deterioration. Water from deeper levels requires more energy to be used to extract it and hence has a larger carbon footprint.
Over-extraction of groundwater also reduces its quality, as the remaining water has a higher amount of dissolved solids in it, which can adversely affect human health.
Rainwater is the purest form of water. However, it often becomes “drain water” before one thinks about treating or managing it appropriately for reuse.
Reusing urban runoff
To tackle these newer urban disasters, a team of hydrogeologists from Manav Rachna International Institute of Research and Studies has modified a commonly used rooftop rainwater harvesting and recharge structure and turned it into an aquifer storage and recovery system.
First, the researchers conducted a detailed investigation of Faridabad, including looking at aspects of its geomorphology, geology and hydrogeology, rainfall patterns across the city, groundwater levels and quality and favourable subsurface recharge zones where the water could easily infiltrate the city’s aquifer.
Based on this survey, an area of nearly 20 square kilometres along both sides of National Highway 19 was found to be vulnerable to urban waterlogging.
Four sites were selected for a pilot study within this area to construct and test the team’s aquifer storage and recovery system.
The system works via a four-step process.
Water harvested from gutter runoff enters first the de-siltation chamber where the suspended load of silt in water settles out.
This silt-free water then moves to the coagulation chamber where an inorganic coagulant is used to get rid of any colloidal substances in the water, substances made of particles so small they won’t settle out of the water over time.
Then the water moves to the filtration chamber where a sand filter removes all possible remaining silt, clay and dirt.
Finally, the filtered water enters the aquifer recharge well and is released to a suitable zone above the groundwater level within the vadose zone: the area between the soil surface to the top of the groundwater table.
From there, the recharged water passes through the soil to mix with the groundwater.
The aquifer storage and recovery system allows for greater and faster recharge of the aquifer and less clogging of the wells.
The recharged water passing through the vadose zone gets further filtered and acclimatised to the aquifer’s temperature before mixing with groundwater, ensuring that it is safe for later human consumption. The recharged water can be recovered from nearby production wells.
Does it work?
Results from the pilot study have suggested the aquifer storage and recovery system is working satisfactorily. Waterlogging in the catchment surrounding the four pilot sites does not persist long.
Water quality at the nearby pumping well has also improved and the pump also fails less often.
Residential parks are available for children to play in even during the monsoon which was not the case before.
Analysis of groundwater samples of nearby production wells found no presence of heavy metals, toxic elements and pollutants like pesticides, pharmaceuticals, personal care products, endocrine-disrupting chemicals, surfactants or micro plastics.
Local newspapers and resident welfare associations have reported that waterlogging is no longer persistent and their domestic reverse osmosis water filters are working more effectively due to the improvements in groundwater quality.
Following the success of the pilot project, the researchers are now looking at how this approach can be scaled up.
The system is equipped to handle larger volumes of runoff water, about two million litres per day, originating from roofs, paved areas, open areas and green land with heavy silt load during rainfall events.
However, it can be installed only in places where urban runoff is free from mixing with external pollution released from disposal sites, sewerage, grease and oil, hospital waste and industrial or commercial effluents.
Therefore, it is better to install the aquifer storage and recovery systems in areas such as parks, playgrounds and residential neighbourhoods. Further, each system has a limited intake capacity and area of influence based on characteristics of when it’s being put.
Several such systems may be required to produce a noticeable change in Faridabad’s urban waterlogging situation.
However, this technology ensures that the twin problems of urban waterlogging and groundwater depletion can be tackled simultaneously in a safe and eco-friendly manner.
Professor Arunangshu Mukherjee, Director, Manav Rachna Center for Advanced Water Technology and Management at Manav Rachna International Institute of Research and Studies has more than 30 years of experience as professional hydrogeologist. Urban hydrogeology, groundwater exploration and development, groundwater recharge are his areas of interest.
This article was originally published on 360info.org
