Topic - Under Ground Water

Cities are home to about half of the global population and urban population has doubled in the last 50 years from 1.5 billion people in 1975 to 3.5 billion people in 2015. This urban population will rise to a predicted 5 billion people by 2050. So, it’s probable that most of us will be reading this from a city. So, city dwellers, do you know how deep your groundwater level is? And is it rising or falling?

It is often written that groundwater is a hidden resource. But in urban areas, when there is a protracted fall in groundwater levels, subsidence may occur. A recent paper published in the journal Science mapped subsidence at a global scale and identified that the most common cause was excessive groundwater extraction.

Many people don’t realize groundwater flooding can also occur. When groundwater rises close to the land surface, it could have unwanted consequences, including damage to infrastructure such as flooded basements and tunnels; loss of amenities, such as flooding of underground venues.

In some cities, continued over-abstraction of groundwater can lead to a fall in groundwater levels. An often-cited example is that of Mumbai, where over-abstraction brought the groundwater level down to around 40 m below the surface, leading to subsidence and saltwater intrusion. This trend was successfully reversed by better regulatory management of the groundwater resource.

Alternatively, groundwater levels may progressively rise closer to the land surface over time. Typically, this is due to the cessation of over-abstraction, firstly as water-intensive manufacturing industries close or move away from an urban area, or secondly, if the urban groundwater resource becomes too contaminated to abstract.

In some locations, where the groundwater level is naturally close to the surface and there is a strongly seasonal climate, a seasonal variation in groundwater level can be expected and planned for.

Then, costs for piping water from the groundwater source to the user can be reduced. However, as with all water resources, it must be managed to avoid overexploitation (i.e., pumping more than the natural recharge, and progressively impoverishing the groundwater stock) and, in some cases, it must also be protected to prevent any degradation of its quality.

By - Nitin Kumar