Fresh water depletion and contamination: a real problem

When speaking about this blog, several people have questioned why water scarcity is an issue. The overall amount of water on the planet is not going to change any time soon and so why should we be worried about potential water shortages affecting two-thirds of the global population by 2030? 

To answer this, we can look at how water arrived on Earth, and what processes change it now that it's here. 

Water has accumulated on the surface of the Earth over the past 4.6 billion years from various sources: volcanism releasing water vapour into the atmosphere; hydrated minerals inside rocks and the deeper Earth releasing water; and a steady bombardment of extra-terrestrial missiles, some of which are carbonaceous chondrites (undifferentiated stony meteorites) containing water.

The following diagram explains the basic principles of the hydrological cycle. Water evaporates from the surface of the Earth, condensates to form clouds, falls as precipitation back to the Earth where it either infiltrates the land and goes into groundwater supply, or forms rivers and lakes of fresh water which eventually run into the sea.

Human activity interferes with this cycle in a number of ways. Firstly, increasing temperatures due to climate change means water cannot infiltrate the ground as much in arid regions as the ground is harder, increasing surface run off and consequently how much fresh water ends up in the sea instead of groundwater. The increased temperatures will also mean there is less surface water available in rivers and lakes for immediate use due to increased evaporation. Secondly, built up areas also increase the amount of surface run off as water is unable to penetrate hard concrete buildings and roads. Drainage is applied in many agricultural areas to prevent water build up, and this increases run off by 20-30% (Kuchment, 2007).

The quality of the fresh water reserves is also affected by human activity. 70% of fresh water withdrawn from the Earth goes towards agriculture where it is used to irrigate crops that are also sprayed with a wide range of fertilisers and chemicals that are then incorporated into rivers, lakes and the water table, sometimes in significant quantities. On top of this, sewage can leak into groundwater from septic tanks, and is also often deposited into rivers (in developing countries, 80% of sewage is dumped untreated) which can cause them to carry a wide variety of diseases. Lastly, a lot of industrial waste is also discarded into the water supply all over the globe, but more so in developing countries that may have fewer regulations. A UN estimate put this figure at 300-400 mega tonnes annually. A report by the USGS points out there is often interaction between surface and sub-surface flows, which would lead to the transfer of these chemicals between rivers, lakes and aquifers thus polluting all sources of freshwater, not just the one that the pollutants were deposited into.

Groundwater is especially polluted in some developing countries such as India, where high levels of fluoride, arsenic and salinity pose health problems (Kumar & Shah, no date). Future posts will discuss areas like this in more detail, as well as covering means of increasing the fresh water supply for countries located in arid regions.

       Two boys collect water from a leaking pipe, surrounded by 

       polluted and contaminated water in India.

       Photograph: Reuters