Groundwater: Earth’s Hidden Freshwater Supply
Hidden beneath our feet, we find groundwater almost everywhere. About 0.65% of the water on Earth is in groundwater stored in aquifers.
If you compare groundwater to lakes and streams (combined), groundwater has more than 100 times the amount.
But the downside with groundwater is that it’s hard to extract from the ground, it takes long for it to recharge and it’s easily contaminated.
So that’s why groundwater is a delicate resource that we use as a rainy day fund.
Aquifers store groundwater
Despite the popular belief that groundwater exists as a huge lake underground, water actually exists in tiny pore spaces within rocks and soil.
Aquifers are basically porous geological formations of rock or unconsolidated materials like sand. Within aquifers, this is where groundwater is stored.
The two types of aquifers are confined and unconfined. The main difference between the two is the porosity of rocks in which water seeps through from the surface.
Unconfined aquifers are able to store and transmit water in useful quantities directly from the ground surface.
Whereas confined aquifers have an impermeable layer preventing water from seeping into the aquifer. Rainwater recharges this type of aquifer at a location farther away where it’s more porous.
Aquifers recharge from rainwater
Aquifers can go dry when they are not recharged. Rainwater and other sources of water recharges aquifers by infiltrating the ground surface. The key to recharging groundwater is porosity which is a measure of how much empty space there is between pore spaces of rocks.
In general, igneous and metamorphic rocks restrict groundwater recharge more so than sedimentary rocks. This is because pore spaces have been squeezed away due to the nature of how metamorphic and igneous rocks are formed.
So for faster groundwater recharge rates, sedimentary rocks like sandstone and limestone make better aquifers. This is because they have pore spaces that water can travel through easily.
Groundwater sustainability issues
Aquifers are important because they can supply water for extended periods of time. Particularly during periods of drought, we draw in times of need.
But when you extract too much water and too often, then sustainability issues arise. For example, the Ogallala Aquifer stretches from Nebraska all the way to Texas and New Mexico.
From 1980-1995, groundwater withdrawal was much faster than it was being recharged. For agricultural center pivot irrigation, farmers over-extracted pulling water from the Ogallala Aquifer.
Currently, its recharging at a more stable rate. This is mostly because farmers are using smarter agricultural techniques. For example, farmers are losing less water from evaporation.