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When the Well Runs Dry - Groundwater Replenishment

© Danita LaSage

Water resources are distributed unequally across the earth, meaning that some of us have more renewable freshwater than we need (Canada has something like 100,000 cubic meters per person, per year) and some of us have far less than we need – for example, Kuwait, which has a mere 75 cubic meters per year, per person of renewable freshwater resources.

The largest supply of freshwater available to humans are underground sources – aquifers like the Ogallala in the High Plains region of the US, that provide water for irrigation, industry, and personal use. As long as recharge replenishes what is withdrawn, groundwater is renewable. But when withdrawal is more than recharge, that is, when we take out more than is replaced naturally, groundwater mining occurs and water levels drop – as much as 12 meters (about 36 feet) or more in places. It’s been estimated the rate of recharge to the Ogallala is only about 3.8 cm (about 1.5 inches) a year, meaning that the aquifer will be exhausted within a hundred years if withdrawal continues at present rates.

That’s why aquifer replenishment is becoming more important in many areas. If natural recharge to shallow, unconfined aquifers is lower than withdrawal, groundwater management may involve adding water to the groundwater system to reduce the impact of overdrafts. Wastewater can be stored in recharge basins and allowed to percolate into the soil, which both cleans it of certain important contaminants, and allows it to recharge underlying aquifers. In the same sense, septic systems act to recharge shallow aquifers.

Surface water (for example, stormwater or snowmelt) can also be spread over large areas of land, allowing it to soak into the ground and effectively storing it there during wet seasons for use later on when it’s needed. Surface spreading works well in flat areas, when the upper soil layers are permeable (that is, water flows easily through them) and the water table is not too close to the surface. Irrigation water that isn’t evaporated or used by plants can also soak into the soil and recharge shallow aquifers. Around Chicago, surface water from Lake Michigan is pumped into the ground to replenish groundwater supplies.

In the case of deeper aquifers, injection wells must be used to pump water into confined aquifers and maintain their pressure. Injection wells are also useful in coastal regions where saltwater intrusion is a risk. A line of injection wells near the coastline can keep the water level above sea level, so that withdrawal wells farther inland are less likely to pull salt water into the aquifer. Unfortunately, rejection wells require frequent maintenance and are not allowed in certain areas (Wisconsin, for example, does not allow injection wells).

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