With every steady rain, a cycle begins which helps to recharge and replenish most aquifers of the world. The water soaks into the ground, makes its way to the aquifer via percolation through the ground and rock. When we take more water than the aquifer receives over a given year, however, we begin to deplete the groundwater source. While conservation and water management can help to decrease the rate at which we consume the water, it is becoming increasingly common for water districts to recharge or replenish their aquifers artificially.
Artificial recharge takes two basic forms; the first being passive recharge. In this method, catchments are created to capture heavy rains that may simply result in runoff and never percolate to the aquifer. By creating large basins to capture the runoff, the captured water has an opportunity to migrate through the ground to recharge the aquifer. This method is widely employed in the southwest United States and in China, as well.
A second type of artificial recharge is active recharge. In this method, water is actively pumped directly into the aquifer or at least into the porous rock above it. This method is clearly more energy-intensive than the passive style of aquifer recharge; this makes it suitable when passive methods are insufficient or impractical due to terrain or geology. Active recharge is often used to prevent saltwater intrusion into a partially-depleted aquifer. Coastal regions of the United States as well as the nation of Singapore take advantage of this technique to protect their freshwater resources. Additionally, active recharge is sometimes used in heavily-populated areas where the demands on the groundwater are particularly severe.
The practice of artificial recharge is part of a comprehensive water management plan for Mexico City and the surrounding region put into place in 2007. The plan, which also includes conservation and irrigation water recycling, was aimed at overcoming many of the water challenges that Mexico City has faced in the last century. The valley in which Mexico City is situated receives 28 inches of rainfall each year, though the vast majority of this precipitation comes in the form of heavy rains in the summer months, often associated with the remnants of hurricanes. The remainder of the year sees very little rainfall; this cycle leaves few permanent rivers and dictates a heavy reliance on groundwater.
To take advantage of the summer rains, the Mexican authorities have begun expanding the stormwater diversion basins to the north of the city. Originally, these basins were created to avert flooding associated with the seasonal storms but studies found that the water was seeping into the aquifer and actually recharging it. To maximize the advantage of this passive means of recharge, the Mexican planners have fully incorporated the basins into their project.
In addition to the fairly well-known practice of stormwater catchments and recharge, Mexico is experimenting with active pumping of treated wastewater as a means to recharge the aquifers in their region. This has been somewhat controversial, though the Mexican government has gone to great lengths to secondarily treat the wastewater prior to pumping it into the aquifer. Additionally, monitoring wells have been established to evaluate groundwater quality near the recharge stations and it is the hope of the Mexican government that any problems will be detected before they cause a health risk. So far, though, no issues have been reported.
Where plausible, artificial recharge appears to offer water planners an effective tool with which they may more effectively manage their groundwater resources. Especially when used in conjunction with conservation; it would seem that humans can help take some stress out of the natural system by helping to recharge the world’s groundwater supply.