When groundwater is depleted, recycled water can be introduced into aquifers by letting it gradually filter through the soil

When you hear the word “percolation,” making coffee probably comes to mind. That’s not incorrect, but generically the word also can be used to describe the movement of a liquid through a medium.

What is Percolation?

In the context of the water cycle, the National Ocean and Atmospheric Administration’s Northwest River Forecast Center describes percolation as “the movement of water though the soil and its layers, by gravity and capillary forces.”

It’s an important natural process that scientists and water managers can calculate and leverage to ensure surface and groundwater purity and potability. It’s a particularly important process in managed aquifer recharge.

Subsurface Water Retention Technology, another water management tool, essentially is the opposite of aquifer recharge. According to Alvin Smucker, a Michigan State University professor who has been working on the system, it stops water from percolating into aquifers by installing a barrier membrane under cropland. This not only helps improve agricultural production, but also helps protect groundwater supplies from excessive fertilizer nutrient contamination.

Modeling Percolation

Water generally percolates through soil and silt strata until it reaches solid rock, where it stops and remains in an aquifer. Several soil characteristics affect how rapidly water passes through. These include soil porosity and how water-saturated the earth may already be. Shale and clay, for instance, are porous, but they typically restrict percolation while sand allows water to percolate quickly.

Percolation in these different media can be modeled mathematically and tested in a percolation or perc test, which determines the rate of percolation based on factors like soil composition and compaction. The information gleaned from the test can help to advantageously site wastewater treatment facilities.

Percolation of Reclaimed Water

Managed aquifer recharge (MAR) has been defined as, “intentional recharge of water to aquifers for subsequent recovery or environmental benefit.” It is an effective strategy to improve groundwater sustainability, and percolation plays a large part, especially in infiltration techniques for recharge of unconfined aquifers. When recharging aquifers, water is diverted to “percolation ponds,” “infiltration pits,” or, “infiltration basins,” from which it seeps into the groundwater. Source water for such programs may be stormwater, excess river flow from wet years, or even highly treated effluent derived from municipal wastewater.

In one 2015 case study, the Clay County (Florida) Utility Authority constructed a series of percolation ponds that would allow as much as 2.2 MGD of reclaimed water to gradually percolate through the ground into the shallow local aquifer. The system eliminates potential problems associated with directly discharging treated water to surface streams.

In California, hydrologists, water authorities, and Central Valley growers are trying, “to capture the atmospheric rivers,” as Tim Quinn, former director of the Association of California Water Agencies, put it. Instead of letting water rush downriver to the sea, the plan is to divert excess water into “parking lots,” where it can percolate into the aquifer. Doing so would help California store water during unusually wet years, when torrential rains from Pacific storms dump water on the state, and then it could be used to mitigate increasingly frequent and severe droughts.

Recycling Wastewater

Also, at Stanford University’s Codiga Resource Recovery Center (CR2C) in California, a new treatment technology called the membrane aerated biofilm reactor (MABR) recently underwent a yearlong testing period to establish compliance with the State’s stringent Title 22 standards. MABR treatment can turn even municipal sewage into high-quality effluent that’s safe for percolation into aquifers depleted by Central Valley agriculture.

Fluence Aspiral™ MABR units are packaged in shipping containers for easy handling and are suited to decentralized treatment, which simply means siting small or medium-size treatment plants near supply, demand, or ideally both to save the cost of lengthy pipelining.

Predicting Leaching

Percolation can be used to predict water transport factors such as the rate of leaching, or the flow of materials into water. This is most often used in agriculture to determine the movement of fertilizers or the salt content of soil. Leaching also refers to the movement of water through substances such as chemicals disturbed during mining, or waste present in landfills, which may affect groundwater supplies.

The term “percolation” is also used in water treatment to describe a technology known as a percolating filter, or trickling filter. Trickling filters are a biological wastewater treatment process — water is oxidized and purified by the process of trickling through a filtration media. This form of aerobic wastewater treatment may be used in lieu of aeration basins in packaged wastewater treatment plants or larger aeration plants.

Percolation is a process that replenishes aquifers, and, like California, many regions are beginning to treat aquifers as precious natural water storage infrastructure that can be managed for water security.

Contact Fluence for the technology and expertise to establish a managed aquifer recharge program.