For decades, millions of gallons of treated sewage water has been pumped into the ground to fight sea-level rise.
While this process sounds harmful, it's actually quite the opposite, according to experts. It is crucial in keeping salt water out of aquifers and keeping groundwater replenished.
To date, nearly 490 billion gallons of recycled water have been recharged into the Central and West Coast Basin aquifers. That is one of many water recharge projects occurring all over the world.
Taking water out of the ground at the current rate has led to sinking of land in some parts of the world. By injecting water, the porous layers soak up the water like a sponge.
Sea-level rise means there could be a greater force of the ocean to push seawater farther underground into groundwater basins, thereby risking freshwater wells becoming salty.
Virtually all coastal aquifers around the world experience seawater intrusion to some degree.
Ted Johnson, chief hydrogeologist of the Water Replenishment District of Southern California (WRD), said recycled water is pumped down into a line of barrier wells near the coast to stop this from happening.
"Our agency puts recycled water into recharge ponds to refill the aquifers and down wells to stop seawater intrusion," Johnson said.
Treated sewage water, also known as recycled water, goes through a rigorous treatment process to remove waste particles and produce clean water that can be reused for other purposes.
"Once the sewer water has been recycled, it can be put underground in two main ways: either directly on the land surface so it can naturally drain underground by gravity to replenish the underlying groundwater aquifers, or in the cases where the water needs to go hundreds of feet deep, it is pumped down wells," Johnson said.
Experts say the ground surface is unlikely to rise in any noticeable amount.
Some of the positives of this process include a reliable source that is locally available instead of having to import water from hundreds of miles away. It also has a lower carbon footprint than importing water and is also more cost-effective than importing water, experts said.
"Maybe most importantly, in a water-short area such as California, the use of recycled water means that we do not have to use drinking water for the seawater barrier wells or recharge ponds anymore – freeing up that drinking water for human consumption and helping to meet that demand," Johnson said.
However, there are some drawbacks of this process. One includes the public perception of using water that was formerly sewage water as a groundwater replenishment supply.
"We counter that by saying that all water on Earth is recycled water, it just keeps going round and round; there is no new water," Johnson said.
Another con is the expense for the startup, which can cost tens to hundreds of millions of dollars.
Steven Phillips, hydrologist and groundwater specialist of the U.S. Geological Survey, said there are some other concerns related to chemical composition of the aquifers as well as area flooding in some cases.
"...the potential for leaching chemical constituents from aquifer materials during infiltration (which can be predicted beforehand), and possibly flooding of basements if the water table rises more quickly, or in different places, than expected," Phillips said.
Agencies such as the WRD provide artificial replenishment to supplement natural replenishment in order to make sure the groundwater basins are balanced (to avoid more water being removed than added).
By using recycled water for recharge, these basins and aquifers will be self-sufficient for local groundwater supply and will be less susceptible to drought and water shortages, experts said.