Why are hydrologists pumping treated sewage water into the ground?

For decades, millions of gallons of treated sewage has been pumped into the ground. It sounds harmful, but it's actually quite the opposite. This process 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 further 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, 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 very rigorous treatment process to remove all the nasty 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 very 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.

"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.

There are also some drawbacks of this process. Some cons might be the public perception of using water that was formerly sewer 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. Tens to hundreds of millions of dollars which gets mitigated by the cost savings over time of not having to use imported drinking water.

"...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," Steven Phillips Hydrologist & Groundwater Specialist U.S. Geological Survey said.

Agencies such as the WRD must provide artificial replenishment to supplement natural replenishment and keep the groundwater basins balanced to avoid more water taken out than going in.

By using recycled water for recharge, these basins and aquifers will be self sufficient for its local groundwater supply and will be less susceptible to drought and water shortages experts said.

The use of recycled water to replace imported water has been a real problem solver for now, but does not solve the problem forever because more groundwater is pumped out than is naturally replenished.

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