LOS ANGELES — Mayor Eric Garcetti signed an agreement May 7 authorizing construction of a $40 million facility aimed at cleaning up polluted stormwater runoff from Los Angeles International Airport before it flows into the Santa Monica Bay.

The underground facility will put 100 million gallons of the water into the groundwater basin each year in an attempt to replenish the water supply.

The five-acre open field where it is to be located will become park space.

About $30 million of the project’s cost will be covered by bond proceeds from the voter-approved Proposition O to pay for water clean-up projects.

Runoff from LAX now flows untreated into the ocean down an open ditch.

The project includes building a facility that diverts the runoff by gravity into a retention basin, where it will be pumped into underground infiltration galleries to put water into the groundwater basin. The agreement also includes a connection that will bring the runoff to the Hyperion Water Reclamation Plant, which treats sewage.

“We must re-imagine our relationship with water,” Garcetti said at the project site near LAX where he signed the agreement. “We must be responsible with how we treat it, across its entire cycle. We can no longer afford to let stormwater run off as pollution into our ocean. We must clean it, we must capture it, and we must put it to good use.”

The agreement, which was also signed by airport and city sanitation officials, lays out the funding and work assignments for the project.

The project is “a cost-effective approach to complying with all the requirements associated with the Clean Water Act, while benefiting LAX and our neighboring communities,” said Gina Marie Lindsey, executive director of Los Angeles World Airport, which operates the LAX and other city-owned airports.