Water purification is the removal of excess nutrients, sediments, and pollutants from wastewater and runoff.

In every year since the Mission Bay/La Jolla Water Quality Improvement Plan was first introduced in 2015, high bacteria levels have been identified as the “highest priority water quality condition in the Mission Bay Watershed.” High bacteria levels can result in illness for beachgoers or complete closures of beaches which would prevent bay-related recreation. This means that improving water quality could reduce the number of waterborne illnesses and beach closures.

There are 10 drainages of stormwater outflow into the northeast portion of Mission Bay. Bacterial advisories for these areas occur when excess nutrients in the water allow bacteria to rapidly multiply. By restoring additional plants to the bay, excess nutrients would be utilized by the plants instead of bacteria which would prevent harmful outbreaks from occurring. In fact, they have the ability to remove as much as 90% of the sediments that are present in the runoff.

We calculate this value by using historic beach advisory and closure data from the CA Water Board for the beaches closest to the restored wetlands. We assume that the restored marsh would prevent all these from occurring and then multiply the annual closures by how many people attend the beaches annually and how much a person would be willing to pay to prevent the closures.

This benefit is low because this estimate only accounts for the avoided beach closures at Mission Bay. Because there are many beaches in San Diego, a closure of one beach isn’t that costly to social welfare. We aren’t able to estimate wastewater treatment or health implications without relying on other studies. Since most of these studies are done in areas that aren’t comparable to Mission Bay, we don’t use them here. Nonetheless, we expect the value of water purification to be fairly high since it could also bring additional recreation to Mission Bay and reduce congestion at other beaches.