Six years ago, state officials embarked on the largest environmental cleanup in California’s history, a sweeping effort to remove lead-contaminated soil from the yards of thousands of properties near a shuttered battery recycling plant in southeast Los Angeles County.

For nearly a century, a succession of companies, most recently Exide Technologies, melted lead-acid car batteries at the Vernon plant. Its emissions blanketed the lawns of nearby homes, schools and parks in a veil of brain-damaging dust, according to the California Department of Toxic Substances Control.

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California’s biggest environmental cleanup leaves lead contamination and frustration

Numerous homes that underwent remediation have been left with lead concentrations in excess of state health standards, according to USC researchers.

State officials say as many as 10,000 properties within 1.7 miles of the plant may have been contaminated. That area includes homes in the neighboring communities of Boyle Heights, East Los Angeles, Commerce, Bell and Maywood.

Since then, companies hired by the state toxics department have overseen the removal and replacement of lead-contaminated soil at nearly 4,400 properties in these communities.

Through a public records request, the Los Angeles Times obtained “letters of completion” for many of these remediated properties. These documents provide a detailed account of work crews’ cleanup efforts, soil sampling and any difficulties they might have encountered. An analysis of data from 3,370 of these records revealed that 559 properties remediated by these crews failed to meet the state’s cleanup standards.

State guidelines call for contractors to excavate layers of soil until the lead concentration is below 80 parts per million (the state health threshold), or remove 18 inches, before putting clean soil on top. In more than 16% of cases, that did not happen, according to The Times’ analysis.

Underground obstructions, such as tree roots or pipes, may have prevented cleanup crews from digging deeper in some areas. The state toxics department insisted that an inability to meet these standards posed no direct health risks because a covering of clean topsoil meant residents would not be exposed to lead unless the buried soil was unearthed.

Homeowners and residents can use the tool below to see a property’s cleanup status and whether work crews met state standards.

You can request a letter of completion from the state toxics department.

USC researchers are providing free lead testing for homeowners in affected communities.